The Blue Marble.

Earth as seen by `Apollo 17`_ in 1972.

Earth (from Old English eoroe; Greek Gaia; Latin Terra) is the third planet from the Sun. It is the only astronomical object known to harbor life. Earth gravitationally interacts with others objects in space, especially the Sun and the Moon.


1   Properties

1.1   Gravitational field


Gravitational field strength at different points on Earth. [32]

1.2   Velocity

The Earth moves at 19 miles per second.

2   Substance

2.1   Shape

The shape of the Earth is roughly a sphere whose circumference is 40,000 kilometers. In fact, in 1790, two years after the `French Revolution`_, the French Academy of Sciences defined the meter_ to be exactly one ten millionth of a quadrant of the Earth.

More precisely, the shape of the Earth is an `oblate ellipsoid`_ (a flattened sphere). This is because the rotation of the Earth generates `centrifugal force`_ which causes mass to accumulate near the equator. In fact, the width of the Earth (12,714km) is 42 kilometers greater than its height (12,756km). (This bulging is also why the `Chimborazo volcano`_ in Ecuador_, and not `Mount Everest`_, is the peak that's actually the farthest from the center of the Earth.) [8]_ The actual circumference is 39,931 km from pole-to-pole and 40,070 km around the equator. The average distance from its surface to its center is 6,371 km (3,959 miles).

The surface of the Earth is smoother than a `billiard ball`_; if a billiard could be scaled to the size of the Earth, its peaks and valleys would be greater.

2.2   Surface

2.2.1   Water


The water hemisphere, containing the largest possible total area of ocean, roughly centered on New Zealand.

Water covers 71% of the Earth's surface. However, as a percentage of the composition of the Earth it's a small amount. Less than 1/50 of 1%. Even on the crust, the mass of the land is 40x greater than that of the oceans.

Surface water is divided into four oceans, the Atlantic Ocean, the Pacific Ocean, the Indian Ocean, and the Arctic Ocean.

Ocean tides are created by the gravitational pull of the moon.

Mean sean level is the height that the ocean would be at a particular point if the Earth was covered in water.

This is important to define so we can know if sea levels are changing.

Defining sea level is complicated. We cannot define it as the average distance from the center of the Earth to the surface of the ocean because the Earth is not spherical. We also cannot define it as the level the sea would settle at if we modeled the Earth as an `oblate ellipsoid`_ because the mass of the earth is not uniformly distributed, so the strength of gravity differs at different points on the globe. Sea level would be higher in places where the strength of gravity is stronger and lower in places where the strength of gravity is weaker. (Empirically, the sea level can differ by up to a hundred meters, depending on the volume and density of the earth beneath it.)

Sea level changes based on climate. It rises as the Earth gets warmer, and lowers as the Earth gets cooler.

Data on the strength of gravity in different locations was gathered by comparing the orbit of satellites to what would be expected if mass were uniformly distributed.

First, there are many places on Earth where there is no sea for many kilometers, so defining sea level at those locations

2.2.2   Land

Land, sometimes referred to as dry land, is the solid surface of Earth that is not permanently covered in water. Earth's total land mass is approximately 148,939,063.133 km^2 (57,505,693.767 sq mi) which is about 29.2% of its total surface.   Continents

The definition of continents is somewhat arbitrary, and different parts of the world define them differently. The English-speaking world divides Earth's landmass into seven continents: `North America`_, `South America`_, Europe, Africa_, Asia, Australia_, and Antarctica_. Some places, particularly and South America, treat North America and South America as a single continent. The United States did so as well until the 1950s. Some places consider Europe and Asia to be a single continent.


Pangea with modern borders.


Earth originally consisted of a single continent called Pangaea. It began to break apart 175 million years ago.

The concept that the continents once formed a continuous land mass was first proposed by Alfred Wegener, the originator of the theory of `continental drift`_, in his 1912 publication The Origin of Continents (Die Entstehung der Kontinente).

Most of Earth's land exist in one hemisphere called the Land Hemisphere (from Greek meaning "half of a sphere"). The Land Hemisphere has just under seven-eighths of the land on Earth. However even in the Land Hemisphere, the ocean area still slightly exceeds the land area.

The major portion of Earth's land is located in the Northern Hemisphere.

2.3   Atmosphere

The atmosphere of Earth is the layer of gases that surround the planet Earth.

The atmosphere has five main layers, organized by temperature. From highest to lowest, they are:

  1. Exosphere
  2. Thermosphere
  3. Mesosphere
  4. Stratopshere
  5. Troposphere

2.3.1   Stratosphere

The stratosphere contains the ozone layer.

The ozone layer does not redistribute itself equally, and instead has holes, because of a polar vortex, a rotating low pressure zone above both the north and south poles, that minimizes the air exchange above these regions. The vortex limits the inflow of ozone to the depleted regions. The polar vortex above Antarctica is typically stronger than the one above the arctic, hence the ozone hole over Antarctica is generally more severe than the one over the arctic.

To be specific, it is because the chemistry that depletes the ozone occurs on the surfaces of tiny ice crystals that only form in high enough concentrations at the altitude of the ozone layer during the late winter and early spring at the center of the polar vortex. In areas outside the center of the vortex the ice crystal are dispersed by the winds of the vortex, but in the center of the gyre, they tend to concentrate and form clouds. The ice crystals serve as a base on which hydrofluorocarbon Chlorofluorocarbon compounds condense and begin acting as a catalyst for the ozone depletion reaction. In effect, it forms a giant catalytic converter hundreds of miles wide, chewing up all the ozone that comes near it.

Looks like the above was not completely accurate. The chemicals condense on the polar ice clouds, but are not active until they are activate by the return of sunlight in the late winter/early spring. The clouds act as traps, gathering the chemicals into high concentrations. Then, when the sun strikes the cloud, they are all photochemically activated and released into the atmosphere in a large pulse that catalytically reacts with ozone, although at that point, no longer on the surface of the ice crystals

2.3.2   Coriolis effect

Planes take different amounts of time to fly the same route in different directions because of winds in upper atmosphere like the jetstream that they either have at their tail or have to fly into. The prevailing direction of these winds is largely caused by the Coriolis effect, which is caused by the fact that parts of the Earth are moving at different speeds, which happens because the Earth is round and spinning.

2.4   Composition

The Earth consists of three layers: the core, the mantle, and the crust. The core, is hottest layer. The crust and upper mantle form the lithosphere, which is subdivided into tectonic plates.

Estimates based upon the averaging of thousands of chemical analyses show that the following elements compose the upper 10 miles of the crust of the Earth in approximately the percentages given (Clarke, The Data of Geochemistry).

Chemical composition of the outer 10 miles of the Earth.
Element Amount in Percent by Weights
Oxygen 46.59
Silicon 27.72
Aluminum 8.13
Iron 5.01
Calcium 3.63
Sodium 2.85
Potassium 2.28
Magnesium 2.09
Titanium 0.63
Phosphorous 0.13
Hydrogen 0.13
Manganese 0.10
All remaining 0.71

2.4.1   Tectonic plates

A tectonic plate is ...

Plates breaking apart is known as rifting, and is the process that creates new oceans with their ridges. For example Africa and South America were once on one plate, that then rifted about 100 million years ago, forming the South Atlantic Ocean between.

Plates can join together as a result of continental collisions. The join is known as a suture. An example is the Iapetus Suture which runs through eastern North America, Ireland, and Great Britain, marking where the Iapetus Ocean closed around 400 million years ago; England and Scotland were once on two seperate tectonic plates before they joined along that suture.

Tectonic plates define the oceans.

2.5   Moon

The full rotation of the Moon as seen by NASA's Lunar Reconnaissance Orbiter


Diagram of the orbit of the moon with respect to the Earth.


Galileo Galilei's drawings of the moon in 1610.

Earth has one moon, called "the Moon".

American astronauts Neil Armstrong and Buzz Aldrin were the first men to walk on the Moon. The men landed on July 20, 1969, as part of the Apollo 11 spaceflight.

The event was broadcast on live TV to a worldwide audience. Armong stepped onto the lunar surface and described the event as "one small step for [a] man, one giant leap for mankind". Apollo 11 effectively ended the `Space Race`_.

3   Orbit


The position of the Sun in the sky over the course of a year as view at a fixed time day from a fixed location on Earth (an "analemma").

Earth's orbit forms an ellipse_ around the Sun (eccentricity). The distance from the sun varies from around 147 millions kilometers to 152 million kilometers on any given year.

3.1   Axial rotation

The Earth rotates counter-clockwise as seen from above the North Pole. This makes the Sun appear to rise in the east and set in the west.

3.2   Axial tilt


A long exposure of the sky at night. The centre half circle is polaris, indicating that is slightly off-position from true north.

Axial tilt (= obliquity) is the angle between an object's rotational axis and its orbital axis. The continuous change in the axial tilt of an object is the called axial precession.

The obliquity of the Earth oscillates between 22.1 and 24.5 degrees on a 41,000 year cycle. For most of history, this angle has been 23.4 degrees (decreasing). This angle has made Polaris_ Earth's "north star", as it does not appear to move during the night. Earth has no "south star".

3.2.1   Seasons


The summer solstice is the day with the most daylight of the year. The winter solstice (Dec 20-23) is the day with the least daylight of the year.

The axial tilt of the Earth gives rise to the seasons. Twice during the year, the direction of Earth's axial tilt will align with the line between Earth and the Sun. A day on which this happens is called a solstice (from Latin sol "sun" + sistere "to stand still"). Similarly, twice during the year the direction of Earth's axial tilt will be orthogonal to the line between Earth and the Sun. A day on which this happens is called an equinox. The boundaries of the four seasons are the solstices and equinoxes.

4   Gravity

Gravity is not the same at different parts of the earth. It varies with elevation above sea level, with the latitude (the weight of a given body on the earth is less by about one part in 200 at the equator than at the poles), and with certain other random disturbing factors. Hence, to be exact, we must defined the value of gravity is to be. This is commonly taken to be 9.8066 meters/sec^2, which is approximately the mean value of gravity at sea level and latitude 45 degrees. (Called "Standard Gravity") [6]

I guess this means gravity should be thought of as every piece of matter being attracted to every other, rather than matter as combined into solid objects, hence why lattitude would matter if earth is an ellipse.

The density of the Earth at different locations affects the strength of gravity at that location.

6   Climate change


In the last 650k years, Earth has gone through 7 periods of glacial advance and retreat. The last was 7k years ago, marking the end of the Ice Age.

CO2 was demonstrated to trap heat in the mid 19th century. In the course of the last 650k years, Earth atmospheric CO2 levels has never been above 300ppm, and we know that through mineral deposits, fossils, and arctic ice leaving telltale predictable signs of how much CO2 must have been in the air at the time. Today, CO2 is over 400ppm. Not only have we kept fantastic records pre-industrial revolution, especially the Swedes for centuries, but arctic ice has acted as a more recent history of the last several dozen centuries. CO2 levels has been growing at unprecedented rates and achieving levels higher than we've ever known to occur that wasn't in the wake of planetary disaster and mass extinction. It follows that if CO2 traps heat, and there's more CO2 in the atmosphere than ever before, it's going to trap more heat than ever before.

Sea levels are rising. 17cm over the last century. The last decade alone has seen twice the rise of the previous century. So not only are the oceans rising, but the rate of rise is increasing exponentially.

The Earth's average temperature has increased since 1880, most of that has been in the last 35 years. 15 of the 16 hottest years have been since 2001. We're in a period of solar decline, where the output of the sun cycles every 11 or so years. Despite the sun putting out less energy, the average continues to rise and in 2015 the Earth's average was 1C hotter on average than in 1890. That doesn't sound like much, but if we go some 0.7C hotter, we'll match the age of the dinosaurs when the whole planet was a tropical jungle. That's not a good thing.

The ice caps are losing mass. While we've seen cycles of recession and growth, you have to consider ice is more than area, it's also thickness and density. Yes, we've seen big sheets of ice form, but A) they didn't stay, and B) how thick were they? Greenland has lost 60 cubic miles of ice and Antarctica has lost at least 30 cubic miles, both in the last decade. Greenland is not denying global warming, they're feverishly building ports to poise themselves as one of the most valuable ocean trading hubs in the world as the northern pass is opening, and it's projected you'll be able to sail across the north pole, a place you can currently stand, year-round.

The number of unprecedented intense weather events has been increasing since 1950 in the US. The number of record highs has been increasing, and record lows decreasing.

The ocean absorbs CO2 from the atmosphere. CO2 and water makes carbonic acid, - seltzer water! The oceans are 30% more acidic since the industrial revolution. 93% of The Great Barrier Reef has been bleeched and 22% and rising is dead as a consequence. The ocean currently absorbs 9.3 billion tons of CO2 a year and is currently absorbing an additional 2 billion tons annually. Not because the ocean is suddenly getting better at it, but because there's more saturation in the atmosphere.

During high school, we thought the ozone hole was going to kill us all.

What does crossing the CO2 levels crossing 400ppm mean for the rest of us?

It means we are quickly running out of time to enact the changes to carbon emissions needed to prevent more than 2 degrees C of warming since the start of the industrial revolution. This threshold is widely accepted the "safe" amount of warming where any benefits of a warmer planet are quickly overwhelmed by the problems. These include but are not limited to more drought, more wildfires and longer wildfire seasons, more extreme rainfall events due to increased atmospheric moisture availability, coral reef bleaching or loss, rising sea levels and coastal flooding, etc etc etc. The list of disruptions gets really long past that warming point and the poorest and the lowest lying nations are impacted disproportionately more than the rich but everyone will have real noticeable climate impacts. - On Camera Meteorologist, The Weather Channel

Addition: This value is important at this time of year because it is typically the minimum point for atmospheric carbon, as the growing season ends in the northern hemisphere and the trees stop using as much carbon. The southern hemisphere is entering spring, but has significantly less land than the north and so the balance is for September to be the minimum. As we continue to emit carbon, there is no clear reason that we will ever be lower than this amount again without new technology and mitigation.

6.1   Global warming


World CO2 emissions from fuel combustion by fuel. [7]


Fuel shares of CO2 emissions from fuel combustion. [7]


World CO2 emissions from fuel combustion by region. [7]

The three major source of greenhouse gas emissions are energy, industry, and agriculture.

6.2   Energy

Air conditioning generates 500MM tons of of CO2 equivalent per year, more than the entire (US?) construction industry, including concrete. [1] Demand is growing so quickly that - even if the optimists are right about possible efficiency gains - there'll be an eightfold increase in energy consumption by 2050. [2]

Artificial cooling consumes 1/6th of global energy output.

6.3   Industry

Concrete production contributes 5 per cent of Annual anthropogenic global `carbon dioxide`_ production mainly because such vast quantities are used. China's booming construction industry produces 3% alone. CO2 is a product of the main reaction that makes cement, concrete's key ingredient. However, if you replace concrete with any other material it would have a bigger carbon footprint. [4] The rule of thumb is that for every tonne of cement you make, one tonne of CO2 is produced. [4]

Concrete production is responsible for so much CO2 because making Portland cement not requires significant amounts of energy to reach 1500 Celsius, but also because the key reaction itself is the breakdown of calcium carbonate into calcium oxide and CO2. Of the 800kg of CO2 released per ton of cement, around 530kg is released by limestone decomposition reaction itself. [4]

6.4   Agriculture

Agriculture, including cattle raising, is our third-largest source of greenhouse gas emissions, after the energy and industrial sectors. [3] Chatham House, the influential British think tank, attributes 14.5 percent of global emissions to livestock — “more than the emissions produced from powering all the world’s road vehicles, trains, ships and airplanes combined.” [3] Livestock consume the yield from a quarter of all cropland worldwide. Add in grazing, and the business of making meat occupies about three-quarters of the agricultural land on the planet. [3]

Beef and dairy cattle together account for an outsize share of agriculture and its attendant problems, including almost two-thirds of all livestock emissions, according to the United Nations Food and Agriculture Organization. That’s partly because there are so many of them — 1 billion to 1.4 billion head of cattle worldwide. [3]

The emissions come partly from the fossil fuels used to plant, fertilize and harvest the feed to fatten them up for market. In addition, ruminant digestion causes cattle to belch and otherwise emit huge quantities of methane. A new study in the journal Carbon Balance and Management puts the global gas output of cattle at 120 million tons per year. [3]

Methane doesn't hang around in the atmosphere as long as carbon dioxide. But in the first 20 years after its release, it's 80 to 100 times more potent at trapping the heat of the sun and warming the planet. [3]

Wouldn't it make more sense to put a carbon tax on fossil fuel, a larger source of greenhouse gas emissions? You bet. But many people who now commute in conventional gas-fueled automobiles have no better way to get home — or to heat their homes when they get there. That broader carbon tax will require dramatically restructuring our lives. A carbon tax on beef, on the other hand, would be a relatively simple test case for such taxes and, according to the French study, only a little painful, at least at the household level: While people would tend to skip the beef bourguignon, they could substitute other meats, like pork and chicken, that have a much smaller climate change footprint. [3]

Food production is a major consumer of energy in Western society. In a survey of Canadian food production, CAEEDAC found that the food sector accounted for 11% of Canada's total energy use. This number includes the direct energy consumed by the agricultural industry, the energy used to produce fertilizers, pesticides, farm machinery, and the energy associated with the processing, packaging, transportation, and cooking of food products. Per capita, it amounts of 13,400 kcal per day. [9]

By comparison, the average amount of food calories consumed per person over the whole age distribution is approximately 2000 kcal per day. One can therefore calculate the overall food product efficiency in Canada as 2000:13,400 or roughly 1:7. [9]

For a person who consumes only locally grown and unprocessed food, the ratio of primary energy to food calories is closer to 1:1. [9]

Read more:

7   History

According to radiometric dating and additional sources of evidence, Earth formed about 4.54 billion years ago. Shortly after forming, the Earth was almost smashed to oblivion by a planet about the size of Mars_ called Theia. Fragments from this formed the Moon.

This Eon is known as the Hadean period, because it was insanely hostile. Rock samples from this period are extremely rare, not only because it was so long ago, but because much of the Earth was liquid or vapourised from frequent bombardment. The only samples we have are on the Moon, and Zircon crystals in South Africa that were formed because the Earth's surface was being glassed. Though the meteorites did contain ice that provided water for later life.

Life started 4.0 billion years ago, as soon as the Earth calmed down enough for there to be liquid water and solid continents.

7.1   Naming

The `Milky Way`_ is so named because Heracles_ bit Hera_ while breastfeeding, and her milk scattered in the sky. The word galaxy_ is related; "galactic" comes from the same root as lactose_.

7.2   Exploration

Pythagoras_ had suggested the idea of a spherical earth in 500 BC, and validated by Aristotle later in 300BC.

Aristotle gave an indirect argument that the Earth was round by looking at the Moon. Aristotle knew that lunar eclipses only occurred when the Moon was directly opposite the Sun. He deduced that these eclipses were caused by the Moon falling into shadow of the Earth. But the shadow on the Moon in an eclipse was always a circular arc. In order for the shadow of the Earth to always be circular, the Earth must be round.

Aristotle also knew there were stars one could see in Egypt but not in Greece. He reasoned that this was due to the curvature of the Earth, so that its radius was finite.


The Greek mathematician Eratosthenes_ (276 BC - 194 BC) made the first recorded measurement of the circumference of the Earth. He was able to do so using simple geometry and a rod.

Eratosthenes lived in Alexandria. He knew that in Syene, a city due south of Alexandria, that the sun cast no shadows at high noon on the summer solstice. (Syene sits on the Tropic of Cancer.) So he stood a rod in the ground vertically in Alexandria and observed if it had a shadow. The rod did have a shadow, so Eratosthenes knew the Earth was curved and not flat.

Further, it meant he could estimate the circumference of the Earth. Let \(C\) denote the center of the Earth and \(A\) denote the top of Eratosthenes's rod in Alexandria. And let \(a\) represent the sun ray strike \(A\) and \(s\) the sun ray passing through Syene. Then the line \(CA\) is a transveral_ that passes through the parallel lines \(a\) and \(s\). This means the angle between the shadow of the rod, the top of the rod, and the base of the rod, \(\alpha\), is congruent to the angle between the top of the rod, the center of the Earth, and Syene. From the height of the rod and the length of its shadow, Eratosthenes calculated this angle as 7.2 degrees.

If the rod had no shadow, that would imply that the Earth was flat. But it did cast one, so

Since the rays of the sun strike the Earth in parallel, then if a vertical rod casts a shadow in Alexandria when it doesn't in Syene, that must mean the surface of the Earth is curved, not flat. And because the rays of the sun are parallel, the transverse line

He knew the distance between the two cities from surveying trips, and was able to find the angle between the two cities by placing a vertical rod in the ground in Alexandria, measuring the length of its shadow at noon (on the summer solstice?), and comparing the result to the height of the rod. Combining the angle between the two cities and the distance between them, he was able to calculate the circumference.

Eratosthenes_ developed the notion of longitude, but it was Ptolemy_ who first used a consistent meridian for a world map in his Geographia. Between 1765 and 1811, Nevil Maskelyne published 49 issues of the Nautical Almanac based on the meridian of the Royal Observatory, Greenwich. Maskelyne's tables not only made the lunar method practicable, they also made the Greenwich meridian the universal reference point.

Aristarchus_ (310 - 230 BCE) computed the distance of the Earth to the Moon. He knew lunar eclipses were caused by the Moon passing through the shadow of the Earth. The width of the shadow of the Earth is approximately the diameter of Earth. The maximum length of a lunar eclipse is three hours. It takes one months for the Moon to go around the Earth. This is enough information to compute the distance. (How?)

As the `United Kingdom`_ grew into an advanced maritime nation, British mariners kept at least one chronometer on the mean solar time at the Royal Observatory in Greenwich, London in order to calculate their longitude from the Greenwhich meridian, which was by conventionally considered to have longitude zero degrees.

In 1884, the United States hosted the International Meridian Conference and twenty-five nations attended. Twenty two of them agreed to adopt the longitude of the Royal Observatory in Greenwich, England as the prime meridian.

8   Representation

8.1   Photography


NASA. Dec 24, 1968. "Earthrise, Seen For the First Time by Human Eyes".

On November 10, 1967, the NASA geostationary weather and communication satellite ATS-3 captured the first color photograph of the whole earth.

Astronauts Frank Borman, James Lovell, and Bill Anders, were the first to see Earth from space during the `Apollo 8`_ lunar mission on Christmas Eve 1968. The event was broadcast live, in what was, at the time, the most watched television broadcast in history. When the astronauts returned three days later, they brought with them photos of the Earth including "Earthrise".

8.2   Projections

Two groups of world maps. One tries to fit the world into a rectangle by distoring the continents. The other corrects the distortion at the cost of the rectangular shape.

The issue is "turning an orange peel into a rectangle".

GPS eliminated need of paper map for navigating sea and sky. Map projections became less about navigation and more about aesthetics.

Arguably, we unconsciously equate size with importance and power. Additionally, people tend to equate places vertically higher as more important than places vertically lower.

8.2.1   Mercator projection

The Mercator projection is a `map projection`_ created by Flemish cartographer `Gerardus Mercator`_ in 1569.

Mercator designed his map in 1569 as a navigation tool for European sailors. Makes it easier to cross an ocean. But it distorts the relative size of nations and continents. For example, it makes Greenland and Africa look nearly the same size, but Africa is in reality 14x larger. South America is almost double the size of Europe. Alaska appears larger than Mexico, when Meixco is larger. Germany appears in the middle of the map when it's actually in the northern most quarter of the Earth.

Mercator preserves shape, direction. It was design so that a line drawn between two points on the map would provide the exact angle to follow on a compass to travel between those points. This line is not shortest route, but it provides a simple reliable way to navigate across oceans. Where Mercator fails is representation of size.

When Mercator was invented, Antarctica was not yet discovered, and the north place was not of interest. Mercator was useful for precisely expressing areas near the equator.

When flying from Tokyo to Brazil, why stop over in Houston? On Mercator, looks like a detour.

8.2.2   Gall-Peter projection

Gall-Peter projection is an equal-area map. Preserves area, but distorts shape.

8.2.3   Dymaxion projection

The Dymaxion projection is a `map projection`_ invented by `Buckminster Fuller`_.

Fuller map preserves size and shape. Invented in 1946 during Cold War. Can see it was not an east vs. west conflict, but conflict between two sides of arctic ocean. However, divides the sea. When trying to explain El Nino currents, which cause extreme weather, arrows describing the ocean current have to be divided.

8.2.4   Authagraph

An authagraph (from authalic + -graph) is an approximately equal-area map projection invented by Japanese architect `Hajime Narukawa`_ in 1999. The map closely preserves size and shape, and it can be tiled to create new maps.

8.3   Global grid system

A global grid system is ...

A global grid system requires a `map projection`_ and a grid laid on top of the map. A map projection transforms a three-dimensional globe to a two dimensional-map. A grid is then overlaid on the map, forming a global grid system. This process can be accomplished in countless ways by combining map projections and grids. The most well known way to do this is with a Mercator projection and a square grid. [30]

There are only three regular polygonal tilings: triangles, squares, and hexagons.

Top three reasons for choosing Hexagons: Neighbor traversal, Subdivision, and Distortion.

Hexagons are convenient for neighbor traversal because each the neighbors of each cell in a hexagonal grid are equidistant. This is not true in triangle grids or square grids.


Triangles have three types of neighbors: edge neighbors, point neighbors centered aligned, and point neighbors center adjacent.


Square have two different types of neighbors, one type which they share an edge (in the four cardinal directions) and another type with which they share a vertex (in four diagonal directions). [30]


In a hexagonal grid, the distance between each cell and each of its neighbors is equal. (In a square grid there are two distances and in a triangular grid there are three distances.) This property simplifies performing analysis. [30]

Squares and triangles are perfect for subdivision. Hexagons do not subdivide perfectly. The best you can do is divide each cell into seven hexagons alternating clockwise, then counter clockwise, rotated 19.1 degrees from the parent. Each child is 1/7 of the area, so the total area is the same. There is a fixed amount of error.



Projecting the Earth onto faces of an icosahedron_ instead of a cube_ reduces distortion because the faces of an icosahedron are smaller.

Non-linear transformations can be applied to reduce distortion even further.

Choosing a `continental layout`_ means all the vertices of the icosahedron_ are in the water. There is one way to do this. This reduces distortion. The vertices are where distortion happens the most.

Grid cells suffer various levels of distortion depending on their location on the Earth. Cells in Google's S2 grid are heavily distorted compared to cells in Uber's H3.

Hexagons are useful for visualization. But it doesn't look as good if hexagons are all skewed.

8.4   Geocoding

Geocoding is the process of converting addresses into geographic coordinates.

Reverse geocoding is the process of converting geographic coordinates into a human-readable address

9   Resources

10   Further reading

11   References

[1]Emmett FitzGerald. Jan 16, 2018. 99% Invisible. Thermal Delight.
[2]Tim Harford. June 5, 2017. How air conditioning changed the world
[3](1, 2, 3, 4, 5, 6, 7) Richard Conniff. March 17, 2018. The Case for a Carbon Tax on Beef.
[4](1, 2, 3) James Mitchell Crow. March 2008. The concrete conudrum. Chemistry World.
[5]mredding. Dec 8, 2016. ELI5: Please explain climate change proof like I am 5.
[6]Technocracy Inc. 1945. Technocracy Study Course. Lesson 2.
[7](1, 2, 3) International Energy Agency. 2017. Key world energy statistics.
[8]Henry Reich (minutephysics). Nov 25, 2013. What is Sea Level?
[9](1, 2, 3) Justin Lemire-Elmore. April 13, 2004. The Energy Cost of Electric and Human Powered Bicycles.
[30](1, 2, 3) Issac Brodsky. 2018-06-27. H3: Uber’s Hexagonal Hierarchical Spatial Index.
[31]Joseph Gilley. 2018-01-22. H3: Tiling the Earth with Hexagons.


Telstar, the first communication satellite.

A nautical mile is a unit of measurement historically defined as one minute of latitude, which is equivalent to one sixtieth of a degree of latitude. The derived unit of speed is the knot, defined as one nautical mile per hour.

The geographical mile is the length of one minute of longitude along the Equator.

The Earth takes in about 174 Petawatts of solar radiation, reflecting about 30% back out into space.

The last large land mass discovered is a 37,000 km2 archipelago north of mainland Russia called Severnaya Zemlya. It was discovered in 1913, but the four major islands weren't mapped until 1931. There are no records of it before this time, and no evidence on any of the islands that humans had previously been there. It's considered to be a polar desert, but there are a number of plants and animals there--mostly birds and lemmings. In the 1970's US satellite imagery was detailed and complete enough to confirm that there are no more undiscovered large land masses. In 1976 these satellites did discover an island a little over 1km2 off the northeast coast of Canada.

Due to general inaccessibility, the area around the north pole represented the true ‘last frontier’ as far the possibility of undiscovered relatively large land masses existing in any serious cartographer's mind and supported by science. Dr. Rollin A Harris theorized the existence of a large arctic landmass in his 1904 publication Manual of Tides (12) and 1911 Arctic Tides (13), in which he claimed a trapezoidal landmass as large as nearly half a million square miles existed north of central canada (14) .

By the 1920’s the ability to explore by air (both in heavier than air and lighter than air craft) had gotten to the point where it was possible, though not not completely reliable, to explore the arctic by air. Although the possibility of land masses still existed and claims still surfaced occasionally (9,10), however as of the Amundsen-Ellsworth 1926 Transpolar Flight on the Norge (11) the question of unknown large landmasses was mainly put to rest.

The "revolution" in Earth imaging came with the start of the Landsat program in the 70s. Though we now know about some military/intelligence satellites used for extensive imaging, such as the Hexagon program, these were intended for spying on military assets and the like, and not for cartographic research. Landsat-1 (originally Earth Resources Technology Satellite (ERTS) when the whole program was still ERTS), launched in 1972, was effectively the first dedicated imaging satellite for civilian use. The launch of Landsat-1 basically marked the beginning of the satellite imaging "era", which has currently culminated in everyone being able to see their backyards from space with a few clicks on google.

Landsat-1 in particular was noted for discovering a tiny, previously unknown island off the Canadian coast, which to this day bears the name "Landsat Island". Whilst I don't know when cartographers felt completely settled as to the shape of the Earth-- and in fact I'm rather sure they don't-- in the simplest terms it was pretty clear at this point that there were no major landmasses of any appreciable size still out there waiting to be found.

You can locate Polaris by finding Ursa Major and Ursa Minor (Big and Little Dipper). Polaris is the end of the spoon on the Little Dipper and the edge of the cup of the Big Dipper points toward it.

Daylight Savings

DST was initially adopted as a measure to save energy during wartime. It first became law during World War I, and then kept popping back up during other wars and energy emergencies. But like many other niceties suspended during wartime -- like, say, a huge chunk of our civil rights -- it eventually became a part of regular life.

But lighting no longer comprises a significant portion of household electricity usage. People's electricity usage mainly stems from air conditioning and electronics. So, when people leave their workplaces to return home during a hotter time of day, they use their home air conditioning more, and the energy savings aren't so clear.

All planets in the solar system fit between the earth and the moon, the sun does not.

From the time it was discovered (1930) to the time it was declassified as a planet (2006), Pluto didn't complete even one revolution around the sun.

The sound made by the Krakatoa volcanic eruption in 1883 was so loud it ruptured eardrums of people 40 miles away, travelled around the world four times, and was clearly heard 3,000 miles away.


This is also for rain droplets amd cloud droplets. They are called condensation nuclei.

Thony Christie.

The first thing to make clear is the situation in terms of astronomy, cosmology and the Church in the first decade of the seventeenth-century before Galileo, Marius, Harriot, Lembo and others changed our view of the cosmos forever with the recently invented telescope. Astronomy and cosmology were not very high up on the Church’s agenda between 1600 and 1610. The vast majority of people, including the experts, still believed in a geocentric cosmos in the form developed by Ptolemaeus, the most modern version being basically that of Peuerbach and Regiomontanus. A very small handful believed in the Copernican heliocentric model, and believed is the right word because it lacked any real form of empirical proof and was burdened by all the physical problems engendered by a moving earth.

The telescopic discoveries were brand new empirical evidence and the biggest shake up in astronomy since mankind first cast its little beady eyes on the heavens. When he started to make his discoveries, late in 1609, Galileo was very much aware of the fact that he was sitting on the Renaissance equivalent of a Nobel prize, a knighthood and the keys to the treasure chest all in one and also very aware that he almost certainly wasn’t the only person making or about to make these discoveries. In the last point he was of course completely right, Harriot was ahead of him and Marius was breathing down his neck. Galileo was fully aware that if his wished to cash in then he had to get his priority claim in tout suite.

To understand this one needs to look at Galileo’s situation. In 1610 he was a forty-six year old professor of mathematics, stuck in the same rather lowly position for the last eighteen years. He was on the down hill slope to ill health, death and anonymity. He had already done his ground-breaking work on dynamics but hadn’t published it. If he were to die tomorrow nobody would remember him beyond a few close friends and his family. Now he had hit the jackpot and needed to cash in fast. He bunged his principal discoveries together in book form, in what was more a press release than a scientific report, the Sidereus Nuncius, and had it printed and published as fast as possible.

The last thing the Galileo wanted to do at this point was to annoy anybody; he wanted fame and fortune not infamy. He spent as much effort on getting permission to dedicate his small book to Cosimo Medici, the ruler of the Duchy of Florence, his home province, and his sometime private pupil, as he did on his telescopic observations. He also made very sure that the Medici would approve of the name he gave to the newly discovered moons of Jupiter; he was after preferment, which he got as a result of his clever tactical manoeuvring. He would have been mortified if his publication had caused problems with the Church in Rome because that would almost certainly have cost him any chance of an appointment to the Medici court, his main aim at the time. The Medici did in fact drop him when he finally collided with the Church in the 1620s.

The telescopic discoveries, which Galileo was the first to publish, shook up the whole of Europe and not just the Catholic Church. However the contents of Sidereus Nuncius neither disproved Ptolemaeus/Peuerbach nor did they prove Copernicus. Of course at first they did nothing at all because like all new scientific discoveries they needed to be confirmed by other astronomers. This proved to be somewhat difficult, as the available telescopes were very poor quality and Galileo was an exceptional telescopic observer. In the end it was the Church’s own official astronomers, the members of Clavius’ mathematical research group at the Collegio Romano, who with the active assistance of Galileo delivered the necessary confirmation of all of Galileo’s discoveries.

Hailed now as the greatest astronomer in Europe Galileo travelled in triumph to Rome where he was feted by the mathematicians of the Collegio Romano, who threw a banquet in his honour, had an audience with the Pope and was appointed a member of the Accademia dei Lincei who also threw a banquet in his honour. No signs of annoyance here. Galileo was appointed philosophicus and mathematicus to the Medici court in Florence, as well as professor for mathematics without teaching obligations at the University in Pisa. The humble insignificant mathematician had become a renowned social figure, almost overnight, feted and praised throughout Europe. High Church officials flocked to make his acquaintance and win his friendship, one of these, the Cardinal Maffeo Barberini, became a close friend and the cause of Galileo’s downfall later in his life.

Although nothing in the Sidereus Nuncius disproved the geocentric model of Ptolemaeus the discovery of the phases of Venus a short time later, by Galileo, Lembo, Harriot and Marius, did. The basic geocentric model was dead in the water and the Church had a problem because Holy Scripture clearly implied a geocentric cosmos. Riding on the wave of his fame Galileo wanted to go for the big one. He wanted to go down in history as the man who proved that the cosmos was heliocentric. Unfortunately he lacked a genuine proof. He had evidence that the cosmos was not geocentric and not homocentric but all the available empirical evidence satisfied both a heliocentric cosmos and a geo-heliocentric Tychonic one and it was the latter that most astronomers, still worried about the physical problems of a moving earth, tended to favour.

In the next couple of years both Galileo and the Carmelite father Paolo Antonio Foscarini tried to tell the Church how to reinterpret those passages of the Bible that contradict a heliocentric interpretation of the cosmos. This was a fundamental failure and guaranteed to annoy the Church extremely, which it did. One should remember that all of this was taking place in the middle of the Counter Reformation and on the eve of the Thirty Years War, which would kill off between one third and two thirds of the entire population of Middle Europe in what was basically an argument about who had the right to interpret the Bible.

The Church set up a commission to investigate Foscarini’s book on the subject and the commission came down very hard on heliocentricity, calling it both philosophically (read scientifically) absurd and heretical. The accusation of heresy was not confirmed by the Pope and so was never official Church doctrine, but the damage was done. Cardinal Roberto Bellarmino was instructed to inform Galileo of the commission’s judgement. In a friendly chat Bellarmino did just this, informing Galileo that he could neither hold nor teach the theory that the cosmos was heliocentric. It is important to note that the theory was banned not the hypothesis. One could continue to discuss a hypothetical heliocentric cosmos, one could not, however, claim it to be fact.

Barberini’s elevation to the Holy Throne gave Galileo the chance he had been waiting and longing for, the chance to go down in history as the man who established the heliocentric cosmos. Using his friendship with the new Pope, Galileo convinced Barberini that the German Protestants were laughing at the Catholic Church because it had rejected heliocentricity because according to those dastardly Protestants the Catholics were too stupid to understand it. Beguiled by his silver tongued friend Barberini gave Galileo permission to write and publish a book in which he would present both the Ptolemaic and Copernican systems to demonstrate the deep astronomical knowledge of the Catholics but by no means was he to favour one of the systems. Galileo wrote the book, his Dialogo, in which he was anything but impartial and unbiased, as instructed, but instead left nobody in any doubt just how superior the Copernican system was in his opinion, however he still lacked any real empirical proof. Through a series of tricks he managed to get his book past the censors and into print. Galileo had erred very badly, you don’t play the most powerful absolutist ruler of your time for a fool, particularly not when that ruler is already displaying strong signs of the paranoia that, sooner or later, effects all absolutist rulers.

Thony Christie. June 22, 2011. But it doesn’t move!

In championship boxing in a title match the challenger, if he wishes to win the title, is expected by the point judges to do more than the champion. He must be more aggressive, he must throw more punches and in general he must be significantly more active than the reigning champion. The theory is that the champion has already proved his worth, after all he is the champion, and the challenger must show that he is truly worthy to replace him. Being as good is not enough he must be better.

The same is true of scientific theories an established theory has already shown that it can explain the phenomena covered by the theory and has over a given period of time proved its worth. A new theory must show that it can explain the phenomena better than the old theory and also stand up to a thorough critical examination, these things take their time and a change of theories does not take place overnight. The transition from geocentrism to heliocentrism took about one hundred and twenty years, which is about par for the course for such a large theory.

In 1610 geocentrism had been the established theory of the universe in Europe since the Pythagoreans in the 6th century BCE. Eudoxus, Plato, Hipparcus, Aristotle and Ptolemaeus had all accepted that the earth was stationary and at the centre of the other celestial bodies that circled it. They all had different explanations of the mechanism of the system but the fundamentals were the same. In the period between the 8th and 15th centuries many Islamic astronomers and philosophers worked on those models but none of them questioned the basic facts. Also as a system for predicting the movements of the planets and the stars the Ptolemaic model had proved remarkably efficient for fourteen hundred years.

In fact it was only when Kepler produced table based on the data of Tycho and his own planetary model, which were significantly superior to everything that had gone before that heliocentricity began to find widespread acceptance.

Using the term physics here is not without problems. I am using it in the modern meaning of the term but at the beginning of the 17th century physics as we know it didn’t exist, which as we shall see is the whole problem, and the word physics meant something completely different in Aristotelian philosophy. Imagine you are sitting on a motorbike on a smooth straight road, you accelerate to 60 kph and then being a skilled and confident biker you take your hands from the handlebars and sit up straight. Now you take a newspaper out of your pocket and read it. This would of course be impossible, the headwind would blow the paper out of your hands and if you did manage to hold it tightly enough to spread it the wind would tear the paper in half. You were only travelling at 60 kph. If the world was spinning on its axis as dictated by heliocentrism someone standing on the equator would be whirling round not at 60 kph but in excess of 1600 kph! Even worse the whole world would be hurtling through space at over 100 000 kph! Now come on who are you kidding?

Now of course people at the beginning of the 17th century did not have motorbikes, or newspapers for that matter, but they did have horses and carriages and knew all about headwinds. If the earth is moving at such frightening speeds why isn’t everything on it blown away? Copernicus already knew the correct answer to this problem. The earth and everything on it is contained in an envelope, we call it the atmosphere, which travels with the earth so there are no headwinds. However the physics necessary to explain this model didn’t exist in 1610. There was no concept of force or of mass, no correct definition of inertia let alone gravity how was this envelope supposed to work? What held it in place? Why wasn’t it blown away? The physics necessary to answer these questions was developed over the 17th century. Galileo in his Discorsi, who gave the laws of fall, was one of those along with Kepler, who gave the first primitive definition of force and the concept of gravity, Stevin, who first described the vector parallelogram of force before there were vectors, Beeckman, who gave the correct definition of inertia, and others who laid the foundation of physics on which Newton would build the theory of universal gravity and thus give scientific substance to Copernicus’ envelope.

Astronomically the big bummer was stellar parallax. If the earth rotated around the sun then it must be possible to detect stellar parallax when viewing the stars from opposite extremes of the earths orbit. All attempts to do so failed miserably up to the 1820s a very black mark against heliocentricity. In fact heliocentricity was accepted long before stellar parallax was finally detected. However at the beginning of the 17th century this failure weighed heavy in the arguments against heliocentricity. The alternative and true explanation that the nearest stars were so far away that the parallax was too small to be detected with the available instruments meant distances so great as to be literally inconceivable at that time. Only over time as the accepted dimensions of the universe grew larger and larger did this alternative become plausible.

All in all to believe in heliocentricity at the beginning of the 17th century was literally an act of blind faith and those that opposed it did so on solid scientific grounds and not purely out of some sense of religious bigotry as is often claimed by those who don’t know their history of science.

From Wikipedia:

Italian scientist Giovanni Battista Riccioli and his assistant Francesco Maria Grimaldi described the effect in connection with artillery in the 1651 Almagestum Novum, writing that rotation of the Earth should cause a cannonball fired to the north to deflect to the east.[7] In 1674 Claude François Milliet Dechales described in his Cursus seu Mundus Mathematicus how the rotation of the earth should cause a deflection in the trajectories of both falling bodies and projectiles aimed toward one of the planet's poles. Riccioli, Grimaldi, and Dechales all described the effect as part of an argument against the heliocentric system of Copernicus. In other words, they argued that the Earth's rotation should create the effect, and so failure to detect the effect was evidence for an immobile Earth.

Scientists knew about ice age cycles long before they knew why they occurred. It confounded them. Then, a century ago, a Serbian scientist named Milutin Milankovic studied the earth’s position relative to the sun, and came up with the theory we now know is accurate: Our planet wobbles just enough to change how much solar radiation is let in, occasionally by enough to wreck havoc.

A few years later a Russian meteorologist named Wladimir Koppen ran with Milankovic’s work, dug a little deeper, and discovered a fascinating nuance. The prevailing idea before Koppen was that ice ages occur when the earth’s tilt supercharges the wrath of cold winters. K0ppen showed that wasn’t the case. Instead, moderately cool summers are the culprit. It begins when a summer never gets warm enough to melt the previous winter’s snow. The leftover ice base makes it easier for snow to accumulate the following winter, which increases the odds of snow sticking around in the following summer, which attracts even more accumulation the following winter. Perpetual snow reflects more of the sun’s rays, which exacerbates cooling, which brings more snowfall, and on and on.

Many conservation efforts exist because humans have a disposition to value rare things.


Sea level. Measure average level of the ocean. But what about parts of the Earth where there is no ocean. How do we know what sea level would be since there's no sea for hundred of kilometers?

So to determine the height of a mount above sea level should we use the height the sea would be if the mountain weren't there? Or the height the sea would be if there mountain weren't there but its grave were?

The people who worry about such things are called geodetic scientists or geodesists. Theydecided that we should define sea-level using the strength of gravity so they went about creating an incredibily detailed model of the Earth's gravitational field called the Earth Gravitational Model. It's incorporated into modern GPS receivers to they won't tell you you're a hundred meters below sea level when you're sitting on the beach in Sri Lanka, which has weak gravity.

The model has allowed geodetic scientists to correctly predict the average level of the ocean to within a meter everywhere on earth, which is why we also use it to define what sea level would be beneath mountains if they werne't there but there gravity were.

The south pole does not have a time zone, and simply uses GMT.

There is only one source of light in the solar system.

As the moon moves around the Earth, our perspective.

Why doesn't the earth block out the light during a full moon? But earth's orbit is tilted by about 5 degrees. So earth's full moon is moving just above or below the earth's shadow.

Still unclear to me.

Moon sometimes looks red because of refracted light by Earth's atmosphere (like a sunset).

Lunar eclipse

People used to believe that hell was physically under the Earth, and that evil spirits lived underground. This made people unwilling to take the subway when it was first opened until it was designed to look clean, bright, and safe.