Astronomy - The Earth

The Earth

Equatorial Diameter: 12,756 km

Polar Diameter: 12,714 km

Mass: 5.97 x 10^24 kg

Moons: 1 (The Moon) and 2 co-orbital satellites

Orbit Distance: 149,598,262 km 1 Astronomical Unit

Orbit Period: 365.24 days

Surface Temperature:  -88 to 58°C

Earth is the third planet from the Sun and is the largest of the terrestrial planets.

The Earth is the only planet in our solar system not to be named after a Greek or Roman deity.

May have derived from old English word Ertha meaning ground or land.

The Earth is thought to have formed from the solar nebula, the disc-shaped cloud of gas and dust left over from the Sun's formation.

The currently accepted method by which the planets formed is accretion, in which the planets began as dust grains in orbit around the central protostar.

Through direct contact, these grains formed into clumps, which in turn collided to form larger bodies.

These gradually increased through further collisions, growing at the rate of centimetres per year over the course of the next few million years.

Protoplanetary Disk

The first facts about the Earth were worked out by the Ancient Greeks.

A good estimate of the Earth's size was made by Eratosthenes (276 BC–194 BC), using trigonometry.

The first estimate of the Earth's age based on evidence was by Benoît de Maillet(1656–1738), a French diplomat, philosopher and naturalist.

He thought the Earth must have developed by slow, natural forces.

In the last quarter of the 19th century there was a long-running debate on the age of the Earth. In Charles Lyell's Principles of Geology (1830–33), he showed that the Earth had changed slowly, and that what we see is the result of gradual changes.

This clearly meant that the Earth was ancient, though Lyell did not try to work out how old.

In 1896 Henri Becquerel discovered radioactivity, he shared the Nobel Prize in Physics with Pierre and Marie Curie

Eventually, it was realized that radioactivity was a major source of heat inside the Earth.

In 1921 came the first modern estimate, using radiometric dating. It was based on uranium-lead dating, the rate of decay of uranium to lead in the crust of the Earth, by Henry Norris Russell. He came up with 2 to 8 billion years.

In 1949, H.E. Suess estimated 4 to 5 billion years, based on a whole array of radioactive isotopes.

This is close to the time we estimate today, which has been refined further to about 4,560 million years.

Many societies assumed the Earth had always been as it is now.

Some religions raised the question of its age: the Hindu religion got closest to the present-day scientific estimate. Some Christians and Jews believe the Genesis creation narrative is literally true, which would mean that the Earth was created between 5000 and 10,000 years ago. However, these days most people think such questions are best answered by scientific methods.

The Earth was once believed to be the centre of the universe.

Due to the apparent movements of the Sun and planets in relation to their viewpoint, ancient scientists insisted that the Earth remained static, whilst other celestial bodies travelled in circular orbits around it.

Eventually, the view that the Sun was at the centre of the universe was postulated by Copernicus, though this is also not the case.

The Earth is the densest planet in the Solar System.

Earth is mostly iron, oxygen and silicon

If you could separate the Earth out into piles of material, you’d get 32.1 % iron, 30.1% oxygen, 15.1% silicon, and 13.9% magnesium. Of course, most of this iron is actually down at the core of the Earth. If you could actually get down and sample the core, it would be 88% iron.

There are three groups of rocks that make up most of the Earth's crust.

Some rock is made when the hot liquid rock comes from inside the earth (igneous rocks); another type of rock is made when sediment is laid down, usually under the sea (sedimentary rocks); and a third kind of rock is made when the other two are changed by very high temperature or pressure (metamorphic rocks). A very few rocks also fall out of the sky (meteorites).

Below the crust is warm and almost-liquid rock that is always moving around (the Earth's mantle).

Then, there is a thin liquid layer of heated rock (the outer core). This is very hot: 7,000 °C or 13,000 °F.

The middle of the inside of the Earth would be liquid as well but all the weight of the rock above it pushes it back into being solid. This solid middle part (the inner core) is almost all iron. This is what makes the Earth magnetic.

Our planet's rapid spin and molten nickel-iron core give rise to an extensive magnetic field, which, along with the atmosphere, shields us from nearly all of the harmful radiation coming from the Sun and other stars.

Earth's atmosphere protects us from meteors as well, most of which burn up before they can strike the surface.

Earths Magnetic Field

Earth's shape is a spheroid: not quite a sphere because it is slightly squashed on the top and bottom.

As Earth spins around itself, the centrifugal force forces the equator out a little and pulls the poles in a little.

The equator, around the middle of Earth's surface, is about 40,075 kilometres or 24,900 miles long.

Earth's water is believed to have come from comets and asteroids hitting Earth, making the oceans.

Only 3% water of the earth is fresh, rest 97% salted. Of that 3%, over 2% is frozen in ice sheets and glaciers.

Means less than 1% fresh water is found in lakes, rivers and underground.

Within a billion years (that is at about 3.6 billion years ago) the first life evolved, in the Archaean era.

Some bacteria developed photosynthesis, which lets plants make food from the Sun's light and water.

This released a lot of oxygen, which was first taken up by iron in solution. Eventually, free oxygen got into the atmosphere or air, making Earth's surface suitable for aerobic life.

This oxygen also formed the ozone layer which protects Earth's surface from bad ultraviolet radiation from the Sun.

Complex life on the surface of the land did not exist before the ozone layer.

The air animals and plants use to live is only the first level of the air around the Earth (the troposphere).

Above this first level, there are four other levels. The air gets colder as it goes up in the first level; in the second level (the stratosphere), the air gets warmer as it goes up.

This level has a special kind of oxygen called ozone. The ozone in this air keeps living things safe from damaging rays from the Sun.

The middle level (the mesosphere) gets colder and colder with height

The fourth level (the thermosphere) gets warmer and warmer.

The last level (the exosphere) is almost outer space and has very little air at all. It reaches about half the way to the Moon.

The three outer levels have a lot of electric power moving through them; this is called the ionosphere and is important for radio and other electric waves in the air. It is also where the Northern Lights are.

The air also keeps the Earth warm, specially the half turned away from the Sun. Some gasses especially methane and carbon dioxide  work like a blanket to keep things warm.

The Earth’s rotation is gradually slowing.

This deceleration is happening almost imperceptibly, at approximately 17 milliseconds per hundred years, although the rate at which it occurs is not perfectly uniform.

This has the effect of lengthening our days, but it happens so slowly that it could be as much as 140 million years before the length of a day will have increased to 25 hours.

Earth doesn’t take 24 hours to rotate on its axis

It’s actually 23 hours, 56 minutes and 4 seconds. This is the amount of time it takes for the Earth to completely rotate around its axis; astronomers call this a sidereal day.

A year on Earth isn’t 365 days

It’s actually 365.2564 days. It’s this extra .2564 days that creates the need for leap years.