Earth – Our Home

Earth is the third planet from the Sun and is also our home. It is the only planet in the universe that we know to have life. The Earth is quite the contrast to the other planets in our Solar System. It is a watery world with an oxygen rich atmosphere.

The Earth taken in July 1979 by Apollo 11. Credit: NASA

The Earth orbits the Sun every 365 days, which is 1 year. (It’s really 365.25 days, which is why we have a leap year every 4 years). The Earth rotates once per day, causing day and night. The best way to imagine this is to think of a spinning top spinning across a table. The movement across the table is like the orbit of the Earth and the spinning is like the rotation. The name for Earth is derived from English and German and means ground.

Earth is a rocky planet that has a solid iron core and liquid outer core, the movement of which causes a magnetic field around the planet. The magnetic field is vital for protecting life on Earth. It is an active planet that has many volcanoes. 71% of the surface is covered in water, while the remaining 29% is land featuring many different landscapes such as mountains, plains, desert, forest and cold polar regions. Temperatures across the surface vary but the average temperature across the surface is 15°C. However, this is slowly rising and although there are natural variations in climate, climate scientists have strong evidence that this rise is as a direct result of damage caused by humans through industrialisation and pollution.

Earth has one moon. It is about one quarter the size of Earth and it is tidally locked. This means that we only ever see one side of the Moon. The Moon affects the Earth in a number of ways but perhaps most significantly is causing tides and controlling the tilt of the Earth. The gravity of the Moon acts to keep the Earth tilted at exactly 23.44o. This tilt causes the seasons and if it was to vary the climate would also vary. It is thought a stable climate is vital to allow complex life to develop. The leading theory about its formation, is that in the early days of the Solar System, the Earth was struck a glancing blow by a smaller object and the impact created a ring of material around the planet which would eventually clump together to form the Moon.

One difference that you may notice when comparing the Earth to other rocky bodies, such as Mercury or the Moon is fewer impact craters. This isn’t because there have been fewer impacts to the Earth’s surface. There are some impact craters as shown in the photo below but there are two reasons for the low numbers of craters. The first is that our atmosphere protects us from the smaller meteorites that hit the Earth. When they pass through the atmosphere the friction against the air causes them to burn up. Only the largest make it to the ground and large collisions are, thankfully, rare. The second reason is our weather. Wind, rain, ice, dust and all the other elements slowly erode craters from the surface of Earth. This process doesn’t happen with the same speed on other planets without an atmosphere, running water or weather. If you look at the picture below of the Manicouagan Crater in Canada you can see the effects water has had on this crater. Many others have completely been wiped from the surface.

Manicouagan Crater and reservoir located primarily in Manicouagan Regional County Municipality in the Cote-Nord region of Quebec, Canada. Scientists believe the crater was caused by the impact of a 5 kilometer (3 mile) diameter asteroid about 215.5 million years ago (Triassic Period). Taken from the International Space Station. Credit: NASA

It’s no surprise that Earth is the best studied of all of the planets. However, there are still lots of areas we know very little about, such as the deep ocean. Earth has been around for approximately 4.5 billion years and yet it is only 59 years since a human left the surface of Earth and ventured into space. In December 1968, Apollo 8 travelled to the Moon for the first time and the astronauts on board, Jim Lovel, Frank Boreman and William Anders, were the first humans to ever see the entire Earth against the blackness of Space. On Christmas Eve 1968, as Apollo 8 orbited the Moon, William Anders took a photo of the Earth rising over the surface of the Moon. It is one of the most iconic photos ever taken. When you stand on the surface of Earth on a clear day you might see 5 to 10 km, depending on where you are. It’s easy from this vantage point to think of the Earth as huge, but go to Moon, look back at Earth, and you can cover it with your fist. The Earth rise photo gives us some context and reminds us just how small and fragile the Earth is in the vastness of the Universe. On 24th December 2018, 50 years after it was taken, William Anders said

Here was everything humans had been, everything we were, and everything we might become — and yet our home planet was physically insignificant in space

We set out to explore the moon and instead discovered the Earth.

William Anders – Apollo 8 Lunar Module Pilot – 24th December 2018.
Earthrise taken by William Anders aboard Apollo 8 on Christmas Eve 1968. Credit: NASA

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