April 22nd: Earth Day

April 22nd is Earth Day, so here are 10 things you may not know about the planet we live on:

Picture by NASA

1. Earth is in the Milky Way galaxy, about 28,000 light years from the centre of the galaxy. A more specific description of where we are in the galaxy is about 20 light years above the galactic plane in the Orion spiral arm.
2. Around the equator, the earth measures 40075.017 km, and around the poles, 40007.86 km. So it's not a perfect sphere but is slightly flattened at the poles with a bulge around the middle.
3. There are, of course, local deviations from the spheroid shape. We call them mountains and canyons and trenches. There are as many, or more, geographical features, mountains, canyons, volcanoes, plains, etc, below the sea as above it. The largest deviations are Mount Everest (8,848m above local sea level) and the Mariana Trench (10911m below local sea level). However, the summit of Mount Everest is not the furthest point from the centre. Due to the equatorial bulge, these are summits of Mount Chimborazo in Ecuador and Huascarán in Peru. While these might seem like massive deviations to us, on a global scale, they are really not. Earth deviates by 0.17%, from the perfect spheroid. In comparison, a regulation billiard ball can deviate by up to 0.22%.
4. Earth's axis is tilted by around 23.4 degrees from the perpendicular. This is what causes the seasons as less heat from the sun reaches the areas which are angled away from it. Also, without the tilt, there would be an eclipse every two weeks, alternating between lunar eclipses and Solar Eclipses.
5. The name of the planet, earth, developed from Middle English erthe, from Old English eorthe which itself came from the Proto-Germanic erthō.
6. The earth is made of rock. The most common element within its rocky structure is iron (32.1%). The ocean floor is largely made of basalt, a volcanic rock.
7. At the centre of the earth, the temperature may be up to 6,000 °C. Only about 20% of the heat is left over from when the planet was formed. Most of it is caused by radioactive substances breaking down.
8. As of 2013, there are about 196 countries on earth, and more than seven billion people.
9. The Earth's atmosphere has no definite boundary, slowly becoming thinner and fading into outer space, so it's hard to tell where Earth ends and space begins. Humans like defined boundaries, though, so 100km above the surface, named The Kármán line, is a used as a working definition for the boundary between Earth's atmosphere and space.
10. The standard astronomical symbol for Earth is a cross circumscribed by a circle. This symbol may be called the wheel cross, sun cross, Odin's cross or Woden's cross. It represents the four Compass points.