|MadSci Network: Astronomy|
Hi, Rosemarie, You've asked quite an important question there, which leads to a lot of different bits of answer. By the time we're finished, you will know a lot more about the Earth. 1) The Earth isn't, in fact, spherical. It nearly is, but not quite. The centrifugal "force" generated by the Earth's slow rotation causes the equator to bulge just a little bit (about 20km) compared to the poles. [note to physics teachers and scientific pedants - I know that centrifugal "force" isn't a force at all, but in terms of everyday experience it is an easily understood concept. The niceties of inertial pseudo-forces and rotating reference frames can come later.] Superimposed on this ellipsoidal shape are some smaller dents and bulges caused by the uneven distribution of mass in the crust and the mantle of the Earth. These are typically only a few metres in height, but may extend over tens to thousands of km in width. 2) The shape of the Earth changes on a daily basis! Just as there is a tide in the oceans, so also is there a solid body tide within the rock beneath your feet. The Earth's surface rises and falls a few centimetres every day, under the tidal influence of the Moon and Sun. 3) The polar spin axis of the Earth is not fixed in space. Instead, the Earth slowly precesses like a spinning top, but it takes around 24,000 years for each slow "wobble". 4) Over geological time, the continents are not fixed on the Earth's surface. They move, along with attached oceanic crust, like conveyor belts across the surface of the Earth. 200 Million years ago, America was attached to Europe. Things only move a few cm each year, but they still move. It is this process (Plate Tectonics) that causes Earthquakes and localises many volcanoes in the Pacific "Ring of Fire". Most volcanoes world-wide have a link to Plate Tectonics. 5) The Earth will keep spinning steadily in space for the next 5 billion years, without any sudden catastrophic changes. Even the impact of the meteor that doomed the Creataceous dinosaurs was like a gnat being squashed on the windscreen of a speeding truck. It would take a body as large as the Moon to significantly change Earth's spin axis, and the Moon's orbit is even more stable than the Earth's spin. 6) Tides on Earth are only half the story - the same forces also act on the Moon and are accelerating it in its orbit. The Moon is steadily receding from the Earth, and tidal friction is slowing the Earth's spin and increasing the length of day. One day in the far future, the Month and the day will be the same length (about 40 days), but by then the Sun will have died, so it won't really matter. (and it's so far in the future that by then there won't be people - we'll probably have evolved to something else and colonised the galaxy). Do some reading on these topics and amaze your friends with some of the strange facts you find. Nick
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