Earth Facts
describe how Eratosthenes made
the first accurate calculation of the circumference of the
Earth
recall the shape (oblate spheroid/flattened sphere) and
diameter (13 000 km) of the Earth
describe the evidence that the
Earth is approximately spherical
recall the rotation period of the
Earth (23 hours 56 minutes) and the time to rotate through 1 degree
(4 minutes)
describe the nature and discovery of the Van Allen
Belts.
There are certain facts about the Earth that you need to remember.
It is a sphere. Its actually a bit squashed so that its diameter is bigger at the equator than it is at the poles. Its average diameter is 13,000km.
How do we know it is a sphere? Here are two early pieces of evidence.
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When you watch a ship sail away the lower half of the ship seems to disappear below the horizon before the top half. This is due to the curvature of the Earth. |
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When you travel north Polaris and the constellations around it appear higher in the sky. Seeing this it is easy to imagine a spherical rotating Earth. Of course now that we have been into space and had a proper look we know for certain that the Earth is a sphere. |
How Big is the Earth?
The first person to make an accurate calculation of the circumference of the Earth was Eratosthenes of Cyrene (276 BC - 195 BC).
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Alexandria is directly 950 km north of Syene. On June 21st, the summer solstice, the sun is directly overhead in Syene at noon. As Syene is on the Tropic of Cancer it is actually at the Zenith, directly overhead. At noon (exactly the same time) in Alexandria the sun's altitude (angle above the horizon) can be measured. This is angle A on the diagram. From this we can get angle B. If we know angle B and the distance between Syene and Alexandria we can calculate the circumference of the Earth. Eratosthenes got a value for angle B of about 7 degrees. What value would this give for the circumference of the Earth? What value does this give for the diameter of the Earth? |
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How long does it take the Earth to rotate?
No doubt you will have been told 24 hours up till now. That is actually wrong.
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The Earth takes 23 hours and 56 minutes to rotate. 4 minutes less than 24 hours. This is known as a sidereal day. (sidereal = star) We can measure this very accurately by observing the stars, i.e. the time it takes for them to travel 360 degrees around Polaris, the celestial north pole. This is a time lapse photo of the sky at night around Polaris. The stars leave trails which are arcs of circles. By measuring how much of an angle they make in a certain time we can calculate how long it would take them to make a complete circle, i.e. the time it takes for the Earth to rotate. |
A solar day (the time between noon and noon) is 24 hours (on average). Why there is a difference will be discussed later.
Questions
What shape is the Earth?
What is its diameter?
How long does it take the Earth to rotate?
What is this called?
Why is Polaris an important star?
What did Eratosthenes calculate?
