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# Angular size of the earth when observed from the moon.

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1. The following is the question from 2014 AS Physics Olympiad, multiple choice, Q.3. What is the geometric meaning of sinking below horizon and angular size?

The question: The Moon takes 2 minutes to sink below the horizon at the equator when observed atnight (about the same time as the Sun takes to set). If the radius of the Earth is 6400km and the radius of the Moon is 1700 km, what is the angular size of the Earth whenobserved from the Moon?
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Answer is 1.9 degrees in the mark scheme.
2. anyone?
3. (Original post by tangotangopapa2)
anyone?
Angular size here means angular diameter... what's the angle between light rays coming from opposite edges of the moon.

The angular diameter of both the sun and the moon at the earth is approximately equal (as hinted in the question) - which is also why total solar eclipses of the type we occasionally get are possible.

time to sink below the horizon is the time taken between the horizon starting to block the moon from view and the horizon completely blocking the moon from view.
4. (Original post by Joinedup)
Angular size here means angular diameter... what's the angle between light rays coming from opposite edges of the moon.

The angular diameter of both the sun and the moon at the earth is approximately equal (as hinted in the question) - which is also why total solar eclipses of the type we occasionally get are possible.

time to sink below the horizon is the time taken between the horizon starting to block the moon from view and the horizon completely blocking the moon from view.
Thank you so much. So, for two objects A and B of diameter a and b the ratio of angle subtended by B on A and A on B is b/a ? Am I following it correctly?
5. (Original post by tangotangopapa2)
Thank you so much. So, for two objects A and B of diameter a and b the ratio of angle subtended by B on A and A on B is b/a ? Am I following it correctly?
well for small angles... fortunately small angles are quite common in astronomy

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