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Astronomy/Sun and Moon angles

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Expert: - 2/1/2004


Question
hello, i was asked this question by a professor in class, and im still stumped. the question is this:

The fact that the Sun and the Moon subtend almost equal angles on the sky is a startling coincidence

True or False?

Thanks a lot


Answer
Hello.

Yes, I myself have always referred to it as a "startling coincidence" given the fact the Moon is barely 2160 miles across, while the Sun is around 864,000 miles across. The Sun is actually 400 times larger than the Moon in diameter.

Nervertheless, the Moon and the Sun have the same angular diameter (about 1/2 degree) in the sky, which is why total solar eclipses are possible. (Moon blocking out the Sun for certain parts of Earth).

Of course, the basis for the "coincidence" had to await astronomers' first finding the distances to each body! Then it could truly be appreciated.

Thus, though the Moon is 400 times smaller than the Sun in actual diamter, it is also 400 times closer to us in distance - and that compensation allows it to have the same angular size in the sky as the Sun.  

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Philip Stahl

Expertise

I have forty years of experience in Astronomy, specifically solar and space physics. My specialties include the physics of solar flares, sunspots, including their effects on Earth and statistics as applied to astronomical investigations.

Experience

Astronomy: more than forty years experience starting with construction of my own simple telescopes. Worked at university observatory in college, doing astrographic measurements. M.Phil. degree in Physics/Solar Physics and more than ten years as researcher.

Organizations
American Astronomical Society (Solar Physics and Dynamical Astronomy divisions), American Mathematical Society, American Geophysical Union

Publications
Solar Physics (journal), The Journal of the Royal Astronomical Society of Canada, The Proceedings of the Meudon Solar Flare Workshop (1986), The Proceedings of the Caribbean Physics Conference (1985). Books: 'Selected Analyses in Solar Flare Plasma Dynamics', 'Physics Notes for Advanced Level'.

Education/Credentials
B.A. Astronomy, M. Phil. Physics

Awards and Honors
American Astronomical Society Studentship Award (1984), Barbados Government Award for Solar Research

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