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Thread: Why would one total solar eclipse last longer than another?

  1. #1 Why would one total solar eclipse last longer than another? 
    Forum Freshman GreatBigBore's Avatar
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    Physicist Brian Cox says that on July 16, 2186, there will be the longest solar eclipse in the last 5000 years: seven minutes. I could understand the durations being different for partial eclipses, as the moon might sometimes just graze the sun and other times cover half of it. But given any two total solar eclipses, why would their durations be different?


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    Moderator Moderator Janus's Avatar
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    The Earth and Moon both follow elliptical orbits. In July, The Earth is is its farthest from the Sun, thus the angular size of the Sun is smaller. When the Moon is at its perigee, the Moon's angular size is larger. If both these things happen during a total eclipse, the Sun takes longer from the moment it totally disappears to when the leading edge appears again.

    If the opposite occurs, the Moon will be smaller and the Sun larger. In this case, we can get what is called an annular eclipse, where the Moon is not large enough to cover the entire Sun even when they are centered on each other. You still see a thin ring, or annulus, of the Sun around the moon.


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    Forum Freshman GreatBigBore's Avatar
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    Quote Originally Posted by Janus
    The Earth and Moon both follow elliptical orbits. In July, The Earth is is its farthest from the Sun, thus the angular size of the Sun is smaller. When the Moon is at its perigee, the Moon's angular size is larger. If both these things happen during a total eclipse, the Sun takes longer from the moment it totally disappears to when the leading edge appears again.
    Ah, now I get it: the moon's apparent size is so much bigger than the sun's apparent size that the sun stays hidden longer. Thanks!
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