## Is the Sun always up for exactly 12 hours at the equator? (Beginner)

What is the length of day at the equator? I recently told a friend with great certainty that at the equator the sun rose and set at exactly 12 hour intervals at roughly 6 a.m. and 6 p.m. (depending on the time zone) every day of the year. Now I'm not so sure.

You're right. It is always exactly 12 hours. There are a couple of different ways you can convince yourself that this is so, but I think the easiest one is this symmetry argument:

In the Northern Hemisphere, the length of the day is longer during the months when the North Pole is tilted towards the Sun and shorter during the months when it's tilted away from the Sun. The reverse is true for the Southern Hemisphere. The Equator is exactly halfway in between the poles. So it wouldn't make any sense for a day on the equator to be longer when one of the poles is tilted towards the Sun, and shorter when the other one is.

#### Christopher Springob

Chris studies the large scale structure of the universe using the peculiar velocities of galaxies.  He got his PhD from Cornell in 2005, and is now a Research Assistant Professor at the University of Western Australia.

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