Good Evening,

my 9yo daughter asked me a curious question about the EArth travelling around the Sun -

I am well studied and pride myself on being a critical thinker, but this has me stumped.

She said... well, if the Earth rotates around the sun once every 24 hrs AND its relative position doesn't change, and it circles the Sun every 12 months, wouldn't the rising and setting of the sun change by about a degree every day? Meaning, every month that goes by the Sun would either set two hours later, until in 6 months from now, if it's 10:47PM here in Miami, it would actually be daytime, (or the equivalent of 10:47AM)?

Does this question make sense?

What is the mechanism where sunset and sunrise change by only a few hours at the equator, not a whole 12 hours?
Is the relative position of the Earth changing as well as it's daily revolution?

Thank you,


The time that the Sun rises and sets WOULD change by two hours per month, if we used the rotation of the Earth (which takes 23 hours and 56 minutes) to keep time. But that "rotation period" is not the clock we use. The 24 hour clock we use is 3 minutes and 56 seconds longer, and "corrects" the time to take account of our motion around the Sun. In other words, the length of a day is not the time it takes the Earth to rotate once, but the time for the Sun to go once around the sky, and includes both our rotation and our motion around the Sun. ("Star time", which does not correct for our motion around the Sun, does change by two hours a month.)

For a detailed discussion of how this works, see my webpage about Rotation Period and Day Length, at (and if that doesn't make things clear enough, feel free to send a follow-up to your question).


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Courtney Seligman


I can answer almost any question about astronomy and related sciences, such as physics and geology. I will not answer questions about astrology and similar pseudo-scientific rubbish.


I have been a professor of astronomy for over 40 years, and am working on an online text/encyclopedia of astronomy, and an online catalog of NGC/IC objects.

Astronomical Journal, Publications of the Astronomical Society of the Pacific (too long ago to be really relevant, but you could search for Courtney Seligman on Google Scholar)

I received a BA in astronomy and physics and a MA in astronomy, both from UCLA. I was working on my doctoral dissertation when I started teaching, and discovered that I preferred teaching to research.

Awards and Honors
(too long ago to be relevant, but Phi Beta Kappa and Sigma Xi still keep trying to get me to become a paying member)

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