Let me illustrate the question with an example. On Feb 2nd 2022, the sun rises in Los Angeles at cardinal direction 110 and sets in direction 250 (according to timedate website). This means that the sun has to travel 250 - 110 = 140 degrees through the sky. Translated in time, this yields 140 / 15 = 9h 20min. But the real daytime length is actually 10h 35min (according to timedate website).

My question is: how to reconciliate the discrepancy between my estimated daytime length based entirely on sunrise/set cardinal directions and the real one?

  • 4
    $\begingroup$ The Sun does not appear to move 15 degrees per hour ALONG THE HORIZON. It moves 15 degrees per hour ACROSS THE SKY. Your simplified calculation is not accounting for the fact that the path across the sky is longer than the path along the horizon. $\endgroup$
    – JohnHoltz
    Commented Feb 2, 2022 at 16:59
  • $\begingroup$ Could you please point me towards a resource that explains this point in detail with worked out numerical examples? Or maybe provide the numerics for the example I provided above? $\endgroup$
    – Rob
    Commented Feb 2, 2022 at 17:18
  • $\begingroup$ You can verify what John said with any planetarium program. Just look at the azimuth of any astronomical object, and move the time ahead by one hour. $\endgroup$ Commented Feb 3, 2022 at 17:07
  • $\begingroup$ Yes, I see. What I was wondering was whether there is a simple equation for the amount of circle? that is covered in the sky so that I could still apply my simple calculation on the real number of degrees traveled by the sun (instead of the sunrise/set as I had originally thought)... $\endgroup$
    – Rob
    Commented Feb 3, 2022 at 18:25
  • $\begingroup$ astronomy.stackexchange.com/questions/14492 might help re formulas and astronomy.stackexchange.com/questions/1116/… might help re the two ways of measuring degrees $\endgroup$ Commented Feb 4, 2022 at 10:56


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