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Can someone please explain this phenomenon? A solid explanation seems to be missing from the entire internet.

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    $\begingroup$ solar-center.stanford.edu/AO/sunrise.html $\endgroup$
    – user21
    Dec 30, 2015 at 0:56
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    $\begingroup$ A correct presumption would be: “… the sun rise[s] north of east between the March and September equinoxes”. $\endgroup$ Sep 10, 2016 at 21:26
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    $\begingroup$ Related: astronomy.stackexchange.com/q/39670/16685 $\endgroup$
    – PM 2Ring
    Jun 9, 2023 at 18:21
  • $\begingroup$ @IncnisMrsi why the assertion that March and September are the correct equinox names? $\endgroup$
    – RonJohn
    Jun 12, 2023 at 2:20
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    $\begingroup$ The “autumnal equinox” comes in March south of the Earth’s equator. Is the titular presumption correct there? $\endgroup$ Jun 12, 2023 at 13:17

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You know, the first time someone told me this, I was absolutely certain they were confused, ignorant, or otherwise mistaken, and I told him he had his facts wrong. To be precise, we were talking about an observer on the equator. I maintained that the Sun would rise due east and set due west, all year round. He said no.

What gave me pause, though, is that this person was far from ignorant, so maybe the other two appellations didn't fit either. I went home and tried to construct a convincing proof of my side, and in the middle of the night a very simple and convincing proof did come to mind. Except that it was a proof that I was wrong.

Imagine that you're on equator at sunrise on the summer solstice. Due east is along the equator. But the plane of the equator is tilted relative to the Earth's orbit, and the Sun is well above (north) of it. You cannot see it due east at sunrise. It has to be $23.5^\circ$ north of east. (Remember, this is for an observer on the equator.)

By the same token, the Sun must rise $23.5^\circ$ south of east on the winter solstice, and gradually transition between these extremes during the year.

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    $\begingroup$ There is no such concept as “summer” on the equator. $\endgroup$ Sep 10, 2016 at 21:25
  • $\begingroup$ @IncnisMrsi the summer solstice (which is what ganbustine wrote) is the at the same time everywhere on Earth (even though you barely notice it). $\endgroup$
    – RonJohn
    Jun 12, 2023 at 2:25
  • $\begingroup$ @RonJohn that’s flatly false: ga.gov.au/scientific-topics/astronomical/… $\endgroup$ Jun 12, 2023 at 11:38
  • $\begingroup$ @IncnisMrsi I read your link. It does not say what you think it says. $\endgroup$
    – RonJohn
    Jun 12, 2023 at 11:40
  • $\begingroup$ @IncnisMrsi now here's my citation, and a quote. nationalgeographic.com/science/article/… "Solstices occur at the same time around the world, but their local times vary with time zones. " Since the equator is part of the world, it happens there, too. $\endgroup$
    – RonJohn
    Jun 12, 2023 at 11:43
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[For this explanation, it's helpful to ignore the slight apparent $1^\circ$ motion of the Sun throughout the day].

The way I like to think about this is imagine standing at the North Pole in the summer time. Over the course of a day, all you're going to do is spin in a circle, and the Sun will make a circle around your zenith. When you're at the one of the poles, your definition of "up" and the Earth's rotational axis are the same. So, of course, the Sun will appear in every direction throughout the day, including directly North.

If you then move 20° to the South, you have rotated yourself 20° from the Earth's rotational axis. So the Sun will still make a circle in the sky, but it will no longer be centered on your zenith. Instead the center will be shifted by the amount you rotated yourself (with respect to the Earth's axis).

If you then move a little further to the South, that circle will begin to intersect the horizon. But only a small piece of the circle is removed.

To exemplify this, the images below show the path of the Sun through the sky at 90°N, 70°N, and 50°N near the summer solstice. You can try different values, and other celestial objects using this Sky Path page.

Path of Sun viewed from North Pole:

enter image description here

Path of Sun viewed from 70deg N.:

enter image description here

Path of Sun viewed from 50deg N.:

enter image description here

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