A comment under this answer states that the apparent angular speed of the Sun is 8% slower during the solstices. This is rather counter-intuitive, since the rotation speed of the Earth is constant (or close enough for the timescales considered).

Why does the Sun appear to move slower in the sky at the solstices?


1 Answer 1


All points on the celestial sphere execute a full circle every (sidereal) day, but the speed of a point with nonzero declination is slower than a point on the celestial equator because it's traveling on a small circle, not a great circle. This is exactly the same as how a point on the terrestrial equator travels at a higher speed than a point not on the equator. Eg, a point with a latitude or declination of 60° travels at half the speed of a point on the equator, because cos(60°) = 0.5

At the solstices, the Sun is on either the tropic of Cancer or Capricorn, so it has its maximum or minimum declination, approximately ±23.4°. So its speed is cos(23.4) $\approx$ 0.9178 relative to a point on the celestial equator, or about 8% slower, as Mike G mentioned in that comment.

Here's a diagram from an answer about latitude speed by SF. on the Space Exploration Stack Exchange:

latitude speed diagram

  • $\begingroup$ I guess I didn't phrase my question right. I wanted to know about the angular speed, because that's what is mentioned in the linked answer. $\endgroup$
    – usernumber
    Mar 2, 2020 at 12:47
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    $\begingroup$ @usernumber Well, "angular speed" can be a bit ambiguous. ;) But my answer is talking about the same thing that Mike G is. Eg, in 1 hour a star on the celestial equator traces out an arc across the sky of 15°, but a star at 60° declination traces out an arc of half that length, and a star close to the celestial pole only moves along a very small arc, although its arc will have a lot more curvature. $\endgroup$
    – PM 2Ring
    Mar 2, 2020 at 13:05
  • $\begingroup$ astronomy.stackexchange.com/questions/1116/… may or may not be helpful $\endgroup$
    – user21
    Mar 2, 2020 at 15:58
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    $\begingroup$ Maybe it's helpful to consider the equivalent terrestrial situation. A nautical mile is defined as the length of an arc of 1 minute on any meridian of an ideal spherical Earth, so 60 NM (nautical miles) equals 1 degree on the meridian. If you travel exactly north or south along a meridian by 60 NM your latitude changes by 1°. Like the meridians, the equator is a great circle, so if you go 60 NM along the equator your longitude changes by 1°. But if you travel along the small circle of latitude 60° for 60 NM your longitude changes by 2°. $\endgroup$
    – PM 2Ring
    Mar 6, 2020 at 14:53
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    $\begingroup$ +1 Useful. @usernumber, the last paragraph of this answer may or may not help. $\endgroup$
    – Mike G
    Mar 7, 2020 at 2:33

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