Home > Calendar, Concepts, Questions answered > More questions about the length of daylight hours

More questions about the length of daylight hours

January 12, 2015

This is the result of a question I got about why the daylight hours change the way they do during the year.  My answer is posted here as “How come hours of daylight changes very slowly around the solstice, but very rapidly around the equinoxes?”

My correspondent has a few more questions.  I’ll boil them down.

I pretty much understand why daylight changes rapidly at the equinoxes and slowly at the solstices based upon your map showing the ecliptic and how the steepest part is at the equinoxes. Also, the figure eight drawing makes sense. But why does the curve of the ecliptic seem to linger for a time at the solstices before plunging? Does it have to do with the speed of the Earth in its orbit?

The analemma, as seen below, is the result of two phenomena.  First, the tilt of the Earth’s axis which would on itself make a figure 8 with equally sized lobes, with crossing point at the equinoxes.  Second, the Earth’s orbit of the Sun is a slight ellipse, meaning for our purposes here that the Earth moves its fastest near perihelion when the Earth is nearest the Sun, around January 4th. and slowest at aphelion, when the Earth is farthest from the Sun, around July 4th.  That makes the bottom lobe larger because the Sun is by reflection moving faster eastward in the sky.  The apparent slowness that the questioner perceives is an illusion because the Sun appears to be moving in a more directly eastward, and changed the actual time of local solar noon.  Wikipedia has a detailed discussion of the analemma.

Analemma

This figure 8 is called an analemma. One can find it on old globes in the Pacific Ocean. Explanation below. Created using my LookingUp program.

I had stated in the prior post that daylight hours would be 12 hours at the equinoxes and also all the time at the equator.  So here’s the other question.

At the equator, day length does change over the course of the year, doesn’t it? At the equinoxes it would be 12 hours long, but at the summer solstice up north it would sink towards the south by 23 degrees and at the summer solstice in the south it would sink towards the north by the same amount.

Other than getting cooperation from someone who either lives on or has visited the equator, I generated a calendar of sunrise and sunset times for the equator, specifically for 0º longitude and 0º latitude.  A link to it is here.  Also read the explanation on that calendar page.

The answer is No, the daylight hours at the equator doesn’t change over the year.  The one minute variance has to do with the Analemma.

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  1. Richard Fidler
    January 13, 2015 at 3:28 pm

    Thanks so much for the explanation. I am trying to see how things work not from the point of view of the analemma, but from the point of view of a person looking upon the Earth going around the sun, a view from above (or below) the plane of the Earth’s orbit, but perhaps that is not a helpful way of looking at things.

    About the day length at the equator: If the day length is the same, surely the path of the sun changes as the seasons (elsewhere) progress, doesn’t it? Wouldn’t the sun cross the zenith at its highest point at the equinoxes and then move south and north of that point during winter and summer?

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