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More questions about the length of daylight hours

January 12, 2015 1 comment

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.

01/12/2015 – Ephemeris – The world’s faorite constellation: Orion

January 12, 2015 Comments off

Ephemeris for Monday, January 12th.  The sun will rise at 8:18.  It’ll be up for 9 hours and 6 minutes, setting at 5:24.   The moon, 1 day before last quarter, will rise at 1:02 tomorrow morning.

For people the world over who look up and recognize the brighter constellations Orion is perhaps the odd on favorite.  The Big Dipper, a favorite in the northern hemisphere, cannot be easily seen south of the equator.  The Southern Cross cannot be easily be seen north of the equator.  Orion, or parts of him can be seen from pole to pole because he straddles the equator of the sky.  It has 7 bright stars like the Big Dipper, but those seven are brighter than those in the big Dipper.  In the early evening Orion is seen is the southeast.  The three stars of his belt now tipped diagonally from upper right to lower left.  They are in the center of a left leaning rectangle of stars with bright red Betelgeuse to the upper left and bright blue-white Rigel to the lower right.

Times are for the Traverse City/Interlochen area of Michigan.  They may be different for your location.

Addendum

Otion as seem from most of the Earth

Orion from mid latitudes north of the equator. Orion would be upside down if viewed south of the equator. Created using Stellarium.

Orion from near the north pole.

Orion from near the north pole. Created using Stellarium.

Orion from near the south pole

Orion from near the south pole. Created using Stellarium.