View Full Version : Solstice

2009-Mar-13, 07:43 AM
Ok, so I noticed this year that around solstice time, sunrise started getting earlier, and then about a week later, sunset started getting later (or maybe I have the 2 reversed). Either way, my question is, "Why doesn't the sunset start getting later and the sunrise start getting earlier on the SAME DAY?!" I've tried to visualize what is going on as our earth orbits to cause this, but I can't figure it out. Can anyone help me out?

2009-Mar-13, 08:23 AM
Welcome to the Board.

Good catch on the sunrise/set thing. There is a reason, and I know what it is, but I don't understand it well enough to really explain it.

Basically it has to do with the orbit of the Earth around the Sun. A lot of stuff is happening and some of it happens at a really co-incidental time. The solstice is caused by the Earth tipping one axis as close to directly at the Sun as possible. By one of those odd coincidences, this is almost, but quite the same time that the orbit around the sun reaches the closest/furthest point.

Lets take The December one. The Earth is actually closest to the sun on about January 3rd. That marks one end of a thing called the Analemma. The Analemma is the path the sun appears to take around a fixed point in the sky. It actually makes a figure 8 shape that's bigger on one end.

Sunrise and sunset aren't really the best markers for the day. Local Noon is. Noon is that time when the sun is directly over the north-south line for your position. "Noon" (in quotes) is 12 PM. Noon can move as much as 45 minutes either side of "noon" if I remember right. This is the midpoint of the day and rise and set times are measrued to and from it.

Because the Earth's orbit isn't really a circle, it goes faster around one part than it does the other. This means that the sun's position can be either "fast" or "slow" compared to where it "should" be if the orbit were a perfect circle. This makes your noon come a little before or a little after 12:00 local time. This reverses, not on the Solstice, but on the Aphelion/Perihelion dates, which are about 2 weeks later.

The days get longer after the winter solstice, but the mid point of the day isn't quite at "Noon".

That probably didn't really help a whole lot, and be sure to see below for more information and corrections to what I've just said. I think I got it right, but as I said, I don't really understand it well enough to explain it.

2009-Mar-13, 10:57 AM
Noon can move as much as 45 minutes either side of "noon" if I remember right. It looks like about 16 or 17 minutes, in November, if I interpret your post right, according to the analemma at analemma.com (http://www.analemma.com/Pages/Summation/SummationEffect/Summation.html). Analemma.com is a nice website for technical details of how the equation of time works.

2009-Mar-13, 09:16 PM
It's because we're not in a circular orbit around the Sun.

2009-Mar-13, 10:38 PM
It's because we're not in a circular orbit around the Sun.
There is more to it than the eccentric orbit. If the orbit was circular, the analemma would be a symmetrical figure-8, and we would see the same pattern around the June and December solstices. The eccentricity exaggerates the effect in December and reduces it in June.

This is an example of a problem in which a picture, especially a moving one, is worth a thousand words or more. The explanation is easy to demonstrate on a globe or a chart, but getting a novice to grasp it and form a correct mental picture from words alone is a real challenge. Much depends on the person's understanding of the appropriate geometry and trigonometry. If I am demonstrating to a novice face to face, I can size up his or her mathematical know-how and adjust my presentation accordingly.

Let my wing it verbally, in a thought experiment with a circular orbit. Let us imagine the celestial sphere (sphere for short) revolving overhead daily, with the fixed stars transiting approximately once every 23 hours and 56 minutes. A bug is crawling eastward along the equator at just the right constant speed, so that the sphere has to continue another four minutes to bring it back to the meridian. We adjust our clock to read exactly 24 hours between successive transits of the bug.

Now let us imagine the Sun moving along the ecliptic at the same speed as that of the bug, and that they leave the March equinox point at the same time. The ecliptic and the celestial equator are great circles, so the Sun and the bug will complete their complete circles at the same time a year later. Since the Sun's motion initially is slantways instead of parallel to the equator, it will lag behind the bug in forward progress in longitude. Since the sphere is revolving in the opposite direction, it will carry the Sun to the meridian before the bug. As the Sun levels off going into June, it finds the lines of longitude closer together than at the equator. Now it is making faster forward progress in longitude and catches up with the bug at the solstice. This faster forward progress continues for a while longer and by mid July the Sun is well ahead of the bug, and as a result the revolution of the sphere is late in getting it to the meridian. As the path slants back toward the equator, the forward progress slows down, and the Sun gets to the September equinox point simultaneously with the bug. The cycle repeats itself from then until the following March equinox.

Clear as mud? I don't begrudge any novice's difficulty in following me from these words alone.