I’ve an iPhone app that, among other things, tracks the visibility of the planets from your location on this one. I like to watch the day shorten and then lengthen throughout the year. The shortest day lasted from 7:27AM to 5:09PM here at 36 degrees N latitude and then began increasing.
Okay, I thought, now the sun will start rising earlier and setting later day-to-day. Except, it hasn’t. Sunset has kept getting later day by day - currently at 5:23PM. But sunrise has kept getting later as well - currently at 7:35AM. I keep trying to visualize why this would be and failing. Is there anyone who knows why this is so? I mean, eventually the Sun will start rising earlier, right?
<Please don’t let me be overlooking something stupidly simple.>
It’s because of the sun’s analemma as seen from Earth.
You said you’re at 36 degrees N latitude. If you put a camera on a tripod and opened the shutter every day at exactly 12:00:00 standard time in your time zone for one whole year, then look at the film, you’d expect to see overlapping images of 365 suns, forming a vertical line from 36-23.5 above the horizon to 36+23.5 overhead. But that’s not what you’d see.You’d see a figure 8, called an analemma.
The reason it’s a figure 8 and not a straight line is that the Earth’s orbit isn’t a perfect circle, it’s an ellipse. When Earth is far away from the sun it moves a bit slower so the sun appears to drift toward the East, and when the Earth is closer in, it moves a bit faster so the sun appears to drift toward the West.
Now imagine that figure 8 moving across the sky and trace it back to the moment of sunrise. Where on the figure 8 is the sun in December and where will it be in January? That tells you whether the sunrise is likely to get earlier or later.
sbunny8 is correct, but here’s a simpler version: Earth’s orbit around the sun is not a perfect circle, nor is the earth a perfect sphere. That and some other stuff means that for us in the northern hemisphere, the earliest sunset is in early December, but the latest sunrise isn’t until early January. In between, they are both getting later. But they’re getting later at different speeds, and around Dec 21-22 is when we end up with the shortest daylight. Click here and the US Naval Observatory will give you a chart of sunrise and sunset for the whole year, and you can see the numbers yourself.
Fear not! Although this phenomenon was known and understood even to the ancients [see SBunny’s links], it’s NOT intuitive, and hardly anyone notices it until they do some actual measurement, as you yourself did.
Thanks - I’d actually heard of and seen an analemma of the sun but didn’t quite understand how it came about. One thing I like about this app is how it shows the ecliptic relative to the Milky Way, which helps me to visualize the orientation of the Solar System as it circles the galactic center. I guess I just like to know how I’m situated, vis-a-vis, well, everything.
Actually the eccentricity of the Earth’s orbit is not sufficient to produce a figure of eight. That’s because the eccentricity changes the orbital speed (and thus the displacement of the Sun disc in the sky from average noon) with a yearly period of unity. A figure of eight needs the displacement of the Sun disc to have two yearly periods with change from positive to negative displacement.
The Earth’s non-spherical shape doesn’t enter into it. But if “some other stuff” refers to the Earth’s tilt then you’re right on. For it’s the Earth’s tilt that introduces a variation of apparent Sun position that has period two over the course of a year. The Sun moves most quickly Eastward (relative to average noon) at the Solstices and back Westward at the Equinoxes. So thus you see a 8-shaped pattern from the seasonal variation alone. The elliptical shape of Earth’s orbit skews it from it being symmetric but it’s the Earth’s tilt that gives it its characteristic shape.
Glad this thread was posted. I had wondered why the earliest sunset of the year was around Dec 9th or so, and not exactly on the winter solstice. I had thought that it might have been because we’re located north of the Tropic of Cancer and that somehow had an effect. Thanks to **Keeve **for supplying the Straight Dope in post #3
I don’t think that’s right - if the Earth circled the Sun in a perfect circle, then it would trace out a simple up-and-down pattern over the year, according to the tile of the axis. It’s the eccentricity of the orbit that causes the Sun to apparently move faster and slower at different times of the year, and that gives some sideways movement to the otherwise purely up-and-down movement.
Well, I did struggle over the wording, so let me rephrase:
The eccentricity of the Earth’s orbit changes the position of the Sun relative to average noon, (average noon is the position at 1200 local standard time) and goes through a cycle of Eastward then Westward once per year. The analemma shows shows two cycles of Eastward and Westward motion over the course of a year. So orbital eccentricity can not alone explain the shape of the analemma.
The change of position from average noon as caused by axial tilt is a non-intuitive thing. Let’s take out eccentricity, as you propose. We would then observe the Sun moving an equal angular distance each day along its path, which is to say along the ecliptic. But the ecliptic is at an angle to the celestial equator. So at the Equinoxes the daily motion of the Sun along the ecliptic is the same as anywhere else during the year, but the net West to East motion (measured as the component parallel to the celestial equator) is less than this.
Picture the ecliptic crossing the celestial equator at the Vernal Equinox. The daily movements of the Sun have a smaller West to East component when the Sun is on a part of the ecliptic at an angle to the celestial equator. At the solstices the ecliptic is parallel to the equator. At the equinoxes the ecliptic is at an angle (23.5 degrees) and the Sun’s motion Eastward parallel to the equator is less than its motion along the ecliptic.
The bit about “a yearly period of unity” was to point out that orbital eccentricity causes a cycle of forward and backward motion that goes through one cycle during the course of a year. The analemma goes through two cycles of backward and forward motion during the course of a year. Although orbital is a significant component of the analemma’s shape, the two-lobed nature is caused by the orbital inclination.
I did literally mean what I said, however, I left something out and for that I apologize. For some years now, my desktop screensaver has been a slideshow of several images starting from the Solar System, continuing on to the Local Stellar Neighborhood, on to the Milky Way Galaxy, the Local Group, etc - all the way to the Observable Universe. Each image has a pointer to where we are.
So, again I apologize. I did not intentionally set you up for a pedantic fail.
I’m not like that. Honest Injun.
For a circular orbit with a tilt, the analemma would be a perfectly symmetric figure 8. The northern and southern lobes would be equal, and the intersection of the lobes would be at the equator.
For an elliptical orbit with no tilt, there would be no analemma per se, since the sun would always shine directly over the equator. The noontime position would oscillate back and forth, left and right, moving in one direction for the six months closest to perihelion and the other for the six months closest to aphelion.
The combination of the elliptical orbit and the tilt gives us the familiar assymetric figure-8 analemma, with the southern lobe wider than the northern.
