A friend of mine (a statistician with way too much time on his hands) has been tracking the time of day when the sun rises and sets for the last couple of months. He claims that the lengthening of days since the winter solstice has been taking place mostly in the morning. For instance, the sun on average rises 2 minutes earlier each day but sets only 1 minute later.
He asked me how this could be possible and I mumbled something about the aberration of starlight and that the earth was rotating toward the sun (right hand rule) so the day would grow more in the spring during the morning and shorten more in the fall during the evenings.
I realize this is complete crap, but could his claim be possible? And if his claim is not just a statistical error, where does this effect come from?
WAG: determining the time of sunrise and sunset is a complicated task, due to the influence of the moon, effects of tides and other factors, probably including Cecil Adams’ mood. If you do all the arithmetic, you get the right answers. But there isn’t a simple answer for observed variations.
However, I’m pulling this from the lower depths of my memory. If anyone has any citations, I’m interested as well.
He’s the sort to stand on a hilltop in a thunderstorm wearing wet copper armor, shouting ‘All Gods are Bastards!’
Yes, it’s rather complex and difficult to visualize. But the main reason the sunrise and sunset times do not move at the same rate is that the earth’s orbit is elliptical. If you measured the position of the sun at 12:00 noon every day over a year, the movement is not a simple up-down caused by the tilt of the earth’s axis. There’s also a sideways motion, because at some point the earth travels along the orbit faster, and at another time of year it travels slower.
If you were to plot the position of the Sun at exactly noon every day, you would find that it is not in the same place each time. Of course, it moves south and north with the seasons, but it also moves east and west, forming a figure eight called an analemma. This is because the earth is closer to the sun around the month of December and so it moves faster than average, making the sun appear to shift west to east (the bottom of the figure eight). Consequently, the sun rises later and sets later–but the lengthening of the day has an effect also, making the sun rise earlier and set later. These two effects offset in the morning, and reinforce in the evening, so there is smaller change in the length of day in the morning than in the evening. That’s the opposite of what your friend claims.
After the sun passes the ‘bulge’ in the figure eight (which the earth has done) and starts heading west, the situation reverses, and there is smaller change in the length of day in the evening than in the morning. That’s currently the case (and so supports his claim), but the accumulated effect since the winter solstice is still greatest in the evening, not the morning.
I made the mistake of thinking of noon as the time of day when the plane defined by the North Pole, the South Pole and my location intersected the center of the sun. I forgot that the time of day is an arbitrary designation based upon the average length of a day. If noon and midnight were based upon the intersection of the aforementioned plane and the sun, the length of the day would probably vary quite a bit.
The time of local apparent noon (when the sun is directly north/south of the observer) fluctuates by about 15 minutes during the year.
Another illustrative example: here in Boston, the time of the earliest sunset is about December 10th, whereas the time of the latest sunrise is about January 5th. The shortest day being halfway between these two dates.