Sunrise/Sunset Asymmetry

[deane]

Once past the winter solstice, the days start getting longer, but for reasons beyond my ken sunset gets later much more quickly than sunrise gets earlier. For example, on Dec 22nd sunrise was at 8:08 and sunset at 16:20. A month later, on Jan 22nd, sunrise was only 9 minutes earlier, at 7:59, while sunset was a whopping 36 minutes later, at 16:56.

This works out well for me. I'm not really a morning person so I'm rarely up at sunrise unless I'm still up from the night before. So while I look forward to the lengthening days, it's really the extra time at the end of the day that I value. Nice to see that nature favours my biological clock.

Comments

[siliconshaman]

The asymmetry is because the clock is an artificial invention. The day itself gets longer symmetrically about local midday... but depending on what time zone you're in, midday might not correspond to 12 o'clock.

[deane]

That means that local noon must be advancing.

[Checks almanac.]

And indeed it is. Cool! I never noticed that before.

That then begs the question of why local noon changes so quickly. I know that the elliptical nature of Earth's orbit means that the length of a solar day varies a bit, but that is by at most just under 8 seconds a day. Yet over the past month solar noon has been advancing by an average of 25 seconds per day. Any idea what's contributing to that?

[siliconshaman]

That would mostly be the result of the axial tilt. 'Noon' is when the sun is at the azimuth, the highest point of it's arc. Which is a combination of the elliptical nature of the orbit and the angle the earth's axis makes relative to the sun's axis.

Basically, the average rate of change is 8 seconds per day, but most of that occurs at the 'ends' of the orbit, when the angle of the axial tilt is changing fastest with respect to the suns axis. [I know, it doesn't change in an absolute sense really.] Hence the actual rate of change is more like a sinusoidal curve.