– How is it possible that the sun rises at the same time (7:50) from December 27th through January 8th in Portland, OR?
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The orbital mechanics aren’t geometrically perfect like they would be in a math problem. Working it all out takes extra work, and the results aren’t evenly spaced.
Two variables that count the most:
1. The Earth’s orbit around the Sun is elliptical and not perfectly circular.
2. The Earth’s axis is not perpendicular to the plane of the orbit.
Taken together, these variables end up contributing to variations that make sunrise and sunset occur at uneven intervals between days.
It’s usually not *exactly* the same time. If you look at when the sun rises to the second or fraction of a second it wouldn’t be exactly the same through all those days.
But we might expect more change than a second or two. There are two things going on here:
First, the day length is changing. That part is usually pretty obvious: days are longer in the summer and shorter in the winter.
Second, when the center of the daylight is changes, too! Solar noon (when the sun is highest in the sky, the middle of the day) isn’t at the same time every day.
So if the days are getting longer we would expect that sunrise would get earlier. But if solar noon gets later proportionally, then sunrise may be at approximately the same time, or even get a little later while the days get longer. The opposite is true for sunset: if the days are getting longer and solar noon is getting later, then sunset will get much later!
Solar time, mean solar time, and universal coordinated time are all different.
Solar time means that every time the sun reaches its peak in the sky it is noon, and it has been 24 hours since the last noon. This means that the length of each minute, hour, and second are slightly different each day because the length of a single day is not consistent.
Mean solar time corrects for this by standardizing 24 hours to be the average (or mean) length of the day. That means solar noon won’t always be at noon, but it will be early just as much as it is late and won’t slowly drift out of place. This makes the length of hours minutes and seconds consistent, but makes coordinating timezones impossible because it will change when you move east or west. Everyone would just stick to the nearest town’s time, essentially every town would be it’s own timezone, off by a few minutes from the previous town.
UCT is mean solar time on the prime meridian, and every timezone is a fixed amount of time away from that. Ie EST is UCT -5:00 because it’s 5 hours behind UCT
The came sunrise time is a result of mean solar time not being centered on solar noon. The sun is setting later, because we get more daylight, but it’s also hitting solar noon later, so instead of an equal amount of earlier and later sunlight, we get the same sunrise, a later (according to our clocks) solar noon, and a doubly later sunset.
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