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WolfKC
2012-Jun-29, 02:09 AM
I read an article over on EarthSky titled "At mid-northern latitudes, latest sunsets of the year in late June", by Bruce D McClure.

He says that "At and around the equinoxes, solar days are shorter than 24 hours, yet at the solstices, solar days are longer than 24 hours." and "The latest sunsets come after the summer solstice because the day is more than 24 hours long at this time of the year."

I don’t doubt this is true. But I would like to better understand why... Obviously it's not a difference in the rotation speed of the earth. I fully understand why the angle of the earth makes longer days in the north hemisphere during summer (in the northern hemisphere). But I also don’t see why the latest sunset is past the solstice. My suspicion is that it has to do with the orbit of the earth being elliptical and how the orbit of the earth around the sun would make a small effect on the time that the rotation takes to get the sun back to exactly the same point as the previous day. If the orbit was a perfect circle then the effect of the earth moving around the sun would have the same effect on the slightly less than 360 rotation that is needed to get back to the same earth sun angle.
Since the earth rotation is the same direction as the revolution, the revolution makes the time needed to get to the same angle less and a faster revolution should mean a faster time to get to that angle.
Earth is furthest from the sun in July, closest in January. So is the earth going around the sun faster in July? Perhaps a better question would be does the earth change angle to the sun any faster as a change in speed might counter the orbit angle (at the narrow point of the ellipse but slower?). Sorry it's been a long time since I’ve thought about physics like that.

So... I'm hoping someone can verify or set me straight regarding the physics of a planet going around the sun and give me a better mental model for why the date of the latest sunset is after the summer solstice.

Thanks in advance for polite answers. :)

Nittany Lion
2012-Jun-29, 04:45 PM
You have a couple of facts backwards.

“If the orbit was a perfect circle then the effect of the earth moving around the sun would have the same effect on the slightly less than 360 rotation that is needed to get back to the same earth sun angle.”

Actually, it’s “more than.”

“Earth is furthest from the sun in July, closest in January. So is the earth going around the sun faster in July?”

Actually, it’s “slower.”


When the Earth is at aphelion in early July it is moving slowest in its orbit around the Sun. So the effect of the Earth’s revolution around the Sun is working against the effect you mentioned.

See: http://en.wikipedia.org/wiki/Solar_time

and especially the discussion about “Apparent solar time.”

At the solstices the Sun’s apparent motion is roughly parallel to the equator. At the equinoxes the Sun’s apparent motion is at the 23.4 degree angle to the equator and so there is a cosine effect. Relative to any parallel of longitude, the Sun moves farther each day near the solstices than near the equinoxes and the Earth needs to rotate further between sunsets.

Jeff Root
2012-Jun-30, 06:50 PM
The website appears to temporarily be down, so I can't
doublecheck what it says, but I agree that it seems to
be in error.

Earth is moving fastest around the Sun at perihelion,
in early January, and slowest at aphelion in early July.
So to rotate from one noon to the next -- a solar day --
it has to rotate farthest close to the December solstice,
and least far close to the June solstice. So days must
be longest in December and January when Earth has to
turn farther, and shortest in June and July when it
doesn't have to turn so far.

The latest sunsets could still come after the June
solstice, though. as the article's headline says, since
Earth hasn't reached its maximum distance from the
Sun yet and still has to turn a bit farther than the
minimum to get to noon again.

-- Jeff, in Minneapolis

WolfKC
2012-Jul-01, 02:54 AM
Great refernce. So although i had the two things backwards, I belive my concept of why the differnce happens is correct and my mental model is working. Thanks! :)