Why aren't seasons different lengths? Or are they?
February 12, 2017 6:27 PM   Subscribe

Because the earth has an elliptical orbit, it travels at different speeds depending how far it is from the sun. Does this make some seasons longer than others? If not, why not?

Ok, so when the earth is closest to the sun (the internet says January), it is moving the fastest, and when the earth is furthest from the sun (I will assume July-ish) it is moving slowest. If this is the case, when the earth is closest to the sun the distance of that segment of the orbit would be shorter and the speed faster, so wouldn't that make northern hemisphere winters shorter? Conversely, in July the distance of the furthest segment of orbit would be longest and slowest, so shouldn't northern hemisphere summers be longer?

Anyway, I googled solstice and equinox dates, and they seem to be exactly six months apart, which seems to imply that the seasons are the same length. Am I missing something?

I realize that this affects the way the length of the day is measures using a sundial, but wouldn't the earth experience more rotations in general on the far side of the ellipse?
posted by Literaryhero to Science & Nature (9 answers total) 1 user marked this as a favorite
 
The Earth's orbit is very close to circular, so the effect you mention isn't noticeable.
posted by It's Never Lurgi at 6:36 PM on February 12, 2017 [1 favorite]


Earth's orbit is elliptical, but only very slightly so.
posted by LionIndex at 6:39 PM on February 12, 2017 [1 favorite]


The answer was right there on Wikipedia! To be fair, I was looking at a variety of science sites, and all of their diagrams of the earth's orbit were seriously egg shaped. Anyway, thanks, It's Never Lurgi and LionIndex.
posted by Literaryhero at 6:40 PM on February 12, 2017


See the table at Wikipedia. This year, we have:

December (2016) solstice to March equinox: 88 days, 18 hours
March equinox to June solstice: 92 days, 17 hours, 56 minutes
June solstice to September equinox: 93 days, 15 hours, 38 minutes
September equinox to December (2017) solstice: 90 days, 2 hours, 21 minutes

So (northern) summer is the longest, then spring, then autumn, then winter, exactly as you'd expect.

(The effect seems smaller than it is because of our calendar... February, the shortest month, happens to be in the shortest season.)
posted by madcaptenor at 6:50 PM on February 12, 2017 [7 favorites]


This elliptical orbit effect does effect the seasons in another way. In northern hemisphere mid-winter, the earth is 3% closer to the sun which means about a 6% increase in solar radiation in winter and correspondingly 6% less solar radiation in northern hemisphere summer. This has a moderating effect on the seasons for the northern hemisphere where almost 90% of people live.

On the other hand, the exact opposite is true for the southern hemisphere. The elliptical orbit and change in annual radiation accentuates the difference between the seasons. In the southern hemisphere the earth is closest to the sun in summer and farthest away in winter.

But there is another factor in that almost 70% of the land surface is in the northern hemisphere. Land areas have much greater seasonal temperature differences than ocean areas because of the moderating effect of water as a heat sink.

So it turns out that we live today in an optimal situation in which the season moderation of elliptical radiation in the the northern hemisphere tends to offset the the seasonal extremes due to more land area.

And in the southern hemisphere, the seasonal extremes of elliptical radiation is moderated by the higher percentage of ocean area.

So we live in a Goldilocks period of climate. The precession of the ellipse has a period of 23,000 years which swaps these seasonal extremes. It may not be coincidental that the human species has proliferated during this period of seasonal moderation around the globe.
posted by JackFlash at 7:17 PM on February 12, 2017 [10 favorites]


Also, we don't all live at the same point on the globe with relation to the other celestial bodies, so our experiences of the seasonal cycles are quite different based on relative positioning on the earth.
posted by Miko at 8:47 PM on February 12, 2017


Anyway, I googled solstice and equinox dates, and they seem to be exactly six months apart, which seems to imply that the seasons are the same length. Am I missing something?

This is computable. Not suggesting you do the math yourself but it is the sort of thing Wolfram Alpha excels at:

2016 winter solstice to 2017 vernal equinox

2017 vernal equinox to 2017 summer solstice
posted by vacapinta at 11:28 PM on February 12, 2017 [1 favorite]


In Australia, they are, because we have weird not-really seasons based on calendar months.
posted by pompomtom at 4:19 AM on February 13, 2017


IIRC, what you're describing actually happens on Mars, which has a more eccentric orbit than Earth.
posted by ROU_Xenophobe at 6:02 AM on February 13, 2017 [1 favorite]


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