Planetarium

"How much does aphelion affect our weather? We're at aphelion in the summer. Would our summers be warmer if we were at perihelion, instead?"


- Steve C., Rockport


Greetings!

Aphelion does affect our weather, but not in the way that one might think. We should begin by explaining that Earth's orbit is not perfectly circular. If it were, Earth's distance from the Sun would never change. However, it is a slightly elongated ellipse, so its distance varies continuously throughout the year. Its distance veers from its minimum distance (perihelion), which it reaches in early January, to aphelion, which it reaches in early July. It is logical to assume that Earth would necessarily be hotter at perihelion than aphelion. However, the difference in the amount of the Sun's energy we receive (called the solar constant) doesn't vary considerably between perihelion and aphelion. After all, the distance difference between perihelion and aphelion is only about three million miles,* a small fraction of Earth's average 93 million mile heliocentric distance.

The solar constant is about 1367 Watts per square meter. Throughout the year, this value varies by only 3.5% due to Earth's small eccentricity. Now, if Earth's orbit were much more elongated, the temperature difference between aphelion and perihelion would be significant. Regard, for instance, asteroid 1566 Icarus. It moves along a highly elongated orbit that brings it to a maximum distance of 185 million miles and then to a minimum distance of 16.75 million miles (less than half Mercury's average distance.) When at or near perihelion, Icarus bakes like an oven. When farther away, it freezes to temperature far below zero.     

One would think that southern hemisphere summers might be a little warmer. However, the southern hemisphere is predominantly water (the land/water ratio is 4/11). Water has a higher heat capacity than land, meaning that it requires more heat energy to increase its temperature than land needs. Consequently, the meager solar constant increase is offset by the higher water to land ratio.

The way aphelion does affect our weather is duration. Earth is farther away from the Sun in summer. Therefore, its orbital velocity is at its lowest and it requires more time to travel from the summer solstice point to the autumnal equinox than it needs to move between the winter solstice and vernal equinox. The winter is about 89 days; the summer is approximately 92 days long.

I hope this answer helps.

*A subscriber suggested that we add a math section to the web-site. It would be devoted to the math that we sometimes perform to write these articles. Or, at least devoted to the math that somebody performed. I was a bit reluctant, initially, because, trust me, I'm no math genius. However, on further consideration, I decided to give it a go. Hence, the Math Zone! The first entry was the calculation performed to determine Earth's distance difference between perihelion and aphelion. http://usm.maine.edu/planet/math-zone (You don't need to tell me that my penmanship is exquisite. Such truths are self-evident.)