THE USM SOUTHWORTH PLANETARIUM
207-780-4249 www.usm.maine.edu/planet
70 Falmouth Street Portland, Maine 04103
43.6667° N 70.2667° W
Altitude: 10 feet below sea level
Altitude: 10 feet below sea level
Founded January 1970
Julian date: 2458667.5
"A gentle ocean breeze wafting through and brilliant summer light illuminating all. The director's going to kill me when he sees this new dome sun roof."
THE DAILY ASTRONOMER
Wednesday, July 3, 2019
Or the Perihelion Effect
Wednesday, July 3, 2019
Or the Perihelion Effect
Today's trip to Pandora brings us back to a highly logical notion: that Earth's changing distance from the Sun could affect the planet's weather. As we prepare to reach aphelion tomorrow, we answer a subscriber's question about the effect of aphelion.
"Are perihelial summers hotter than aphelial ones? Are aphelial winters colder than perihelial ones? Did I just make up those words?"
D.G.
Greetings!
You did make up those words, but they are splendid ones. Remember, every single word in our language was made up at some point.
First, a brief background:
Earth, like all the other planets, travels along an elliptical orbit. Each year, the planet reaches its nearest point (perihelion) and its most distant point (aphelion). Earth will actually be at aphelion tomorrow!
Earth is closest to the Sun in January and farthest
away in July! However, as the distance difference
is merely 3 million miles, a small fraction of the
Sun's average distance of 93 million miles, Earth's
heliocentric distance has little measurable effect on
the weather.
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.
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.
As for the perihelion winter. Regard the graphic above that shows the solar flux for the northern hemisphere throughout the year. The autumnal equinox is listed at the left of the chart. Notice that the solar flux decreases soon after the autumnal equinox? Even as Earth approaches the Sun, the solar flux in the northern hemisphere decreases. This decrease is a consequence of Earth's tilt: the Solar energy striking the northern hemisphere is diminished more by the changing angle of Earth than it gains by the closer proximity.
As Earth's orbit is nearly circular, the aphelion effect or the perihelion effect on the weather is very slight.