THE USM SOUTHWORTH PLANETARIUM
207-780-4249 www.usm.maine.edu/planet
<http://www.google.com/url?q=http%3A%2F%2Fwww.usm.maine.edu%2Fplanet&sa=D&sntz=1&usg=AFQjCNHulkHuLP13bOG2PkNrPazsGWFs2A>
70 Falmouth Street Portland, Maine 04103
43.6667° N 70.2667° W
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
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!
[image: index.jpg]
*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.
[image: main-qimg-43f6570d0faf9c3db29ca7aa57211f71.png]
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.
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