Archive for August 9, 2014

Supermoon, Smoopermoon

August 9, 2014 2 comments

Pardon me if I don’t get excited by the fact that we are going to have a “supermoon” August 10th. On the night of the full moon it will be at perigee, its closest point to the Earth in its orbit.  The distance according to our Celestial Calendar page is 356,897 kilometers.  That’s 221,766 miles.  At apogee this month, on the 24th, the moon will be 406,523 kilometers, or 252,602 miles away.  That’s somewhat larger than 11 percent difference in distance, due to the Moon’s elliptical orbit.  The name for the smallest moon is micromoon.  Either is an exaggeration of terms.

A supoermoon compared to a micromoon.  Credit: Michael Myers.

A supoermoon compared to a micromoon. Credit: Michael Myers.

I don’t remember the supermoon term growing up.  Wikipedia says it was coined by astrologer Richard Knolle in 1979 according to his web post from 2011.  Oooo, an astrologer.
There’s a profession astronomers can respect. </snark>*

Being a relatively old guy, 1979 was well past my formative years as an amateur astronomer and even four years after I started producing my Ephemeris programs for Interlochen Public Radio.  Yet I only remember supermoon being a big deal or any deal at all for the last few years.
The actual size of the supermoon aside, folks mistake the normal optical illusion of an enlarged moon rising as the supermoon.  The moon always looks larger when it’s near the horizon than when it’s high in the sky.  The same thing happens to the sun, it looks larger rising and setting, the when higher in the sky.  Caution:  Use a solar filter to observe the sun.  In photographs the Moon is the same size whether on the horizon or high in the sky.  Actually the horizon moon will appear slightly smaller on the horizon.  One, it will be squished vertically by the action of the refraction of the earth’s atmosphere. Two, it is nearly 4,000 miles farther away at the horizon than at he zenith, where we’re the radius of the Earth closer to the Moon.

I challenge anyone to be able to actually detect, by looking at the moon in the sky, whether they are looking at a supermoon or not.  There’s nothing of comparable size out there.  The same thing will happen when one thinks the full moon is so white.  OK, there’s some gray too.  However the Moon’s total albedo of reflectance is 0.136 or 13.6%.  Some say 0.07 or 7%, comparable to a charcoal briquette.  If one could get Saturn’s moon Enceladus, with nearly a 100% albedo, next to the Moon without it turning into a comet by sublimating away, the dinginess of our Moon would be immediately obvious.