CMC professor Jimmy Westlake’s Celestial News column appears Tuesdays in the Steamboat Today. Catch his sum up of this weekend’s full moon, first printed in the Steamboat Today, below. For more columns by Westlake here.
Steamboat Springs — Ordinarily, I would be writing this week to tell you all about the upcoming Perseid meteor shower, arguably our best annual meteor shower.
As luck would have it, the Perseid meteor shower will be washed out by the nearly full moon this year. You might witness a few of the really bright Perseids through the moonlight, but the usual numbers will be reduced greatly.
This year’s meteor shower is due to peak on the afternoon of Aug. 12, so observing before dawn in Aug. 12 and 13 will offer the best chances of seeing a few bright meteors. The good news is that next year’s Perseid shower will happen in moonless skies — something to look forward to.
So, I’ll tell you, instead, about that big, bright, full moon that will be drowning out the meteor shower. The second full moon of summer is sometimes called the Green Corn moon. It just so happens, that this year’s Green Corn moon also will be a so-called “super moon.” A super moon is the result when the full moon happens on the night of the moon’s perigee, or closest point to Earth.
Of course, we have a full moon every month, so there’s nothing unusual about that. Sometimes, like next July, two full moons will fit into one calendar month and the second one is dubbed a “blue moon.”
Unusual? Yes, but nothing to get all that excited about.
And, due to its lopsided orbit around the Earth, the moon swings in to its perigee, or closest point, once a month, then swings out to its apogee, or farthest point, about two weeks later. Happens every month, so nothing unusual about that either.
But, when the full moon and lunar perigee occur on the same night, the result is a full moon that appears 7 percent larger than average and 14 percent larger than the puny little apogee full moon. One would be hard pressed to notice this paltry difference in size, though, unless the perigee and apogee full moons could be viewed side by side.
The sun’s gravity tugging on the moon causes its orbit to shift a little each month so that super moons occur about a month later each year. Next year, September will get the super moon.
Sometimes, when seen on the horizon, the rising full moon can look abnormally large.
The moon’s swollen appearance when seen on the horizon is a famous optical illusion called the “moon illusion.” It is really no larger when seen near the horizon than it is when seen overhead.
But, don’t take my word for it. Prove it to yourself.
This weekend, when you first see that big full moon rising in the southeast, hold up your pinky finger at arm’s length. You can totally eclipse the moon with that tiny appendage. Later in the evening, when the moon has risen higher in the sky, perform the same experiment.
Seeing is believing — the results will be the same.
To compound the moon illusion mystery, when you see that big full moon, turn around, bend over and look at it upside down from between your legs. Lo and behold — it looks normal size again. No foolin’.
Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College’s Alpine Campus. His “Celestial News” column appears weekly in the Steamboat Today newspaper and his “Cosmic Moment” radio spots can be heard on local radio station KFMU. Check out Jimmy’s astrophotography website at www.jwestlake.com.