Tip ‘o the backyard telescope to the folks over at EarthSky, who have a skywatcher alert that this Friday, December 12, the full moon will be closer to Earth than it has been for the past 15 years.
If the weather cooperates, viewers will see the whole round, shining face of the moon at its closest approach around 9:45 p.m. UT.
In astro-speak, the moon will be in perigee—the closest the body gets to Earth during its elliptical orbit around our planet. At it’s farthest, the moon is considered to be at apogee.
—Illustration courtesy Pearson Scott Foresman
[And now fans of ancient Disney films will know where Angela Lansbury got the words to her spell for turning poor David Tomlinson into a fluffy white rabbit … ]
The moon’s orbit is also eccentric, or a bit wobbly, meaning that its exact distance to Earth as it spins ’round varies over time.
The closest we’ve ever seen it get was 221,441 miles (356,375 kilometers) away and the most distant was 252,724 miles (406,720 kilometers) away.
—Image courtesy NASA/JPL/Northwestern University
Clearly with the length of a month being linked to a full lunar orbit, perigees and apogees are happening with enough frequency that you might not be inclined toward excitement.
But on Friday, at a mere 221,559 miles (356,566 kilometers) away, the moon will shine more intensely than it has since March 1993.
Although the moon reflects a mere 7 percent of the sunlight that hits it, proximity to Earth means it’s the brightest natural object we see in the night sky. Being closer to us than usual increases the intensity of the light, making it seem noticeably brighter.
Friday’s moon will also seem bigger than usual, but seeing this effect can be a tad trickier. Most people might not think much about changes in the relative size of the moon, as it can appear to shrink and swell over the course of a single night thanks to its shifting angle as it rises and sets.
The trick here is that once it’s high in the sky, the full moon should no longer be a slave to viewers’ perspective, and yet it does change size from month to month due to orbital eccentricity.
Check out this page for a good set of comparison shots.
The upcoming perigee is probably most exciting for skywatcher photographers and anyone who does not have access to major observatories and wants to get a good, live look at features on the moon.
For example, a bigger, brighter moon should really highlight Tycho crater and its expansive system of rays on the moon’s southern limb.
Unfortunately, if you miss this week’s show, you won’t see a moon this close again for another eight years.
But there will be other perigees between now and then, and if you care to look, here’s a nifty little moon calculator for finding out when the next one will occur.