The biggest and brightest full Moon of 2017 is coming this weekend, on Sunday night, Dec. 3rd. It’s a perigee “supermoon,” almost 8% wider and 16% brighter than an average full Moon.
Full moons vary in size because the Moon’s orbit is not a circle, it’s an ellipse:
One side of the Moon’s orbit, called “perigee,” is 50,000 km closer to Earth than the other side, “apogee.” This Sunday’s Moon becomes full only 16 hours away from perigee, closer than any other full Moon of 2017.
How super will it actually look? A 16% difference in brightness can easily be masked by clouds or the glare of urban lights. Also, there are no rulers floating in the sky to measure lunar diameters. Hanging high overhead with no reference points to provide a sense of scale, one full Moon looks much like any other. To get the most out of Sunday’s lunar apparition, try to catch the Moon just as it is rising or setting. This will activate the Moon Illusion and make the perigee Moon of Dec. 3rd look super, indeed. Via (NASA spaceweather.com)
When this month’s full moon arrives, it will perform an optical illusion that has baffled onlookers since Aristotle. As with many moonrises — but especially full moons — it will look bizarrely large when it’s near the horizon, then seem to shrink as it ascends.
This is the “moon illusion,” and it’s all in your head. The moon isn’t changing sizes, and while its distance from Earth does change slightly over time — producing an occasional “supermoon,” which really does appear up to 14 percent larger than usual — that happens too slowly to yield such a dramatic transformation in one night.