June 22, 2013
Supermoon Coming June 23rd
John P. Millis, Ph.D. for redOrbit.com - Your Universe Online
The Moon appears to be in a nearly circular orbit around the Earth. But that word "nearly" means that there are slight variations in its motion across the heavens.As it turns out, the distance from the Earth to the Moon varies by about 30,000 miles. This sounds like a lot, but it only represents about a 6-7 percent deviation from the average distance between the two bodies.
On Earth, the difference between when the Moon is at its closest point (perigee) and its farthest position (apogee) causes the Moon to appear slightly smaller or larger in the sky. [Note: these terms can be a little confusing, because perigee and apogee vary after each orbit, which means they change from month to month and year to year. So they really represent the nearest and farthest points in the lunar orbit over a specific period of time.]
On June 23rd, the Moon will be in a nearly full moon phase as it reaches perigee, making it appear slightly larger in the night sky. At 7:32 a.m. EDT, the Moon will be the closest to Earth that it will be for all of 2013. Such approaches, when perigee coincides closely with a full moon, are known as supermoons. But the apparent size difference is very difficult to see; only careful measurements reveal the difference.
This particular supermoon is actually not that great. Occurrences in each of the coming years will be even better. The best one of the century won´t happen until December 6, 2052. And the Moon will not cross within 356,400 kilometers until January 1, 2257 (356,371 km), a truly rare approach!