Image credit: Ingrid Taylar
The moon reaches perigee – its closest point to Earth for the month – on August 19, 2013, at 1:00 Universal Time. For U.S. time zones, that places this month’s perigee on August 18, at 9:00 p.m. EDT, 8:00 p.m. CDT, 7:00 p.m. MDT or 6:00 p.m. PDT. At this particular perigee, the moon lies 362,264 kilometers (225,100 miles) away.
Let’s contrast this month’s lunar perigee to lunar apogee – the moon’s most distant point in its orbit. The moon last swung out to apogee on August 3, 2013, when our neighboring world was 405,832 kilometers (252,172 miles) distant. This month, the moon will swing out to apogee for a second time on August 31, when the moon will reach a distance of 404,881 kilometers (251,581 miles).
Because the moon’s orbit around Earth is not perfectly circular – but slightly oblong in shape – the moon’s distance from Earth varies throughout the month. The closest point to Earth in the moon’s orbit is called lunar perigee, whereas its farthest point is referred to as lunar apogee.
However, the moon’s complex relationship with the Earth and sun causes the moon’s orbit to somewhat change shape throughout the year. Sometimes, the moon’s orbit becomes more elongated. At other times, it’s closer to being circular. Therefore, the moon’s apogee and perigee distances vary from month to month. Extreme perigees and apogees happen when the moon’s orbit is most eccentric (elongated).
For instance, this year’s nearest lunar perigee (356,991 kilometers) came to pass on June 23, 2013. Two weeks later, the moon staged the year’s most distant apogee on July 7, 2013 (406,490 kilometers).
In contrast, the year’s farthest lunar perigee took place on March 5, 2013 (369,957 kilometers). Two weeks later, the year’s closest apogee occurred on March 19, 2013 (404,261 kilometers). At this time, the moon’s orbit was closest to being circular for the year.
Enjoy tonight’s lunar perigee, as it’ll be the moon’s closest pass to Earth until December 4, 2013!