The only Supermoon of 2017 arrives on Sunday; it’ll appear in the night sky a bit brighter and larger than all of the other full moons of the year. The moon will be at full phase at 10:47am ET on Sunday and at perigee on Monday at 3:42 am ET, less than 24 hours apart, hence the supermoon phenomena.
Apogee and Perigee
The moon’s orbit around Earth is far from a perfect circle. The 27 day voyage of Earth’s satellite is actually an elliptical orbit with an ever changing distance. The casual observer will notice the phases of the moon as it makes this orbit but the more subtle change, and also noticeable, is the slight change in distance as well. During the course of any given month the distance between the Earth and the moon will vary by close to 30,000 miles. The farthest point is called apogee, the moon being a distant 252,712 miles from Earth. Perigee is the closest point with that distance shrinking to 221,519 miles. A full moon at perigee will appear a bit larger and much brighter than a full moon at apogee.
This Year’s Supermoon
This year’s only supermoon won’t be nearly as spectacular as last year’s. The November 14, 2017 full moon occurred within 2 hours of the point when the moon was at perigee. Before 2016, this happened last in January of 1948 and will not happen again until November of 2034.
The supermoon classification is a term that was coined by astrologer Richard Nolle in 1979. Nolle used the supermoon term to describe a new or full moon that occurs when the moon is in the top 10% of it’s closest distance to Earth. There was never a reason given why the 10% value was chosen and the term “supermoon” is not recognized in the astronomical community, but has certainly taken hold with the general population and media reports. Given that every 27 days there is a full and new moon along with perigee and apogee point the supermoon is not that uncommon.
There will also be some unwanted effects of the full moon/perigee combination. The ebb and flow of the tides is a direct result of the moon’s gravitational pull on the Earth’s waters. During a new and full moon this force is not competing with the Sun’s smaller gravitational pull and can lead to higher than normal tides. These tides occur typically twice a month and are never really noticed by anyone. This gravitational force however can change and is directly related to the distance between two objects, shorten that distance and that force will increase. So when you have a regular tide along with a supermoon then you may be in for minor flooding on or near each high tide.
The best time to really appreciate the supermoon will be as the moon is close the the horizon and will appear larger due to the moon illusion phenomenon.
See how one photographer captures the moon in stunning pictures around New York City here: Photographer Wows with NYC Imagery