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Why you won’t see the new supermoon

Image at top via memrise.com.

The new moon of May 25, 2017 is a supermoon. In fact, it’s 2017’s closest and largest supermoon. This is the first time since 2009 that a new moon (not a full moon) is the closest and largest moon of the year. And, paradoxically, we can’t see this very large moon, although Earth’s oceans will feel it. What’s a supermoon? Check out the timing of the events below …

New moon (moon most nearly between Earth and sun): May 25, 2017 at 19:44 UTC
Lunar perigee (moon closest to Earth): May 26, 2017 at 1:23 UTC

See? New moon and lunar perigee are separated by only about 6 hours. In recent years, this coincidence in time has given rise to the word supermoon.

When it’s a full supermoon, the moon really is slightly larger than usual in our sky, and it’s definitely brighter. So full supermoons are much-watched events. But the new supermoon? We won’t see it because the lighted half of today’s new moon faces entirely away from us … and because, like all new moons, this one crosses the sky with the sun during the day. A new moon is too close to the sun’s glare to be visible with the eye.

And yet – although we won’t see it – the May 25, 2017 new moon will be the largest moon of this year. Sound paradoxical? It is, but it’s also true. Here’s one way to think about it. If there were a total eclipse of the sun on May 25 (which there isn’t) – an event where the body of the moon passes in front of the sun – it would be a particularly long eclipse. That’s because the moon will be so close to us on that day and hence – although we won’t see it – so large in our sky.

Composite image of a 2006 solar eclipse by Fred Espenak.  Read his article on the August 21, 2017 total solar eclipse, first one visible from contiguous North America since 1979.

The next solar eclipse will be total and visible from North America. It’ll be the first total solar eclipse visible from contiguous North America since 1979. Read more about the August 21, 2017 total solar eclipse. Composite image of a 2006 solar eclipse by Fred Espenak.

This year will have 13 lunar perigees. Sometimes the year’s closest perigee is called proxigee. In any year, it’s either the new moon or the full moon that aligns with the closest perigee (proxigee). So watch out for any proxigean new or full moon because that’s when large perigean spring tides (or proxigean spring tides) are likely to occur.

For the first time since 2009, the centers of the Earth and moon will not come closer than 221,830 miles (357,000 km) in 2017. Although it’s the proxigee new moon that brings the moon closest to Earth for the year on May 25, 2017, it’s actually the absence of a proxigee full moon in 2017 that enables the proxigee new moon to lay claim to the closest supermoon crown in 2017.

The full moon loses out to the new moon this year because proxigee full moons recur in periods of 14 lunar or synodic months (14 returns to full moon). This 14-lunar-month cycle is appreciably longer than one calendar year in length, representing a duration of about one year and 48 days. Since the last proxigee full moon came late in the year in 2016, the following proxigee full moon won’t come until after 2017 has passed, or in early 2018. The most recent proxigee full moon (356,509 km) took place on November 14, 2016, and the next proxigee full moon will happen on January 2, 2018 (356,565 km).

Proxigee full moon cycle

Amazingly enough, proxigee full moons recur every 14 lunar months (14 returns to full moon) because 14 lunar months are nearly commensurate to 15 returns to perigee:

14 lunar months x 29.53059 days = 413.428 days
15 returns to perigee x 27.55455 days = 413.318 days

This 413-day period of time is approximately equal to one year, one month and 18 days. Therefore, the proxigee full moon comes about one month and 18 days later each year, as shown on the list of proxigean full moons below.

Proxigean full moons from 2010 to 2020

2010 Jan 30 (356,593 km)

2011 Mar 19 (356,575 km)

2012 May 06 (356,955 km)

2013 Jun 23 (356,991 km)

2014 Aug 10 (356,896 km)

2015 Sep 28 (356,877 km)

2016 Nov 14 (356,509 km)

2018 Jan 02 (356,565 km)

2019 Feb 19 (356,761 km)

2020 Apr 08 (356,907 km)

Alas, the proxigean full moon skips the year 2017 altogether because the most recent proxigee full moon fell on November 14, 2016, and the next one won’t occur until January 2, 2018.

Bottom line: For the first time since 2009, the new moon – instead of the full moon – will coincide with the year’s closest perigee (proxigee); and also for the first time since 2009, the year’s closest perigee (proxigee) will NOT come closer than 357,000 km (221,830 miles).

Resources:

Lunar perigee and apogee calculator

Moon at perigee and apogee: 2001 to 2100

Phases of the moon: 2001 to 2100

Bruce McClure

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