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| You won’t see the May 25-26 new moon at perigee – the year’s largest “supermoon” – but Earth’s oceans will feel it. Expect higher-than-usual tides in the days following this supermoon. |
EarthSky.org
For the first time since 2009, a new moon – not a full moon – will be the closest and largest supermoon of a year.
In recent years, the year’s largest supermoon has been a full moon. But not in 2017. The moon will be new on May 25, and it’ll swing to lunar perigee – and be closest to Earth for all of 2017.
We say “largest” despite the fact that you won’t see this moon. We can’t see it because every new moon travels across the sky with the sun during the day. However, if there were a total solar eclipse at this new moon (which there isn’t), it would be a particularly long eclipse, due to this close and large new moon.
Plus this closest and largest supermoon of 2017 will be felt by Earth, in the sense that it will strongly affect earthly tides in days following May 25.
Astronomers sometimes call the year’s nearest perigee moon a proxigee moon.
How close will this May new moon supermoon be? The 2017 proxigee, or closest perigee, moon will lie 357,207 km (221,958 miles) away. That gives 2017 another first since 2009. It’ll be the first time since 2009 that the centers of the Earth and moon will not come closer than 357,000 km (221,830 miles).
Why do we have no proxigee full moon in 2017? It’s because – like so many things in the heavens – proxigee full moons happen in predictable cycles.
They tend to recur in periods of 14 lunar months (413 days, or 14 returns to full moon), which is considerably longer than one calendar year. The last proxigee full moon came late in the year in 2016. Thus there’s no proxigee full moon in 2017, and the following proxigee full moon will arrive in early 2018.
This 413-day period of time is approximately equal to one year and 48 days. The most recent proxigee full moon (221,524 miles or 356,509 km) took place on November 14, 2016. So the next proxigee full moon won’t happen until after the year 2017 has passed, or on January 2, 2018 (221,559 miles or 356,565 km).