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Supermoon tonight, but you won't see it |
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May 25, 2017 |
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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. |
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By Bruce McClure
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). |
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