© NASA Image Collection / Alamy Stock PhotoMoon tour
William Herschel was one of history’s
truly great astronomers. A hugely
skilled observer of the night sky, he is
best known for discovering the planet
Uranus in 1781, but he also discovered
that planet’s icy moons Oberon and
Titania and discovered Saturn’s moons
Enceladus and Mimas. It’s no wonder
that his amazing life and important
discoveries have been officially
recognised by modern astronomers,
who have named not one but three
cratersonbodiesafterhim.
The largest is on Mars – the
enormous Herschel impact basin is
more than 300-kilometres (186-miles)
wide and was named in honour of
both William and his son John. The
smallerbutbestknowncraternamed
after William dominates Saturn’s
moon Mimas and gives it its popular
nickname of ‘the Death Star moon’.
Much closer to home Earth’s own
Moon has a crater named after William
too, and that’s our destination for this
month’s Moon Tour.
Ofthethreecratersinourlittlepart
of the galaxy named after William
Herschel,theoneontheMoonisthe
smallest. Only 39-kilometres (24-miles)
across and less than four-kilometres
(2.5-miles) deep, it is a classic lunar
crater – a roughly circular pit with
steep, sharp walls and a mountain
peak in the middle. To be honest, if it
were anywhere else on the Moon it
would hardly warrant a second glance
because it would look just like dozens
or hundreds of other craters around it.
WhatmakesHerschelstandout–and
makes it worthy of your attention on
the next beautifully moonlit night – is
itsverycloseproximitytoatrioof
much larger craters.
Just below the centre of the Moon,
alittletothesouthoftheequator,lies
agreatchainofthreehugecraters,all
joined together along their rims. At
thebottomofthechain,Arzachelis
47-kilometres (29-miles) across. Above it
is Alphonsus, more than twice as wide
asArzachelwithadiameterofover110
kilometres (68 miles). At the top of the
chain, connected to Alphonsus along
its southern rim, is the huge crater
Ptolemaeus. 154-kilometres (96-miles)
across,Ptolemaeushasaveryflatand
almost featureless floor, making itlook almost like a round tray or plate.
Little Herschel, a quarter as wide as
Alphonsus, sits a short distance away
from Alphonsus’ northern rim, and as
it is much smaller and deeper it really
stands out when you look at it.
Herschel is a young crater in lunar
terms, just 400 million years old.
Through a telescope at low power it
stands out starkly from the rugged
southern-highland landscape of
theMoonasaroughlycircular,sharp-
edgedpit.Highermagnificationsreveal
thecrater’sinnerwallsareterraced,
with ledges and benches of slumped
and collapsed rock. In the crater’s
centre its central mountain peak rises
sharply from its f loor. The main appeal
of viewing Herschel is comparing how
deep and detailed it is compared to the
much broader, f latter crater to its south.
So when can you see Herschel this
month?AttheendofJanuaryHerschel
crater will not be visible because it will
beintotaldarkness.Thefirstchance
to see Herschel will come on the
evening of 12 February, when the
Moon is at first quarter and shining
closetotheHyadesandPleiadesstarclusters in the evening sky. On that
evening the terminator, the line that
divides lunar day and night, will have
just passed over the crater, so it will
be visible as a black hole in the Moon
surrounded by a brighter rim. By the
following evening Herschel will be in
full sunlight, and on Valentine’s Day
evening it will be obvious even through
a pair of binoculars.
As the days pass the Sun will climb
higher and higher in Herschel’s sky,
illuminating more of its f loor and inner
walls to us here on Earth. By full Moon
on the 19th Herschel will have been
reduced to a white ring with a grey
interior above Ptolemaeus, but soon the
terminator will sweep over it again. By
the 27 February it will be swallowed up
by darkness once more.Herschel will be
easiest to spot, and
at its most striking,
when it is close to
the terminator on 12
and 13 February, and
again on the 25th
and 26th.Top tip!
Find the smallest of the three craters in the Solar System
named after the great astronomer William Herschel
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