Skywatch - 2021 - 26

M AY | 2021

To the
Moon
Our complex yet delightful
celestial neighbor continues
to fascinate.

fter the Sun, the Moon is likely
the fi rst celestial object that we
become aware of. At the dawn
of time, our distant ancestors would
already have noticed that the Moon
regularly cycles through a series of
phases and then seemingly disappears
for a short while, only to reassuringly
return to grace the evening sky. At fi rst
a shy crescent that gradually becomes
bolder, it eventually dominates the night
sky as a blazing orb, only to dissolve
back into an ever-thinning crescent
until evanescing completely. And then
all this repeats. Ad infi nitum.
A lunation - the time it takes for the
Moon to go from new Moon to new
Moon again - is around 29.5 days
long. This is a smidgen shorter than
our calendar months (barring February) - our word month comes from
the Old English mōnath, from mōna, or
moon. Consequently, some months can
have two full Moons (as in October last
26 SK Y WATCH 2021

year, for example), the second of which is
often called a "Blue Moon." Sky & Telescope is, in fact, responsible for this dubious
term, which fi rst appeared in the March
1946 issue. The original defi nition is the
third full Moon in a season with four full
Moons (instead of only three).
Waxing and waning. Referring to the
diagram on page 27 let's walk through one
cycle of lunar phases. The Moon is in synchronous rotation, which means that it rotates
once about its axis in the same time that
it takes it to orbit the Earth once - essentially, we only ever see the same lunar
hemisphere. However, thanks to a number
of factors - including the elliptical shape
of the Moon's orbit - small oscillations
known as librations bring hidden craters
into view, allowing us to see nearly 60% of
the total lunar globe. Librations are listed
in many places, including the monthly
issue of Sky & Telescope magazine.
Let's start with the new Moon. That's
when it's between the Sun and the Earth

and consequently the Moon's Earthfacing hemisphere is completely in
shadow - so we simply don't see it.
As the Moon continues in its orbit
around the Earth, its reflective surface
gradually starts growing - waxing -
into a crescent. After about a week,
the Moon arrives at a point where it
makes a right angle with the Sun and
Earth: We see a perfect semicircle,
an orb that's half sunlit and half in
the dark. The Moon has now covered
approximately one-fourth of its orbit,
hence the term first-quarter Moon.
Trotting along, the Moon "grows"
and is now gibbous (from the Latin gibbus for hump). When it fi nally arrives
at a spot directly opposite the Sun, we
are privy to the glory of the full Moon,
its whole surface bouncing solar photons back to us here on Earth.
Thereafter the scenario plays out
backwards: The Moon starts shrinking - waning - then passes through

GARY SERONIK

Earth's natural satellite is covered in a dusty layer called regolith. Craters from ancient impact
events and maria (from the Latin
for sea, mare - early astronomers mistook these features
for actual seas such as those on
Earth) cover the surface.

Skywatch - 2021

Table of Contents for the Digital Edition of Skywatch - 2021