Skywatch - 2021 - 28

M AY | 2021
Binoculars are great for observing
the Moon - see the article on page 66
for info on what to see and how. Telescopes allow you to zoom in on specific
features in detail. A neat thing to do is
pick a favorite crater and check it out
over several nights as the illumination
increases (or decreases) and shadows
lengthen (or shorten). Features nearest
the terminator can even change dramatically in just a few hours!
Syzygy is the word. The Moon's
orbit is elliptical, which means that at
times the Moon is a bit farther from
Earth (apogee) and at times it's closer
( perigee). At apogee, the apparent diameter of the Moon is smaller than average, while at perigee it's larger. What's
commonly known as a "supermoon,"
or a full Moon when it's closer to Earth,
is what we prefer to call by its more
scientific name, a perigee-syzyg y (syzygy
refers to the alignment of three celestial
bodies). The diﬀerence in size between
a perigee and apogee full Moon is only
some 14%, and the eﬀect is even smaller
when comparing with an average full

Moon. Most will say this isn't a noticeable eﬀect, but test it for yourself: Find a
spot, preferably with a foreground landmark, and note the full Moon around
May 26th (near perigee). Then return to
the same spot on December 18th (near
apogee, and yes, we'll remind you). Can
you see a diﬀerence?
Courtesy calls. As the Moon traverses the sky, it will regularly come
close to other celestial objects, which
makes for pretty sights. On the mornings of May 3rd through 5th, early-risers
will see the last-quarter Moon above the
southeastern horizon producing a pretty
- and changing (remember the 12° per
day?) - tableau with Jupiter and Saturn. After the new Moon on the 11th,
the waxing crescent Moon pays a visit to
three planets at dusk in quick succession:
Venus, Mercury, and Mars (see below).
On the evening of the 19th, high above
the southwestern horizon, the fi rst-quarter Moon isn't far from Regulus, in Leo.
A few evenings later, on the 23rd, the
waxing gibbous Moon is close to bluewhite Spica, Virgo's lucida.

Dawn, May 3 - 5

Dusk, May 12 -15

1 hour before sunrise

30 minutes after sunset
Mars

Moon
May 15

Moon
May 14

CAPRICORNUS
Saturn

TA U R U S
Jupiter
Moon
May 3
Moon
May 4
Moon
May 5

Moon
May 13
Mercury

Betelgeuse

PLANETS IN
MAY
Mercury puts on its best 2021 display this
month and remains visible at dusk through
the 26th. On the 13th, the ﬂeet-footed little
world is joined by a narrow crescent Moon
sitting roughly 3° left of the planet - both
objects can be seen together in the same
binocular view. On the 17th, Mercury has its
greatest elongation from the Sun, when it
shines at magnitude +0.4 and hovers about
11° degrees above the west-northwest horizon 45 minutes after sunset. This is your
best chance to catch Mercury this year.
Sharing evening twilight with Mercury is
brilliant Venus. At magnitude -3.9, Venus is
hard to miss even in bright twilight. On the
12th, a very thin earthlit crescent Moon is
just 1° to the planet's left. The best time to
look is shortly after sunset, but you'll need
an unobstructed horizon (and perhaps binoculars) to catch this close conjunction.
Throughout May, Mars is visible at
dusk and sets around midnight. Shining at
magnitude 1.7, it's still a conspicuous sight
in Gemini. On the evening of the 9th, Mars
is only a little more than ½° from Epsilon
Geminorum, and on the evening of the
15th, it's visited by a waxing crescent Moon
sitting roughly 1½° to its right. Binoculars
will enhance the view of both pairings.
After being a close couple for many
months, there's now plenty of space
between Jupiter and Saturn. By the end
of May, they're 18° apart - a gap that
hasn't been this wide since December
2019. That spacing is reﬂected in their rise
times. For telescope users, both worlds are
magniﬁcent sights and at their best during
morning twilight when they're highest.

10°

Aldebaran

Venus
Moon
May 12

Looking Southeast
S Left:

Looking West

The waning crescent Moon glides past Saturn and Jupiter in the early hours of the morning. The
Moon is still at last quarter on May 3rd, when it's about 6° from Saturn. Right: You won't want to miss the
waxing crescent Moon as it climbs higher each evening shortly after sunset as of May 12th. On its way up
it pays a visit to three planets. See the Planets sidebar at right for more info on how to view these sights.
28 SK Y WATCH 2021

Historical Events
5/3: Charles
Messier makes his
first discovery of a
deep-sky object, the
globular cluster M3 in
Canes Venatici, 1764

5/5: Gottfried and
Maria Magarethe
Kirch discover M5,
a globular cluster
rich in variable
stars, 1702

Skywatch - 2021

Table of Contents for the Digital Edition of Skywatch - 2021