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M5M338 ASTRONOMY • DECEMBER 2021
Reappearing act
COMET SEARCH I Best comet since NEOWISE
TAKE EVERY CHANCE you get to observe the brief appearance of
Comet C/2021 A1 (Leonard). Visible to the unaided eye by the second
week of December, it will rapidly fade to 8th magnitude by month’s
end. But what a show: An emerald blade shoots out on the 9th as its
fan swishes in front of us!
The gas part of a comet’s tail flows straight out, away from the
Sun. Almost certainly it will be green, but blue — like C/2020 F3
(NEOWISE) — is possible. The dust component spreads out into a
relatively flat fan, which becomes edge-on to our line of sight when
Earth passes through the comet’s orbital plane. The lightsaber
likeness should be present for one night on either side of Dec. 9.
It’s not to be missed and worth a drive to darker skies.
Immediately after, the dust begins to blaze, peaking on the 14th.
Dust forward-scatters light really well — best when the particles lie
between us and the Sun — then fades night to night as the angles
change. Don’t be misled by thinking Gregory Leonard’s comet will
be best when it comes closest to the Sun (perihelion) in early
January. More important is the increasing Earth-comet separation.
Brightness drops by the square of the distance between two objects,
meaning Leonard will fade to binocular brightness by the time we
hit New Moon.
Beware of the confusing information on when to view Leonard.
Technically, it is visible both after sunset and before sunrise in
different parts of the sky. In a nutshell, here’s when it’s best: Up to
Dec. 12, look southeast as dawn is breaking. From the 12th onward,
switch to early evening and look low in the southwest. Find observing
spots with as low a horizon and as minimal light pollution as possible.Comet C/2021 A1 (Leonard)
Saturn’s disk spans 16"; the long
axis of its rings span 35" and
tilts 19° to our line of sight.
The easiest of Saturn’s
moons to spot is Titan, shining
at magnitude 8. The fainter
moons become more difficult to
find in poor seeing conditions
when the planet is at low alti-
tude. Saturn sets shortly after7 P. M. local time on Dec. 31, so
be sure to catch it early.
Jupiter starts December in
the eastern part of Capricornus,
about 16.5° east of Saturn. Itshines at magnitude –2.3, but
quickly dims 0.1 magnitude and
then crosses into Aquarius
Dec. 14. The waxing Moon
stands below Jupiter on Dec. 8,
while Jupiter stands 10° west of
the Moon on Dec. 9.
Jupiter’s 37"-diameter disk
looks great through a telescope
— your best views will be as
twilight descends and for the
first hour or so after dark. By
the end of December, Jupiter
dips below 20° around 7 P. M.^
local time and sets by 9 P. M.
Check the configuration
of its four Galilean moons —
their relative positions change
nightly. On Dec. 1, an occulta-
tion and eclipse reappearanceoccur close together. First,
Callisto reappears from behind
Jupiter at 9:13 P. M. EST, followed
by Europa exiting Jupiter’s
shadow at 11:15 P. M. EST.
Watch Io disappear behind
Jupiter Dec. 4 at 9:35 P. M. EST.
The following night, both Io
and its shadow travel across
the face of Jupiter shortly afterSKY THIS MONTH
Note that our illustration shows the comet’s progress only through
Dec. 12. Visit http://www.Astronomy.com/skythisweek later this month for
charts and viewing tips through the latter half of December.
Other visible comets this month include, in order of brightness,
67P/Churyumov-Gerasimenko, 9P/Borrelly, and C/2019 L3 (ATLAS).— Continued from page 33
At 8:24 P. M. EST on the 26th, Europa is poised to emerge from Jupiter’s large,
dark shadow. Callisto, meanwhile, is closing in and will begin transiting the
disk at 8:53 P. M. EST.
Early this month, Comet Leonard swipes past globular cluster M3 —
compare their appearance! After midmonth, you’ll want to switch from
morning to evening viewing. Our friends south of the equator see only the
second half of the performance.EVENING SK Y
Mercury (southwest)
Venus (southwest)
Jupiter (south)
Saturn (southwest)
Uranus (east)
Neptune (south)MIDNIGHT
Uranus (west)MORNING SK Y
Mars (southeast)WHEN TO
VIEW THE
PLANETS
Comet C/2021 A1 (Leonard) skims within
2.6 million miles of Venus on Dec. 18.