Sky & Telescope - December 2020 - 47

Jupiter that occurred over 3 and 2 BC.
That series of encounters included a
mutual occultation on June 17, 2 BC!

December
solstice

Uranus

EVENING
Mars reaches the meridian before 8:30
p.m. on December 1st, and around 7
p.m. on the 31st. As Earth rapidly leaves
Mars behind in their orbital race, the
Red Planet fades from magnitude -1.1
to -0.3 this month. As a rule of thumb,
when Mars is about zero magnitude,
its angular diameter is approximately
10″ - often considered the limit for
useful observations of the planet in
medium-sized telescopes. This month
Mars appears distinctly gibbous as its
diameter decreases from 14.6″ to 10.5″.
The planet passes through its ascending node (is north of the ecliptic) on
December 1st, and glides eastward in
Pisces all month.
Neptune is a magnitude-7.9 speck
in Aquarius, and by mid-December
transits the meridian during evening
twilight and sets before midnight.
Observe it early in the evening when it's
still fairly high. Uranus glows at magnitude 5.7 in Aries and culminates in the
early evening. (A Neptune ﬁnder chart
appeared in the September issue on page
48; for a Uranus ﬁnder chart, turn to
page 51 of the October issue.)

Moon
Dec 18

Mars

Earth

Neptune
Saturn

June
solstice

ORBITS OF THE PLANETS
The curved arrows show each planet's movement during December. The outer planets don't
change position enough in a month to notice at this scale.

DAWN
Venus rises in the southeast well over
2 hours before the Sun as December starts, but less than 1½ hours by
month's end. The planet shines at magnitude -3.9 throughout December and
shrinks a bit, from 11.7″ to 10.7″ as it
waxes from 89% lit to 94% lit. Optical
aid will help show the ﬁne double star
Beta (β) Scorpii only 10′ south of Venus
on the morning of the 18th. Venus
shines 5.6° north of Antares (and about
100 times brighter) on the morning of
the 23rd. For some observers, Venus will
be occulted by the Moon on the 12th

Dusk, Dec 21

45 minutes after sunset

45 minutes after sunset

CAPRICORNUS

CAPRICORNUS
Saturn

Saturn
Jupiter

Jupiter
Moon
Dec 16

Looking Southwest

Sun
Venus
Mercury

Dusk, Dec 16 -18

Moon
Dec 17

Sept.
equinox

March
equinox

Jupiter

Looking Southwest

(see page 50 for details).
Mercury is visible in binoculars very
low in the southeast at dawn for the
ﬁrst few days of December. The planet
starts the month at magnitude -0.8
and rises only 45 minutes before the
Sun. Mercury passes through superior
conjunction on December 20th.

SUN AND MOON
The Sun arrives at the December
solstice at 5:02 a.m. EST on the 21st,
marking the beginning of winter in the
Northern Hemisphere and summer in
the Southern Hemisphere. The Sun also
experiences a total eclipse, observable
from parts of Chile and Argentina on
the 14th (see page 48 for details).
The Moon is a waning gibbous less
than 6° to the upper right of Regulus
on the morning of December 6th, and
a slim lunar crescent ﬂoats about 4° to
the upper right of Venus at dawn on the
12th. The Moon is essentially absent
during the peak of the Geminid meteor
shower on the night of December 13-14
(see page 14). Back in the evening sky, a
slender lunar crescent shines roughly 5°
below the tight Jupiter-Saturn pair on
the 16th. On the evening of the 23rd,
a waxing gibbous moon is about 5½°
below Mars.
 FRED SCHAAF started writing his first
book, Wonders of the Sky: Observing
Rainbows, Comets, Eclipses, the Stars
and Other Phenomena, 40 years ago.
sk yandtelescope.org * DECEMBER 2 02 0

47
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Sky & Telescope - December 2020

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