Winter is coming as the Solstice on 21 December at 3:03 pm heralds the start
of the astronomical winter season which lasts for the next 88 days, 23 hours
and 42 minutes. On the Solstice, you will cast your longest shadow of the year.
The Sun leaves Ophiuchus (The Serpent Bearer) on 17 December at 10:22 pm and
enters Sagittarius (The Archer). Daylight shortens from 07:24 (7.401 hours) on
1 December to 07:04 (7.069 hours) on 31 December so we gain 20 minutes of
night time by month’s end. In fact, we gain 27 minutes of night time up to the
Solstice (when the days get shorter) and then lose 7 minutes afterwards (when
the days start to get longer). On the Solstice, the Sun’s lowest altitude of
the year will be 10.7 degrees at 12:10 pm with the Sun some 147,174,568 km
distant. This day, the shortest of the year, lasts 6 hours, 57 minutes and 37
seconds. The earliest sunset of the year is on 14 December at 3:37 pm and the
latest sunrise of the year is on 28 December at 8:44 am.
The Moon begins the month in Pisces (The Fishes) and ends in Taurus (The Bull)
but comes closer by 6,380 km. Lunar perigee (closest to Earth) on 4 December
at 11:15 am finds the Moon some 356,967 km away from Earth—around 27,433 km
closer than average—subtending an angle of 33.5 arc-minutes. The full Cold
Moon makes an appearance on 4 December at 11:14 pm in Taurus (The Bull). This
is the last supermoon of the year. The Moon enters last quarter on 11 December
at 8:52 pm in Leo (The Lion). Lunar apogee (furthest from Earth) that occurs on
17 December at 6:22 am and takes the Moon to 406,301 km away from Earth—some
21,901 km further than average—subtending an angle of 29.4 arc-minutes. The
new Moon appears on 20 December at 1:43 am in Sagittarius (The Archer)
beginning a new Lunar (synodic) month which will last 29 days, 18 hours and 9
minutes. Finally, the first quarter of the new Lunar cycle shows up on 27
December at 7:10 pm in Pisces (The Fishes).
On 4 December, the Moon will conjunct with the Pleiades star cluster (M45),
around 3 am, being separated by less than one degree. It will re-visit this
cluster on the last day of the year in the evening hours but separated by a
larger margin. On 10 December, the Moon will conjunct with Regulus in Leo (The
Lion), before sunrise, also being separated by less than one degree.
For the inferior planets: Mercury begins the month in Libra (The Scales) and
ends in Sagittarius (The Archer) but recedes by 79,952,837 km (0.52AU).
Contrarily, the ‘Swift Planet’ increases in magnitude from 0.00 to -0.56 (1.69
times in brightness) over the month. On 4 December, Mercury reaches dichotomy
when exactly half the surface is illuminated by the Sun. Mercury reaches
greatest western elongation on Pearl Harbour Day. Venus begins the month in
Libra (The Scales) and ends in Sagittarius (The Archer) but recedes by
4,177,228 km (0.03AU). This barely affects the magnitude which hovers around
-3.8. It is still viewable as the ‘Morning Star’ planet but less so as the
month goes on as it approaches superior conjunction in the new year.
For the superior planets: Mars remains an unobservable daylight object. The
‘Red Planet’ begins the month in Ophiuchus (The Serpent Bearer) and ends in
Sagittarius (The Archer) but comes closer by 1,970,962 km (0.01AU). Mars
increases in magnitude from 1.31 to 1.12 (1.18 times in brightness) over
December. Jupiter remains in Gemini (The Twins) all month but comes closer by
32,729,317 km (0.21AU). The ‘Gas Giant’ rises later and later in the evening
all month so, as long as the clouds stay away, should provide excellent
viewing. Jupiter increases in magnitude from -2.54 to -2.67 (1.13 times in
brightness) over the month.
Saturn remains in Aquarius (The Water Bearer) all month but recedes by
75,963,014 km (0.59AU). The ‘Ringed Planet’ appears after sunset, in the
evening sky, all month but decreases in magnitude from 0.91 to 1.00 (1.09
times in brightness) as the month goes by. Uranus remains in Taurus (The Bull)
all month but recedes by 34,636,471 km (0.23AU). Neptune remains in Pisces
(The Fishes) all month but recedes by 79,196,018 km (0.53AU).
Comet C/2025 T1 ATLAS passes perihelion on 2 December at 9:08 am in Vulpecula
(The Fox). It is very well placed in the south west all month but, alas, is
very faint to all but the best telescopes.
There are 5 meteor showers available for keen observers. For the hopelessly
optimistic, the Monocerotids (019 MON) and the sigma-Hyrids (016 HYD) peak
on the same night in, roughly, the same area of sky viewable around midnight
for several hours. However, activity is low and the waning gibbous Moon is
illuminated to approximately 70%. We may fare better with the Coma Berenecids
(020 COM) which will be available after midnight on 16 December with a low ZHR
and the waning crescent Moon only at 12% illumination once it rises a few
hours before sunrise.
The highly anticipated, and usually glorious, Geminid (004 GEM) meteor shower,
radiant near Castor in Gemini (The Twins), peak on the night of 13–14 December
for Edinburgh and Lothian. They are viewable after astronomical twilight ends
(around 6 pm) for the next 12 hours and reach high altitude so observability is
excellent. The Geminids can produce up to 150 meteors and fireballs per hour.
The parent body is not a comet but the asteroid 3200 Phaethon and was the first
asteroid discovered in IRAS satellite data in 1983. The waning crescent Moon,
illuminated to 37%, should not interfere until is rises in the wee, small,
hours of 14 December.
The Ursids (015 URS), radiant near Kochab in Ursa Minor (The Little Bear), are
a poorer show compared to the Geminids but will peak on 22–23 December and are
circumpolar so viewable all night. The waxing crescent Moon sets early so won’t
interfere. The parent comet is 8P/Tuttle discovered in 1858.
At the time of our sky map, some constellations visible are Perseus (The Hero)
at zenith, Draco (The Dragon) in the north, Cancer (The Crab) in the east,
Pegasus (The Winged Horse) in the west, and Eridanus (The River) in the south.
The ecliptic hosts Leo (The Lion), Cancer (the Crab), Gemini (The Twins),
Taurus (The Bull), Aries (The Ram), Pisces (The Fishes) and Aquarius (The Water
Bearer).
The ‘Winter Hexagon’—Rigel in Orion (The Hunter), Aldebaran in Taurus (The
Bull), Capella in Auriga (The Charioteer), Pollux in Gemini (The Twins),
Procyon in Canis Minor (The Lesser Dog) and Sirius in Canis Major (The Great
Dog)—rises in the south-east. This superstructure also contains the ‘Winter
Triangle’ comprising the stars Procyon and Sirius but joined by Betelgeuse in
Orion (The Hunter). Circumpolar constellations—always above the
horizon—include Cepheus (The King), Camelopardalis (The Giraffe), Cassiopeia
(The Seated Queen) and Ursa Minor (The Lesser Bear).
