December 2021 Observing Guide

Hi all,

3 things to note for December.

At the end of the month, you may be able to see 6 planets (4 of which will be naked eye) in the evening sky. A flat South-Western/Western horizon will be necessary to catch the first 2 (Mercury & Venus).

There is a chance to see the Geminids peak on the night of the 13th/14th with an observing window for those who are night owls/early risers.

There is POTENTIALLY a naked eye comet visible during the 1st half of the month, we will see whether Comet C/2021 A1 (Leonard) actually delivers.

OBSERVING GUIDE 
(Please note all times are UT and are based on an observing location of Belfast and covers the month of December)

The Sun

At the start of the month, the Sun rises at 08:20 and sets at 16:00. By month's end, it rises at 08:45 and sets at 16:05. 

The Planets  

Conjunctions 

29th pm – Mercury lies SE of Venus (Flat South Western Horizon needed). 

Regular Stuff 

Mercury is visible during the last week of the month very low in evening skies. At month’s end, it sets at 17:20 in Sagittarius and is mag -0.7.

Venus is visible very low in the evening sky in Sagittarius during the month. At the start of the month, it sets at 18:30 and by month’s end, it sets at 17:25. It fades from mag -4.5 to mag -4.2 during the month.

Mars is visible in the morning sky during the month when it moves from Libra to Ophiuchus. It rises at 06:50 during the month and brightens from mag +1.6 to mag +1.5.   

Jupiter is visible in the evening sky during the month when it moves from Capricornus to Aquarius. During the month, it is visible as soon as darkness falls and by month’s end, it sets at 20:40. It fades from mag -2.1 to mag -2.0 during the month. 

Saturn is visible in the evening sky in Capricornus. During the month, it is visible as soon as darkness falls and by month’s end, it sets at 18:50. It maintains its brightness at mag +0.7 during the month. 

Uranus is visible in the evening sky in Aries during the month. During the month, it rises during daylight hours and by month’s end, it sets at 03:40. It maintains its brightness at mag +5.7 during the month. It lies 2/3 of the distance on an imaginary straight line between Hamal (Alpha (α) Arietis, mag +2.0) and Mu (μ) Ceti, mag +4.3.

Neptune is at eastern quadrature on the 12th and is visible in the evening sky in Aquarius. During the month, it is visible as soon as darkness falls and by month’s end, it sets at 22:40. It maintains its brightness at mag +7.9 during the month. It lies to the East of Phi (φ) Aquarii, mag +4.2. 

The Moon

The new moon is on the 4th (07:43) with the first quarter moon on the 11th (01:36). The full moon is on the 19th (04:35) with the last quarter moon on the 27th (02:24).

Regular Stuff 

1st am the 13% waning crescent lies E of Spica (Alpha (α) Virginis, mag +1.0) at 05:00. 
6th pm the 8% waxing crescent lies SW of Venus (close to horizon) at 17:00. 
7th pm the 16% waxing crescent lies E of Venus (close to horizon) and SW of Saturn at 18:00. 
8th pm the 26% waxing crescent lies E of Saturn and SW of Jupiter at 18:00. 
9th pm the 36% waxing crescent lies SE of Jupiter at 18:00. 
10th pm the 47% waxing crescent lies SW of Neptune at 18:00.11th pm the 57% waxing gibbous lies SE of Neptune at 18:00. 
14th pm the 84% waxing gibbous lies W of Uranus at 18:00. 
15th pm the 90% waxing gibbous lies SE of Uranus at 18:00.
16th pm the 95% waxing gibbous lies SW of M45 – The Pleiades and NW of Aldebaran (Alpha (α) Tauri, mag +0.9) at 18:00. 
17th pm the 98% waxing gibbous lies NE of Aldebaran (Alpha (α) Tauri, mag +0.9) and SE of M45 – The Pleiades at 18:00. 
23rd pm the 80% waning gibbous lies N of Regulus (Alpha (α) Leonis, mag +1.4) at 22:00.
24th pm the 71% waning gibbous lies SE of Regulus (Alpha (α) Leonis, mag +1.4) at 22:00.
28th am the 33% waning crescent lies N of Spica (Alpha (α) Virginis, mag +1.0) at 03:00.
29th am the 27% waning crescent lies SE of Spica (Alpha (α) Virginis, mag +1.0) at 04:00.
31st am the 9% waning crescent lies NW of Mars at 07:00.

Meteors

The best time to observe meteor showers is when the moon is below the horizon; otherwise its bright glare limits the number you will see especially the fainter ones. Below is a guide to this month's showers. 

The Geminids peak on the night of 13th/14th with a ZHR of 150. The radiant is visible from 18:00 on the 13th, with approximately a 3 hour window to observe the shower after the waxing gibbous moon sets at 03:20 on the 14th.

There may be additional minor showers this month, details of which can be found in the below Information Sources and Links Section. The ZHR or Zenithal Hourly Rate is the number of meteors an observer would see in one hour under a clear, dark sky with a limiting apparent magnitude of 6.5 and if the radiant of the shower were in the zenith. The rate that can effectively be seen is nearly always lower and decreases as the radiant is closer to the horizon. The Zenith is the overhead point in the sky. 

Asteroids

Asteroid (44) Nysa is at opposition on the evening of the 10th at mag +9.1. It can be seen in Taurus and is visible as soon as darkness falls on the 10th.

Finder charts and further information about other fainter asteroids can be found in the below Information Sources and Links Section.

Comets

Comet 67P/Churyumov-Gerasimenko peaked last month and is slowly fading. It is currently mag +8 and can be found in Cancer in December.  It is visible from 20:00 at the start of the month, by mid-month, it is visible from 19:00 and by month’s end from 18:00. It stays within 5 degrees of Iota (ι) Cancri, mag +4.0 during the month. 

Comet C/2019 L3 (ATLAS) is currently mag +10 and is predicted to peak at mag +10 in January. During the month, it is visible from 17:00 and moves from Lynx to Auriga to Gemini. It lies to the N of Castor (Alpha (α) Geminorum, mag +1.6) and the Geminids radiant mid-month.

Comet C/2021 A1 (Leonard) is currently mag +8 and is predicted to reach mag +4 when it is at perigee (its closest approach to the earth) on the 12th. It starts the month in Coma Berenices, then moves into Canes Venatici, Bootes, Serpens and Ophiuchus by the 12th. It is visible from midnight at the start of the month, by the 10th, it is visible from 04:00. It could be visible for the 1st half of the month. On the morning of the 1st, it passes near to Beta (β) Comae Berenices, mag +4.2. On the morning of the 3rd, it passes near to M3. On the mornings of the 6th and 7th, it passes near to Arcturus (Alpha (α) Bootis, mag -0.1). It may be visible on the 12th itself, setting 2 hours after sunset VERY LOW in the West.

Finder charts and further information about the above and other fainter comets can be found in the below Information Sources and Links Section. Any of the above estimates are based on current information at the time of writing the guide and can be wrong - “Comets are like cats; they have tails, and they do precisely what they want”, David H Levy. “If you want to have a safe gamble, bet on a horse - not a comet”, Dr Fred Whipple.

Deep Sky

On the deep sky front this month, galaxies M81 and M82 can be observed in Ursa Major. In Andromeda, M31 - The Andromeda galaxy can be observed along with its satellite galaxies M32 and M110. In Perseus, there is the open cluster M34 and the excellent Double Cluster - NGC 869 and 884. In Triangulum, there is the galaxy M33. In Auriga there are three open clusters M36, M37 and M38 and also M35 in Gemini. Taurus has the excellent Pleiades - M45, the Hyades and also M1 - The Crab Nebula. Orion returns to our skies with M42 - The Great Orion Nebula and also Cancer with M44 - The Beehive Cluster.

General Notes

Always keep an eye out for Aurorae. The winter solstice is on the 21st which sees the shortest day of the year and after this date the nights shorten and the days lengthen. This also sees the beginning of winter. Other interesting naked eye phenomena to look out for include the Zodiacal Light and the Gegenschein. Both are caused by sunlight reflecting off dust particles which are present in the solar system.

The Zodiacal Light can be seen in the West after evening twilight has disappeared or in the East before the morning twilight. The best time of year to see the phenomenon is late-Feb to early-April in the evening sky and September/October in the morning sky - it's then that the ecliptic, along which the cone of the zodiacal light lies, is steepest in our skies. The Gegenschein can be seen in the area of the sky opposite the sun. To view either, you must get yourself to a very dark site to cut out the light pollution. When trying to observe either of these phenomena, it is best to do so when the moon is below the horizon. A new appendix has been added explaining some of the more technical terms used in the guide.

Clear Skies

Neill McKeown

Information Sources and Links

Sky at Night Magazine Observing Guide – All Rounder
Stardust Magazine – All Rounder
in-the-sky.org/– All Rounder
www.nightskyhunter.com/  - All Rounder
Philip's Stargazing 2021 – All Rounder
Collins 2021 Guide to the Night Sky – All Rounder
Night Sky Almanac: A Stargazers Guide to 2021 – All Rounder
Yearbook of Astronomy 2021 – All Rounder
  www.heavens-above.com   – All Rounder
Sky Safari App – All RounderStellarium App – All Rounder
ww.timeanddate.com/astronomy/ - All Rounder
www.irishastronomy.org  - Irish Federation of Astronomy Societies Website and Calendar – All Rounder
irishastro.org.uk/- Irish Astronomical Association website – All Rounder
www.eaas.co.uk  - Northern Ireland Amateur Astronomy Society – All Rounder
eco.mtk.nao.ac.jp/cgi-bin/koyomi/cande/phenomena_en.cgi – Sun/Planets/Moon Only
International Meteor Organisation -  www.imo.net/files/meteor-shower/cal2021.pdf  - Meteors Only
britastro.org/computing/charts_asteroid.html – Asteroids Only
www.cobs.si  – Comet Observation Database
www.aerith.net  – Comets Only
www.ast.cam.ac.uk/%7Ejds/  - Comets Only
astro.vanbuitenen.nl – Comets Only
theskylive.com/ - Comets/Asteroids
messier.seds.org/ - The Messier Catalogue website – Deep Sky Only
www.spaceweather.com  – Aurorae Forecasts/Naked Eye Atmospherics 

Appendix

The radiant is the point in the sky, from which (to a planetary observer) meteors appear to originate, i.e. the Perseids, for example, are meteors which appear to come from a point within the constellation of Perseus. When the radiant is quoted as "circumpolar", it is never below the horizon and visible all night, otherwise the times quoted are when the constellation in which the radiant lies rises above the horizon in the East.

A fireball is defined by the International Astronomical Union as a meteor brighter than any of the planets, i.e. magnitude -4 or brighter. The International Meteor Organisation alternatively defines it as a meteor which would have a magnitude of -3 or brighter at the zenith.

The full moon’s width when viewed from the Earth is 30 arc minutes or ½ a degree. This should give an idea for judging any distances quoted in the guide.

An asterism is a collection of stars seen in Earth's sky which form simple patterns which are easy to identify, i.e. the Big Dipper. They can be formed from stars within the same constellation or by stars from more than one constellation. Like the constellations, they are a line of sight phenomenon and the stars whilst visible in the same general direction, are not physically related and are often at significantly different distances from Earth. 

A conjunction is when two objects appear to be close to each other in the sky according to the perspective of the observer. 

Mag is short for magnitude which is the measure of an object's brightness. The smaller the number, the brighter the object. The brightest object in the sky is the Sun at mag -26, the full moon is mag -12 and Venus the brightest planet is mag -4. The brightest stars are mag -1. If there is a 1 mag difference between two objects - there is a difference in brightness of a factor of 2.5 between the two objects. For example, the full moon is eight magnitudes brighter than Venus on average, which means it is 1,526 times brighter than Venus. Objects down to mag +6 can be seen with the naked eye under very dark skies. 

Local time is always quoted in the guide and this means for November - February - universal time (UT)/GMT is used and for April to September - daylight savings time (DST, = GMT+1). For the months of March and October when the clocks go forward/back respectively, both times will be used and attention should be paid to any times at the end of these months for that change.

Deep Sky Objects such as galaxies, nebulae and star clusters are classified in catalogues such as the Messier catalogue for objects like M44 - M for Messier. Another example of a catalogue would the New General catalogue whose objects have the prefix NGC. There are links for websites to both catalogues in the section above.

Perihelion is the point in the orbit of a planet, asteroid or comet where it is at the nearest point in its orbit to the sun. It is the opposite of Aphelion, which is when the object is at the farthest point in its orbit from the sun. For the earth, the comparative terms used are perigee and apogee and for the moon, pericynthion and apocynthion are sometimes used.

The Planets

From Earth - Mercury and Venus are the inner planets in the solar system and Mars, Jupiter, Saturn, Uranus and Neptune are the outer planets. Below is a short guide as to how both the inner and outer planets move around the sun. 

The Inner Planets

These are best seen when at Greatest Eastern/Western elongation and are not visible when at either Inferior/Superior conjunction. Greatest Eastern elongation is when the inner planet is at its furthest point east from the sun as seen from Earth and visible in the evening sky in the West after sunset, Western elongation is when it's at its furthest point west from the sun as seen from Earth and visible in the morning sky in the East before sunrise. Inferior conjunction occurs when the inner planet is between the Sun and the Earth. Superior conjunction occurs when the inner planet is on the other side of the Sun as seen from Earth.

From our Northerly latitudes, the ecliptic, along which the planets move, lies at a very shallow angle to the horizon after sunset in the autumn and before sunrise in the spring. This means that any of the planets will be difficult to see when fairly close to the Sun in the evening sky in the autumn or in the morning sky in the spring. In particular, Mercury is more or less invisible from here when at Eastern elongation in the autumn or at Western elongation in the spring, because it lies so close to the horizon and is never above the horizon except in daylight or bright twilight.

The normal cycle for an inner planet is Superior Conjunction - Greatest Eastern Elongation - Inferior Conjunction - Greatest Western Elongation - Superior Conjunction. After superior conjunction, the planet moves away from the Sun as seen from Earth and becomes visible in the evening sky after a period of time. It then moves past the point of Greatest Eastern Elongation and moves back towards the Sun as seen from Earth until a point when it is not visible and at Inferior Conjunction. After this the planet appears in the morning sky for a time, before again slipping into the Sun's glare as seen from Earth. The duration of this cycle will depend on the planet's closeness to the Sun, i.e. Mercury completes the above cycle in around 4 months.

The Outer Planets

These are best seen when at opposition and are not visible when at conjunction. Opposition occurs when the earth is between the sun and the outer planet. It is the best time to observe them because the planet is visible all through the night and it is due south and at its highest at about midnight. The planet is also at its closest point in its orbit to Earth - making it appear brighter. Conjunction occurs when the outer planet is on the other side of the Sun as seen from Earth.

If the planet is at or near its furthest point south along the ecliptic, then it won't get very high in the sky even at opposition - just as the Sun never gets high in the sky in midwinter. This happens when opposition occurs near midsummer when the planet is opposite the Sun in the sky and in midsummer the Sun is high, so the planet will be low. The opposite of course applies in winter.

The normal cycle for an outer planet is Conjunction - Western Quadrature - Opposition - Eastern Quadrature - Conjunction. After conjunction, the planet moves away from the Sun as seen from Earth and becomes visible again. The planet from this point on rises earlier and earlier in the morning sky and eventually becomes visible in the evening sky. At Western Quadrature it is at its highest at sunrise and by opposition, it is in the same position by midnight. By Eastern Quadrature, it is past its best and is at its highest at sunset, meaning it is rising in daytime and setting earlier and earlier until a point when it sets too close to the Sun as seen from Earth and is no longer visible. The duration of this cycle will depend on the planet's closeness to the Sun, i.e. Jupiter completes the above cycle in around 13-14 months.