December 2019 Observing Guide

3 things to keep an eye on in December. Comet C/2017 T2 (PanSTARRS) visible all night, Venus and Saturn conjunction on the 11th and the waxing crescent moon and Venus close on the 28th and 29th.

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 08:25 and sets at 16:00. By month's end, it rises at 08:45 and sets at 16:05.

The Planets

Conjunctions

11th pm Venus and Saturn (very low to horizon) – Venus 1.8 degrees to the South of Saturn.

Regular Stuff

Mercury will appear in the morning sky at the start of the month when it rises at 06:25 in Libra and is mag -0.5.

Venus is in the evening sky albeit very low. At the start of the month, it sets at 17:30 and is mag -3.8 in Sagittarius. By month’s end, it sets at 19:00 and is still mag -3.9 but now in Capricornus. It has a close encounter with Saturn (see above).

Mars is visible in the morning sky in Libra. It rises at 05:25 and brightens from mag +1.7 to mag +1.6 during the month.

Jupiter is at conjunction on the 27th. It is visible in the evening sky in Sagittarius at the start of the month when it sets at 17:10 and is mag -1.7.

Saturn visible in the evening sky in Sagittarius. At the start of the month, it sets at 18:40 and by month’s end it sets at 16:55. It brightens from mag +0.6 to mag +0.5 during the month.

Uranus is visible in the evening sky in Aries. It rises during daylight hours during the month and sets at 02:55 by month’s end. It maintains its brightness at mag +5.7 during the month.

Neptune is at eastern quadrature on the 8th and is visible in the evening sky in Aquarius. During the month, it rises during daylight hours and by month’s end it sets at 22:15. It maintains its brightness at mag +7.9 during the month.

The Moon

The first quarter moon is on the 4th (06:58) with the full moon on the 12th (05:12). The last quarter moon is on the 19th (04:57) with the new moon on the 26th (05:13).

Regular Stuff

4th pm the 55% waxing gibbous lies S of Neptune at 18:00.

8th pm the 88% waxing gibbous lies S of Uranus at 18:00.

10th pm the 98% waxing gibbous lies S of M45 – The Pleiades and NW of Aldebaran (Alpha (α) Tauri, mag +0.9) at 18:00.

11th pm the near full moon lies E of Aldebaran (Alpha (α) Tauri, mag +0.9) at 18:00.

16th pm the 75% waning gibbous lies N of Regulus (Alpha (α) Leonis, mag +1.4) at 23:00.

17th pm the 64% waning gibbous lies SE of Regulus (Alpha (α) Leonis, mag +1.4) at 23:00.

21st am the 28% waning crescent lies E of Spica (Alpha (α) Virginis, mag +1.0) at 04:00.

23rd am the 10% waning crescent lies E of Mars at 06:00.

28th pm the 7% waxing crescent lies S of Venus at 17:00.

29th pm the 12% waxing crescent lies E of Venus at 17:00.

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 during daylight hours on the 14th with a ZHR of 140 (the actual visible rate is always less than this). The radiant is visible from 19:00 on both the nights of 13th/14th and 14th/15th, but unfortunately the shower this year is ruined by a bright waning gibbous also rising in Gemini on both nights.

There may be additional minor showers this month, details of which can be found in the below Information Sources and Links Section.

Asteroids

There are no bright asteroids at opposition this month.

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

Comets

C/2017 T2 (PanSTARRS) is slowly brightening and is predicted to be mag +10 in December. It will be circumpolar all month. It starts in Auriga, before into Perseus. It then spends a short time in Camelopardalis before ending the month in Perseus. On the night of the 13th/14th, it lies NW of NGC 1545 (mag +6.3 open cluster in Perseus). On the night of the 15th/16th, it lies NW of NGC 1528 (mag +6.4 open cluster in Perseus). On the night of the 19th/20th, it lies NW of NGC 1496 (mag +9.6 open cluster in Perseus).

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.

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 22nd 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 – very good for finder charts for comets and asteroids
www.nightskyhunter.com/ - All Rounder
Philip's Stargazing 2019 – All Rounder
Collins 2019 Guide to the Night Sky – All Rounder
www.heavens-above.com – All Rounder
www.timeanddate.com/astronomy/ - All Rounder
www.nakedeyeplanets.com - Planets
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
staratlas.com - All Rounder (Planetarium software)
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/cal2019.pdf - Meteors Only
britastro.org/computing/charts_asteroid.html – Asteroids Only
www.aerith.net – Comets Only
www.ast.cam.ac.uk/%7Ejds/ - Comets Only
messier.seds.org/ - The Messier Catalogue website – Deep Sky Only
www.spaceweather.com – Aurorae Forecasts/Naked Eye Atmospherics
asa.usno.navy.mil/SecA/occns.html - Lunar Occultations

Appendix

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.

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 it 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.