December 2 – 69P/Taylor reaches its brightest. Comet 69P/Taylor is forecast to reach its brightest, at around mag 11.9. It will lie at a distance of 2.40 AU from the Sun, and at a distance of 1.53 AU from the Earth.

December 3 – C/2018 L2 (ATLAS) at perihelion. Comet C/2018 L2 (ATLAS) will make its closest approach to the Sun, at a distance of 1.72 AU.

December 3 – Conjunction of the Moon and Venus – 18:43 UTC. The Moon and Venus will share the same right ascension, with the Moon passing 3°37′ to the north of Venus. The Moon will be 26 days old. At around the same time, the two objects will also make a close approach, technically called an appulse. The Moon will be at mag -10.5, and Venus at mag -4.6, both in the constellation Virgo. The pair will be too widely separated to fit within the field of view of a telescope but will be visible to the naked eye or through a pair of binoculars.

December 3 – Close approach of the Moon and Venus – 21:11 UTC. The Moon and Venus will make a close approach, passing within 3°23′ of each other. The Moon will be 26 days old. The Moon will be at mag -10.4, and Venus at mag -4.6, both in the constellation Virgo. The pair will be too widely separated to fit within the field of view of a telescope but will be visible to the naked eye or through a pair of binoculars. At around the same time, the two objects will also share the same right ascension – called a conjunction.

December 5 – C/2018 L2 (ATLAS) reaches its brightest. Comet C/2018 L2 (ATLAS) is forecast to reach its brightest, at around mag 9.1. It will lie at a distance of 1.72 AU from the Sun, and at a distance of 2.43 AU from the Earth.

December 5 – Conjunction of the Moon and Mercury – 21:07 UTC. The Moon and Mercury will share the same right ascension, with the Moon passing 1°52′ to the north of Mercury. The Moon will be 28 days old. The Moon will be at mag -8.6, and Mercury at mag 0.5, both in the constellation Libra. The pair will be too widely separated to fit within the field of view of a telescope but will be visible to the naked eye or through a pair of binoculars.

December 7 – New Moon – 07:22 UTC. The Moon will pass close to the Sun and become lost in the Sun’s glare for a few days. At new moon, the Earth, Moon and Sun all lie in a roughly straight line, with the Moon in the middle, appearing in front of the Sun’s glare. In this configuration, we see almost exactly the opposite half of the Moon to that which is illuminated by the Sun, making it doubly unobservable because the side we see is unilluminated. This is the best time of the month to observe faint objects such as galaxies and star clusters because there is no moonlight to interfere.

December 7 – Close approach of Mars and Neptune – 14:02 UTC. Mars and Neptune will make a close approach, passing within 0°02′ of each other. Mars will be at mag 0.1, and Neptune at mag 7.9, both in the constellation Aquarius. The pair will be close enough to fit within the field of view of a telescope, but will also be visible through a pair of binoculars. At around the same time, the two objects will also share the same right ascension – called a conjunction.

December 7 – Conjunction of Mars and Neptune – 14:40 UTC. Mars and Neptune will share the same right ascension, with Mars passing 0°02′ to the north of Neptune. At around the same time, the two objects will also make a close approach, technically called an appulse. Mars will be at mag 0.1, and Neptune at mag 7.9, both in the constellation Aquarius. The pair will be close enough to fit within the field of view of a telescope, but will also be visible through a pair of binoculars.

December 8 – Asteroid 40 Harmonia at opposition – 19:12 UTC. Asteroid 40 Harmonia will be well placed for observation, lying in the constellation Taurus, well above the horizon for much of the night. Regardless of your location on the Earth, 40 Harmonia will reach its highest point in the sky at around midnight local time.

December 9 – Conjunction of the Moon and Saturn – 05:19 UTC. The Moon and Saturn will share the same right ascension, with the Moon passing 1°07′ to the north of Saturn. The Moon will be 2 days old. The Moon will be at mag -9.0, and Saturn at mag 0.4, both in the constellation Sagittarius. The pair will be too widely separated to fit within the field of view of a telescope but will be visible to the naked eye or through a pair of binoculars.

December 11 – 38P/Stephan-Oterma reaches its brightest. Comet 38P/Stephan-Oterma is forecast to reach its brightest, at around mag 9.7. It will lie at a distance of 1.63 AU from the Sun, and at a distance of 0.76 AU from the Earth.

December 11 – Mercury at dichotomy – 12:31 UTC. Mercury will be well placed for observation in the dawn sky, shining brightly at mag -0.3. Mercury’s orbit lies closer to the Sun than the Earth’s, meaning that it always appears close to the Sun and is very difficult to observe most of the time.

December 12 – LMC is well placed. Across much of the world the Milky Way’s dwarf companion, the Large Magellanic Cloud (LMC), in Dorado will be well placed for observation. It will reach its highest point in the sky at around midnight local time. At a declination of -69°45′, it is easiest to see from the southern hemisphere but cannot be seen from latitudes much north of 0°N. At magnitude 0.9, LMC is visible to the naked eye, but best viewed through a pair of binoculars.

December 13 – 46P/Wirtanen at perihelion. Comet 46P/Wirtanen will make its closest approach to the Sun, at a distance of 1.05 AU.

December 14 – Geminid meteor shower. Although the Geminid meteor shower will reach its maximum rate of activity on December 14, some shooting stars associated with the shower are expected to be visible each night from December 7 to 16.

December 15 – Conjunction of the Moon and Mars – 23:23 UTC. The Moon and Mars will share the same right ascension, with the Moon passing 3°33′ to the south of Mars. The Moon will be 7 days old. At around the same time, the two objects will also make a close approach, technically called an appulse. The Moon will be at mag -11.7, and Mars at mag 0.2, both in the constellation Aquarius.

December 15 – Close approach of the Moon and Mars – 01:53 UTC. The Moon and Mars will make a close approach, passing within 3°21′ of each other. The Moon will be 8 days old. The Moon will be at mag -11.8, and Mars at mag 0.2, both in the constellation Aquarius. The pair will be too widely separated to fit within the field of view of a telescope but will be visible to the naked eye or through a pair of binoculars. At around the same time, the two objects will also share the same right ascension – called a conjunction.

December 15 – Moon at First Quarter – 11:51 UTC. The Moon will be prominent in the evening sky, setting around midnight.  At this time in its monthly cycle of phases, it appears almost exactly half illuminated.

December 15 – Mercury at greatest elongation west – 15:17 UTC. Mercury will be well placed for observation in the dawn sky, shining brightly at mag -0.5. Mercury’s orbit lies closer to the Sun than the Earth’s, meaning that it always appears close to the Sun and is very difficult to observe most of the time. It is observable only for a few days each time it reaches greatest separation from the Sun – moments referred to as greatest elongation. These apparitions take place alternately in the morning and evening skies, depending whether Mercury lies to the east of the Sun or to the west. When it lies to the east, it rises and sets a short time after the Sun and is visible in early evening twilight. When it lies to the west of the Sun, it rises and sets a short time before the Sun and is visible shortly before sunrise. On this occasion, it lies 21° to the Sun’s west.

December 15 – NGC 1981 is well placed. The open star cluster NGC 1981 in Orion’s sword will be well placed for observation. It will reach its highest point in the sky at around midnight local time. At a declination of -04°25′, it is visible across much of the world; it can be seen at latitudes between 65°N and 74°S. At magnitude 4.2, NGC1981 is tricky to make out with the naked eye except from a dark site but is visible through a pair of binoculars or a small telescope.

December 16 – 46P/Wirtanen reaches its brightest. Comet 46P/Wirtanen is forecast to reach its brightest, at around mag 3.9. It will lie at a distance of 1.05 AU from the Sun, and at a distance of 0.07 AU from the Earth.

December 17 – Asteroid 433 Eros at opposition. Asteroid 433 Eros will be well placed for observation, lying in the constellation Camelopardalis, well above the horizon for much of the night.

December 21 – Conjunction of Jupiter and Mercury – 14:41 UTC. Jupiter and Mercury will share the same right ascension, with Jupiter passing 0°51′ to the south of Mercury. Jupiter will be at mag -1.8, and Mercury at mag -0.5, both in the constellation Ophiuchus. The pair will be a little too widely separated to fit comfortably within the field of view of a telescope but will be visible to the naked eye or through a pair of binoculars.

December 21 – December solstice – 22:09 UTC. December 21 will be the shortest day of 2018 in the northern hemisphere, midwinter day. This is the day of the year when the Sun’s annual path through the constellations of the zodiac reaches its most southerly point in the sky, in the constellation of Capricornus at a declination of 23.5°S. On this day, the Sun is above the horizon for the less time than on any other day of the year in the northern hemisphere. This is counted by astronomers to be the first day of winter. In the southern hemisphere, the Sun is above the horizon for longer than on any other day of the year, and astronomers define this day to be the first day of summer. At the solstice, the Sun appears overhead at noon when observed from locations on the tropic of Capricorn, at a latitude 23.5°S.

December 22 – Ursid meteor shower. The Ursid meteor shower will reach its maximum rate of activity on December 22, but some shooting stars associated with the shower are expected to be visible each night from December 17 to 25. The parent body responsible for creating the Ursid shower is 8P/Tuttle. The maximum rate of meteors expected to be visible is around 10 per hour (ZHR).

December 22 – Full Moon – 17:50 UTC. At this time in its monthly cycle of phases, the Moon lies almost directly opposite the Sun in the sky, placing it high above the horizon for much of the night. The sequence of full moons through the year are often assigned names according to the seasons in which they fall. This month’s will be the first to fall in winter 2018 – the Old Moon. Over the nights following December 22, the Moon will rise around an hour later each day, becoming prominent later in the night. Within a few days, it will only be visible in the pre-dawn and early-morning sky. By the time it reaches last quarter, a week after full moon, it will rise at around midnight and set at around noon. At the exact moment when the Moon reaches full phase, it will lie at a declination of +21°07′ in the constellation Orion, and so will appear highest in the northern hemisphere.

December 25 – Close approach of the Moon and M44 – 05:19 UTC. The Moon and M44 will make a close approach, passing within 0°17′ of each other. The Moon will be 18 days old. The Moon will be at mag -12.7, and M44 at mag 3.1, both in the constellation Cancer. The pair will be close enough to fit within the field of view of a telescope, but will also be visible through a pair of binoculars.

December 26 – Puppid–Velid meteor shower. The Puppid–Velid meteor shower will reach its maximum rate of activity on December 26. However, some shooting stars associated with the shower are expected to be visible each night from November 17 to January. The maximum rate of meteors expected to be visible is around 15 per hour (ZHR). However, this assumes a perfectly dark sky and that the radiant of the meteor shower is directly overhead. The parent body responsible for creating the Puppid–Velid shower has not been identified.

December 28 – NGC 2232 is well placed. The open star cluster NGC 2232 in Monoceros will be well placed for observation. It will reach its highest point in the sky at around midnight local time. At a declination of -04°50′, it is visible across much of the world; it can be seen at latitudes between 65°N and 74°S. At magnitude 4.2, NGC2232 is tricky to make out with the naked eye except from a dark site but is visible through a pair of binoculars or a small telescope.

December 29 – Conjunction of Venus and Ceres – 06:17 UTC. Venus and 1 Ceres will share the same right ascension, with Venus passing 3°05′ to the south of 1 Ceres. Venus will be at mag -4.5, and 1 Ceres at mag 8.9, both in the constellation Libra. The pair will be too widely separated to fit within the field of view of a telescope but will be visible through a pair of binoculars.

December 29 – Asteroid 6 Hebe at opposition – 09:36 UTC. Asteroid 6 Hebe will be well placed for observation, lying in the constellation Monoceros, well above the horizon for much of the night. Regardless of your location on the Earth, 6 Hebe will reach its highest point in the sky at around midnight local time.

December 29 – Moon at last quarter – 09:36 UTC. The Moon will be prominent in the dawn sky, rising at around midnight. Over coming days, the Moon will rise later each day, so that it is visible for less time before sunrise and it less far above the eastern horizon before dawn. By the time it reaches new moon, it will rise at around dawn and set at around dusk, making it visible only during the daytime.

December 29 – NGC 2244 is well placed. The open star cluster NGC 2244, in the rosette nebula in Monoceros will be well placed for observation. It will reach its highest point in the sky at around midnight local time. At a declination of +04°56′, it is visible across much of the world; it can be seen at latitudes between 74°N and 65°S. At magnitude 4.8, NGC2244 is too faint to be seen with the naked eye from any but the very darkest sites but is visible through a pair of binoculars or a small telescope.