SkyEye

The Calendar

One of the best meteor showers, the Geminids, benefit from moonless skies when the shower peaks mid-month. The only asteroid which reaches naked-eye visibility, 4 Vesta, comes to opposition later in December.

The Solar System

The word planet is derived from the Greek word for 'wanderer'. Unlike the background stars, planets seem to move around the sky, keeping mostly to a narrow track called the ecliptic, the path of the Sun across the stars. Dwarf planets and small solar-system bodies, including comets, are not so constrained, often moving far above or below the ecliptic.

Sun Ophiuchus → Sagittarius

Although Ophiuchus is not a member of the zodiac, the ecliptic passes through it.

Mercury Sagittarius → Ophiuchus

Mercury is visible for most of December in the west after sunset but it is brightest at the beginning of the month with greatest elongation east (21.3°) taking place on 4 December. The planet heads back toward the horizon, reaching inferior conjunction on 22 December and ending the year in the dawn sky. It has a distant encounter (3.6° apart) with Mars before sunrise on 28 December.

Venus Virgo → Libra → Scorpius

The morning star drops a tenth of a magnitude this month, ending the year at −4.1. It is still ascending above the eastern horizon for southern hemisphere observers but is best seen from equatorial regions. It is still easily visible for those inhabiting more northern latitudes.

Earth and Moon

Earth reaches it second solstice in this year on 22 December. The word solstice means 'sun stands still' so that on this day, the solar declination reaches an extreme. In this case, the Sun appears directly over the Tropic of Capricorn in the southern hemisphere.

The Phoenicid meteor shower early in the month may be affected by moonlight but the Puppid-Velids should fare better a few days later. The peak of the ever-reliable Geminids occurs around the time of New Moon but the Ursids in the latter half of the month may suffer from the light of a waxing gibbous Moon. Lunar occultations taking place in December include the bright star Antares on 12 December (invisible due to the proximity of the Sun), Neptune on 19 December (visible from Antarctica) and the Pleiades on 24 December. The Moon also skims past first-magnitude star Pollux twice, on the first day of the month and again on 28 December, and is found 2.3° north of Spica on 8 December.

Mars Scorpius → Ophiuchus → Sagittarius

The red planet was at conjunction last month and is very low in the east at sunrise. Although it is a first-magnitude object and slowly brightening, it may be lost in dawn skies. Its track passes close to a number of third- and fourth-magnitude stars this month, the waning crescent Moon is just under 4° distant on 12 December and Mercury once again swings by, approaching to 3.6° on 28 December. The two planets last met in October, just before conjunction. Mars finishes 2023 at its most southerly declination for the year.

Jupiter Aries

Jupiter is visible for most of the night, already above the eastern horizon as night falls and setting before dawn. The waxing gibbous Moon is found 2.6° north of the bright planet on 22 December. Jupiter returns to direct motion on the last day of the year.

Saturn Aquarius

Visible in the evening sky, Saturn now sets two or three hours before midnight. The first-magnitude planet is visited by the Moon on 17 December when our satellite is found 2.5° south of Saturn.

Uranus Aries

Uranus is an evening sky object, setting before dawn. The waxing gibbous Moon moves past the faint planet on 23 December, coming to within 2.7°. However, observations of Uranus are best undertaken on a moonless night earlier in the month.

Neptune Aquarius → Pisces

Neptune returns to direct motion during the first week of December and reaches east quadrature on 17 December. A lunar occultation takes place on two days later but this event is only visible from the Antarctic. The eighth-magnitude blue ice giant is an evening sky object and sets around midnight. A small telescope is always necessary to view this distant planet but choose a moonless night.