SkyEye

The Calendar

Comet 46P/Wirtanen reaches perihelion on 12 December and approaches close to Earth four days later. The comet is expected to reach naked-eye visibility. This is also an excellent year to observe the Geminid meteor shower which peaks on 14 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 Libra → Ophiuchus

This tiny planet reappears in the morning sky at the beginning of the month, with observers in the northern hemisphere getting the best views. The waning crescent Moon passes north of Mercury on 5 December and the following day, the planet moves from retrograde back to direct motion. Greatest elongation west occurs on 15 December. Look for Mercury and Jupiter less than a degree apart in the pre-dawn sky on 21 December.

Venus Virgo → Libra

The morning star continues to rise higher in the east ahead of the Sun. This turns into a slight descent for northern hemisphere observers by the end of the month. The planet reaches a maximum magnitude of −4.7 on the second day of the month.

Earth and Moon

The Earth is at solstice on 21 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. From now until the solstice in June, days will be getting shorter in the southern hemisphere and longer in the northern hemisphere.

Dark skies make this an excellent year to watch the Phoenicid meteor shower on 2 December. The same can be said five nights later when the poorly-studied Puppid-Velid meteor shower comes to maximum. The waxing crescent Moon on 14 December does not unduly affect observations of the ever-reliable Geminid meteor shower which should peak around 12:30 UT. However, the Full Moon obliterates the Ursid meteor shower on 22 December.

The Moon glides past Mercury on 5 December and actually occults Saturn four days later.

Mars Aquarius → Pisces

Zero-magnitude Mars is at east quadrature on 3 December and comes within 0.4° of Neptune four days later. Optical aids will be necessary to view this appulse since Neptune is only magnitude +7.9.

Jupiter Scorpius → Ophiuchus

Now a morning sky object, Jupiter remains easiest to see from the southern hemisphere. Indeed, observers in temperate northern latitudes may struggle to see the planet this month. Mercury and Jupiter come within a degree of each other on 21 December.

Saturn Sagittarius

The ringed planet is getting difficult to see as it approaches the Sun and conjunction. Parts of eastern Russia will see the Moon occult Saturn on 9 December.

Uranus Aries → Pisces

Uranus is well-placed for viewing in the evening, not setting until after midnight.

Neptune Aquarius

A small telescope is necessary to view the most distant planet in the solar system but you will need to look for it in the early evening as it sets well before midnight. It is at east quadrature on 5 December and has a close encounter with the substantially brighter Mars two days later.