SkyEye

The Calendar

A spectacular total solar eclipse occurs on the same day as the closest perigee of the year. The minor meteor showers early in the month have favourable dark skies but the stronger showers later on are diminished by moonlight. Will comet C/2021 A1 (Leonard) attain naked-eye brightness by mid-month? Earth reaches solstice on 21 December and Mars is occulted twice by the Moon.

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 Ophiuchus → Sagittarius

Following last month's superior conjunction, Mercury returns to the evening sky. The planet never gains much altitude in the west when viewed from northern temperate latitudes but it is the beginning of a fair apparition for southern hemisphere astronomers. The New Moon occults Mercury on the fourth but both objects are too close to the Sun to be visible (except during the total solar eclipse that day). Near the end of the month, on 29 December, Mercury moves 4.2° south of Venus.

Venus Sagittarius

The evening star is at its brightest this month, blazing away at magnitude −4.7 in the west after sunset. It is rapidly dropping toward the horizon as viewed from the southern hemisphere but remains fairly level, albeit quite low, when seen from northern temperate latitudes. The waxing crescent Moon is in attendance on 7 December. Retrograde motion begins on 18 December, a sure sign that inferior conjunction is not far away. Mercury joins Venus low to the horizon on 29 December although the two planets will be over 4° apart.

Earth and Moon

The two minor meteor showers at the beginning of the month, the Phoenicids and Puppid-Velids, benefit from dark skies. The waxing gibbous Moon interferes somewhat with the showcase shower of the month, the Geminids, but the Ursids are largely lost to moonlight.

The waning crescent Moon occults Mars twice this month and Mercury once. The nearest perigee of the year coincides with the New Moon which totally eclipses the Sun on 4 December. However, the Full Moon on 19 December is the smallest this year in apparent size and is designated the Micro Moon of 2021.

Comet C/2021 A1 (Leonard) was the first comet discovered in 2021 and will be nearest to Earth (and at its brightest) on 12 December. Perihelion occurs early next year, after which the comet will be ejected from the solar system on a hyperbolic orbit.

Earth reaches it second solstice in this year 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.

Mars Scorpius → Ophiuchus

Twice the waning crescent Moon occults Mars before sunrise this month. The first instance, on 2 December, begins around 23:00 UT and is visible from parts of Asia. The second occurence takes place on that last day of the month and begins around 18:30 UT. This event is visible from southern Australia. Mars reaches its descending node on 19 December, moving to a position south of the ecliptic.

Jupiter Capricornus → Aquarius

The largest planet in the solar system is visible in the evening but look for it as soon as the sky gets dark; it sets ever earlier as the month wears on.

Saturn Capricornus

The next conjunction occurs in early February and Saturn is already getting near the horizon as night falls.

Uranus Aries

The waxing gibbous Moon is 1.5° south of Uranus on 15 December but the best time to look for this evening sky object is when the Moon is absent. Northern hemisphere observers have the best views of this faint planet, with Uranus not setting until well after midnight.

Neptune Aquarius

A small telescope is necessary to view the most distant planet in the solar system. Neptune completes the retrograde portion of its path amongst the background stars and returns to direct motion on the second day of the month. Ten days later it reaches east quadrature. Northern hemisphere planet watchers have the best views of this faint object but look for it as soon as the sky is properly dark because the ice giant sets before midnight.