SkyEye

December 2018

Welcome to SkyEye, your guide to this month's celestial events.

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.

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Date Body Event
1
2 Earth Phoenicid meteor shower
3 Mars east quadrature
4
5 Moon, Mercury 1.9° apart
Neptune east quadrature
6 Mercury stationary point: retrograde → direct
7 Earth Puppid-Velid meteor shower
Moon new
Mars, Neptune 0.4° apart
8
9 Moon, Saturn occultation of Saturn: visible from parts of eastern Russia
10 Moon descending node
11
12 Moon apogee
46P/Wirtanen perihelion
13
14 Earth Geminid meteor shower
15 Mercury greatest elongation west (21.2°)
Moon first quarter
16 46P/Wirtanen closest approach to Earth
17
18
19
20
21 Mercury, Jupiter 0.8° apart
Earth solstice
22 Earth Ursid meteor shower
22 Moon full
23
24 Moon perigee
Moon ascending node
25 Moon 0.5° south of the open star cluster M44 (known as Praesepe or the Beehive Cluster)
26 Venus perihelion
27
28
29 Moon last quarter
30
31 Saturn minimum ring opening (25.5°)

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.

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Sun OphiuchusSagittarius

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

Mercury LibraOphiuchus

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 VirgoLibra

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 AquariusPisces

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 ScorpiusOphiuchus

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 AriesPisces

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.

The Celestial Sphere

Constellations are patterns of stars in the sky. The International Astronomical Union recognises 88 different constellations. The brightest stars as seen from the Earth are easy to spot but do you know their proper names? With a set of binoculars you can look for fainter objects such as nebulae and galaxies and star clusters or some of the closest stars to the Sun.

Descriptions of the sky for observers in both the northern and southern hemispheres are available for the following times this month. Subtract one hour from your local time if summer (daylight savings) time is in effect.

Local Time Mid-month Northern Hemisphere Equator Southern Hemisphere
1730 hours (1830 hours summer time) 60° N 50° N 40° N 30° N 20° N 10° N 10° S 20° S 30° S 40° S
1930 hours (2030 hours summer time) 60° N 50° N 40° N 30° N 20° N 10° N 10° S 20° S 30° S 40° S
2130 hours (2230 hours summer time) 60° N 50° N 40° N 30° N 20° N 10° N 10° S 20° S 30° S 40° S
2330 hours (0030 hours summer time) 60° N 50° N 40° N 30° N 20° N 10° N 10° S 20° S 30° S 40° S
0130 hours (0230 hours summer time) 60° N 50° N 40° N 30° N 20° N 10° N 10° S 20° S 30° S 40° S
0330 hours (0430 hours summer time) 60° N 50° N 40° N 30° N 20° N 10° N 10° S 20° S 30° S 40° S
0530 hours (0630 hours summer time) 60° N 50° N 40° N 30° N 20° N 10° N 10° S 20° S 30° S 40° S