The Planets this Month - December 2015

Mercury

Mercury the innermost and smallest planet reaches greatest eastern elongation (20 degrees) on December 29th and therefore is visible as an early evening object during the latter part of the month. For observers at tropical and southern latitudes it appears low down above the western horizon from about the middle of December. Try looking for the planet from about 30 minutes after sunset. Mercury is more difficult to spot from mid-latitude northern locations. Only during the last few evenings of December will observers have their latest change to glimpse this elusive World, low down towards the southwest, 35 minutes after sunset.

During December the magnitude of Mercury fades slightly from -0.8 to -0.5 with the thin waxing crescent Moon passes 7 degrees north of the planet on the 12th.

Venus

Venus remains a superb morning object although its period of visibility is now decreasing. At the start of December from northern temperate latitudes the planet rises over 4 hours before the Sun decreasing to about 3 hours by months end. For observers located further south, Venus is visible for less time. For example from Sydney, Australia (33.5 S) it rises 2.5 hours before the Sun on December 1st, decreasing to 2 hours by December 31st.

Venus fades slightly from magnitude -4.2 to -4.0 with its illuminated phase increases from 67% to 77% this month. It continues its rapid eastwards motion crossing the constellation boundary from Virgo into Libra on December 11th. By the end of the month, Venus is now 30 degrees southeast of much fainter Mars (mag. +1.4).

On December 7th, the waning crescent Moon passes 0.7 degrees north of Venus and an occultation is visible from North and Central America at 16:56 UT.

Venus during December 2015 - pdf format (credit:- freestarcharts)

Mars

Mars is located in Virgo and continues to gradually improve in brightness and apparent size as the distance between the "Red planet" and Earth continues to decrease. It remains a morning object throughout December, starting it at magnitude +1.5 and ending it at magnitude +1.3. The apparent size increases from 4.8 to 5.5 arc seconds.

From northern temperate latitudes you can almost set you watch by Mars this month. As a consequence of its current eastwards and southwards motion Mars rises at approx. the same time each morning; between 2 am and 3 am depending on location. For observers at tropical and southern latitudes the planet rises slightly earlier each morning and by the end of the year it's visible just after midnight.

It's generally regarded among amateur astronomers that an apparent size of 5 arc minutes is the minimum required to begin serious Martian telescopic work. Mars crosses that threshold this month and although obviously still small in diameter, a medium 150mm (6-inch) scope at high powers on nights of good seeing should reveal the more prominent markings.

The waning crescent Moon passes 0.1 degrees south of Mars on December 6th with an occultation visible from Australia, Central and East Africa, Indonesia, Southern India (2:42 UT). Later on December 21st, Mars passes 4 degrees north of Spica (α Vir - mag +1.0).

Mars during December 2015 - pdf format (credit:- freestarcharts)

Jupiter

Jupiter the major dominant planet in the Solar System is now a brilliant morning object moving direct in southeastern Leo. The giant planet rises after midnight at the start of the month improving to before midnight by months end and remains visible until dawn. It increases in magnitude from -2.0 to -2.2 with the apparent size growing from 36 to 39 arc seconds as the month progresses.

Through a pair of binoculars the four large Galilean moons, Io, Europa, Ganymede and Callisto are easily visible. They constantly change position as they orbit the planet and sometimes all four can be seen at once but more often than not some are hidden from view as they pass behind or in front of Jupiter's disk. On rare alignment occasions none of the four can be seen at all, just the planet itself.

When viewed through a telescope, Jupiter is a stunning sight. Even a small 80mm (3.1 inch) refractor will show the main Northern and Southern equatorial cloud belts. Larger telescopes reveal much more finer details, including smaller belts, ovals, festoons, darker regions and of course the famous "Great Red Spot".

The waning crescent and last quarter Moon pass 2 degrees and 1.5 degrees south of Jupiter on December 4th and 31st respectively.

Jupiter during December 2015 - pdf format (credit:- freestarcharts)

Saturn

Saturn passed through solar conjunction late last month and subsequently re-appears low down in the southeastern sky before sunrise during the second half of December. The "Ringed" planet is located in Ophiuchus, shinning at magnitude +0.6.

Uranus

Uranus, mag +5.8, is now two months past opposition but remains well placed for observation in the evening sky among the stars of Pisces. During December, the second furthest planet from the Sun is visible as soon as it's dark enough, remaining so until around midnight. It's easy to spot with binoculars.

Uranus starts the month moving slowly retrograde. It then reaches its second stationary point on December 26th - signaling the end of this year's opposition period - after which direct motion is once again resumed. The planet is positioned 15 degrees south and 20 degrees east of the centre of the "Great Square of Pegasus" and 2 degrees south of epsilon Psc (ε Psc - mag. +4.3).

On December 20th, the waxing gibbous Moon passes 1.2 degrees south of Uranus with an occultation visible from Southern South America, Falkland Islands at 0:50 UT.

Uranus during December 2015 - pdf format (credit:- freestarcharts)

Neptune

Neptune, mag. +7.9, is located in Aquarius and is visible with binoculars and small telescopes. The Solar System's distant far planet can be seen towards the west for a few hours after sunset. It's positioned some 30 degrees southwest of the "Great Square of Pegasus" and two degrees northeast of sigma (σ) Aqr (mag. +4.8).

On December 17th, the waxing crescent Moon passes 3 degrees north of Neptune.

Neptune during December 2015 - pdf format (credit:- freestarcharts)

Solar System Data Table December 2015