The planets in December 2020 and January 2021

For UK observers Mercury starts December very low in the south-eastern morning sky and will be hard to catch before sunrise. It moves into Superior conjunction, behind the Sun, on the 20th and may just be glimpsed in the western evening sky after Christmas. By the New Year Mercury is stretching further to the east of the Sun with each sunset. Starting January at magnitude -1.0 it fades noticeably to +1.1 through the month but can be found very low in the south-west shortly after sunset. On the 8th and 9th of January Mercury passes 1.6 degrees south of Saturn and then 1.4 degrees below Jupiter over the following 2 evenings. Greatest eastern (evening) elongation from the Sun is on the 24th of January when Mercury may be found at 10 degrees of elevation in the south-west at 1700 UT for mid-UK latitudes making this a good evening apparition in the northern hemisphere.

Venus is moving ever closer to the Sun with each sunrise so is best viewed early in December but it can be followed, just, throughout this period. Venus rises at 0520 UT in early December below and to the east of the star Spica in Virgo and at magnitude -3.9 it will be very obvious. At 89% illuminated and still 12 arcseconds in apparent size, early December is a good time to image Venus in both infrared and ultraviolet light to follow the cloud patterns in its atmosphere. By Christmas and up to New Year’s Eve Venus reaches barely 10 degrees of elevation by sunrise but can readily be followed into full daylight and observed at higher altitudes if care is taken. In January the apparent size of Venus falls below 10 arcseconds and it rises in morning twilight. Separation from the Sun is 17 degrees mid-month and 13 degrees by months-end by which time Venus rises less than 30 minutes ahead of the Sun.

Despite being well past opposition Mars is still beautifully placed for observation in the mid-evening period. South transit in early December is around 2030UT at nearly 45 degrees of elevation. Mars will be very obvious as the brightest object in Pisces, shining at magnitude-1.1, fading slightly through the month; its 14.5 arcsecond disk will still show plenty of surface detail. By early January south transit is at 1900 UT with magnitude faded to a still bright and obvious -0.22 and size reduced somewhat to 10.5 arcseconds. At this time Mars will not set until 0200 UT so can be followed for extended periods and the slow rotation of its surface observed. Late in January Mars transits at the end of twilight, around 1800 UT and more than 50 degrees above the southern horizon. At 8 arcseconds in size surface detail should still be visible but after this detailed observation becomes increasingly difficult. For a few nights either side of the 20th of January Mars passes very close by Uranus. On that date Mars sits just 1.5 degrees above the gas giant but the close proximity of the Moon may make observation of the pair difficult; better to try two or three days before or after when the separation is around 2 degrees.

Jupiter and Saturn sit very close together, low in the south-western sky shortly after sunset. They have a not-to-be-missed close conjunction during the 21st of December when they are just 6 minutes of arc, 1/10th of a degree, apart with Jupiter passing below Saturn. Separated from the Sun by some 30 degrees this event can be seen from the UK shortly after sunset but if you are comfortable in finding the planets in daylight they can be seen at a slightly higher altitude due south at around 1415 UT. After this Saturn leads Jupiter across the sky with the pair sinking ever lower in the south-west as darkness falls. Saturn passes behind the Sun on the 24th of January with Jupiter following on the 29th, both becoming morning objects in the next period.

The ice-giants of Uranus and Neptune are best seen as evening objects early in December. Neptune transits in darkness at 1840UT early in the month; it sits on the eastern edge of Aquarius at above 30 degrees of elevation and will set around midnight UT. Uranus is higher and brighter at the head of Cetus the Whale, transiting at 2140 UT above 50 degrees of elevation, and still more than 40 degrees up as Neptune is setting.

By early January Neptune is transiting in early twilight so its tiny 2.2 arcsecond disk, shining at magnitude +7.9, is not realistically visible until around 1800 UT when it has already started to decline in altitude. Uranus transits at 1940 UT around 9 degrees east and a little above Mars. Slightly larger than Neptune and noticeably brighter at magnitude +5.7, Uranus is an excellent target still above 50 degrees of elevation at transit. Mars acts as a good reference to find Uranus, the pair drawing closer as January progresses with a conjunction of less than 2 degrees on the 20th. Neither Uranus nor Neptune will show atmospheric features to visible observers but photographic ones may capture storm features, especially when imaged in the near infrared.

Alan Clitherow.