The Planets in December 2019 and January 2020

In this period it is the innermost and outermost pairs of planets which are best placed for detailed observations while the more normal planetary targets, from the middle of the solar system, languish low in twilight skies.

Mercury starts December well placed for morning observation from the UK having been at its greatest western elongation from the Sun in late November. It will be readily visible low in the south-east in morning twilight for the first week of December shining at magnitude 0.5, but is rapidly lost after that so see it as early in this period as possible. Mercury is at superior conjunction behind the Sun on the 10th of January and reappears in the south-western evening sky very late in that month shortly after sunset.

Venus is improving in visibility as an evening object throughout this period as it stretches slowly from the Sun at each sunset. The visible phase starts at 89%, falling to 82% by New Year’s Day and down to 74% in late January, making this an excellent period to observe the bright phase for any dark cloud features which may be visible in deep-blue or purple filters, or photographically using ultra-violet band filters. This work can be done in daylight as Venus is bright enough to find using a telescope mount fitted with setting-circles or having goto capability, as long as the Sun itself is studiously avoided while seeking out the planet. Venus lies close to Saturn on the 11th of December, and very close, around 1/10th of a degree, south and east of Neptune on the 27th of January, with closest approach shortly before the pair set as seen from the UK.

Mars is an early morning object, rising a little south of due east around 0510 UT in early December and perhaps only 10 minutes before this in late January. It remains below 5 arcseconds in apparent size throughout the period so only the largest surface features will reveal themselves to careful observers. Mars brightens steadily from magnitude 1.7 to 1.4 but reaches a little more than 15 degrees of elevation by sunrise for mid-UK latitudes.

Jupiter is in conjunction (behind the Sun) on the 27th of December so is very hard to observe. You may just catch it close by Venus in evening twilight during very early December and rising around 0640 UT in the morning sky of late January. Saturn follows Jupiter closely in this period, reaching its own solar conjunction on the 13th of January. Again, early December is the best time to seek it out during early evening twilight; Venus sits close by especially during the second week of the month. Like Venus (and, indeed, like Jupiter) Saturn is relatively easy to find in daylight as long as sensible safety precautions are taking concerning the Sun!

Undoubtedly the best placed planets for evening observation in this period are the outer “Ice-Giants”, Uranus and Neptune, simply due to their high elevation compared with the other planets. Of these Neptune transits, due south, at 1835 UT in early December and 2 hours earlier by New Years Day; it will be above 30 degrees of elevation at transit with its small 2.3 arcsecond disk shining at a faint and telescopic magnitude of 7.9, needing care and patience to pick it out from the background stars of Aquarius. By late January Neptune transits in full daylight but may be caught in darkness close to Venus especially on the 27th when the pair are exceptionally close; a real photographic challenge given the huge difference in brightness between these two planets.

Uranus follows Neptune and is a much easier proposition; transiting at 2130 UT in early December, 1930 at the start of the New Year and by 1730 in late January. In each case south-transit occurs at 50 degrees of elevation for mid-UK latitudes, close to the border between Pisces and Cetus. At magnitude 5.8, Uranus is significantly brighter than Neptune and may just be visible to the naked eye from an exceptionally dark site. At the telescope both planets will show small but obvious blue, or blue-green disks ‘though little more can be seen visually. Photographically both planets respond well to planetary cameras fitted with filters passing deep-red or near-infra red light and may show some banding or obvious shading as well as the occasional bright storm feature. These are of great interest and I would welcome any such observations of these somewhat neglected planetary targets.

Alan Clitherow