Dundee Astro - Planetary Notes

Venus is a brilliant evening object, improving steadily throughout December and reaching its Greatest Eastern Elongation (GEE) from the Sun on 10 January. Early in December it will first appear perhaps 15 degrees high in the south-south-west, shining at -4.18 magnitude and it will be the first ‘star’ visible on any given evening. Elevation, when first seen, will steadily improve, being some 15 to 18 degrees high by mid-December and noticeably higher than 20 degrees by New Year’s Eve, especially from the south coast.

By GEE Venus will be obvious, more than 25 degrees high, still in the south-south-east, from 16.25UT, shining at a dazzling -4.5 magnitude and showing a 50% phase with an apparent size of some 24 arcseconds (24”) from pole to pole. Venus will remain an easily observed target throughout January and for a few days around the 19th it sits close to and above Saturn, so the pair make an attractive option for early evening observation around that date. By late January Venus will be obvious in the south-west from around 17.00UT sitting more than 30 degrees high. Visible phase will have fallen to 38% but brightness will be more than magnitude -4.6.

Mars rises in the North-East from 20.00UT in early December and is then visible for the rest of the night, climbing steadily higher and culminating in the south at nearly 60 degrees of elevation by 04.00UT. On the 17th into the 18th the Moon is close-by with the pair due-south around 02.45UT on the 18th and visible together in low-powered eyepieces in wide-field telescopes. An occultation will occur shortly after dawn that day but will be very low in the west and, perhaps, unobservable. By New Year’s Eve Mars transits around 01.40UT and it is obvious we are moving ever closer to opposition.

This occurs on 16 January when Mars shines at -1.42 magnitude and shows a disk 14” across. This isn’t terribly large as Mars is moving slowly further from the Sun in its rather eccentric orbit so these ‘perihelic’ oppositions can be a little disappointing, however plenty of surface and cloud detail should be on show and Mars is a ‘must see’ target throughout these two months. By late January Mars transits around 23.00UT so is still visible from almost dusk till dawn.

Mars is the target you will turn to after observing mighty Jupiter. Jupiter has its own opposition on 17 December and shows a disk never smaller than 43.4” across and more than 48” across in the week leading up to opposition. It follows the same high arc as Mars so the chances for observing at high altitude in steady air are very good. South transit is around 22.00UT late in December and 19.55UT by the end of January, then making Jupiter a target for the more convenient evening hours. A large rift has appeared in the south-equatorial belt recently and it will be important to watch how this develops.

Saturn is well past opposition but remains observable in the early evening hours, transiting due-south around 18.15UT in early December. At some 30 degrees of elevation for mid-UK latitudes it should be sought out soon after sunset and followed through transit and into the south-west. Initially the disk will be around 17” across with the rings stretching to 40” but these apparent sizes will fall a little throughout the period. By the start of the New Year, transit will be shortly after sunset so Saturn will already be sinking south-westerly when first visible, but the rings will be at their best presentation towards the Earth around this time; for the rest of 2025 they close up, with the Earth passing through the plane of the rings in springtime. This is not to say much as the rings will be tilted towards us by barely 3 degrees but if you wish to see them at all then keep watching Saturn into early January. As mentioned, Saturn is close to Venus around 19 January.

Of the ice giants, Uranus starts December just past its own opposition so is visible at high altitude for most of the night. If you wish to find it, Uranus sits 7 degrees to the right and a little below the Pleiades cluster (M45) and 22 degrees ahead of Jupiter along the path of the ecliptic. South-transit in early December is around 22.45UT at more than 50 degrees of elevation and the planet will be an easy binocular target shining at +5.8 magnitude. Its disk will be only 3.8” in apparent size but its north pole, which is tilted towards us (Uranus flies through its orbit laying on its side!) is brightening so some contrast with its equator should be obvious in a telescope. Transit time, mid-period, will be around 20.40UT and at 18.40UT by late January; brightness and apparent size will hardly change throughout the period.

Neptune is accessible during the early to mid evening, especially in December. It sits 14 degrees behind Saturn along the path of the ecliptic, but at magnitude +7.75 you will need larger binoculars or a telescope to ensure this tiny blue-green ‘star’ does actually show a planetary disk. South transit times is around 19.05UT in early December at some 35 degrees of elevation for mid-UK latitudes. Its disk will be noticeably smaller than that of Uranus, at 2.3”, but imaging in the blue end of the spectrum may capture any new dark spots and in the IR may show bright storm cloud features.

By early January south transit is in twilight but Neptune will still be above 30 degrees high until 19.00UT and at 18.10UT by mid month; after that Neptune will be too low for detailed observation in darkness.