Alan Clitherow is the Planetary Section Director of the Society for Popular Astronomy and an active member of DAS, and writes regularly on planetary observations. This article is reproduced from the Society For Popular Astronomy and with the kind permission of Mr Clitherow.
Mercury moves into superior conjunction behind the Sun on the 17th of February and is not visible again until the early evening skies at the end of the month. However for UK observers, March gives the best opportunity to see Mercury during 2018, particularly around the time of its greatest eastern (evening) elongation from the Sun, on the 15th. Earlier, on the 3rd of March, Mercury can be found in the evening sky, very close to Venus. Look west very shortly after sunset, from around 1745 ut, and the bright beacon of Venus should soon appear; Mercury is level with it and less than 1 degree further west. On subsequent nights Mercury rises higher and by the 12th sits almost directly over Venus, 4 degrees further up. Greatest elongation occurs on the 15th when Mercury lags the setting Sun by 18 degrees; at 1815 ut the planet sits 14 degrees up on a bearing of 260 degrees and can then be followed until it sets around 1945 ut. Night-on-night Mercury then declines in elevation so is best observed a few days either side of the 15th.
Venus was in solar conjunction on the 9th of January, so it will become visible as an evening object slightly before Mercury. From late February you may just catch it (close to Neptune!) on a compass bearing of 250 degrees from 1745 UT; however it improves steadily in visibility for the rest of the period. In March Venus shines at magnitude -3.9 and it will be a brilliant beacon above the western horizon shortly after sunset, rising in elevation night-on-night, slowly growing in apparent size and starting to show a decrease in its illuminated phase; virtually full in early March, falling to 94% by the end of the month. On the 28th of March Venus passes less than ½ a degree from distant Uranus but seeing them together will require binoculars and very steady seeing in the low altitude air of early evening.
Mars is a morning object that initially shines at magnitude +1.4 and grows steadily in apparent size and brightness through this period. Detail should start to become visible on its surface and it will therefore repay prolonged observation. At the beginning of February it rises around 3.30 ut from mid UK latitudes and reaches around 18 degrees of altitude as it transits due south, shortly before sunrise. At that time it can be found roughly half way between the brighter and more obvious light of Jupiter and the comparably bright star Antares; the obvious redness of both Antares and Mars will help you pick them out; since Antares can be translated as ‘rival of Mars’ it would be a shame to miss their close passing. Through the period Mars moves steadily prograde (easterly), passing over the top of Antares on the 7th of February and reaching 10 degrees east of it by early March.
By then Mars will rise around 0300 ut but due to the slowly changing angle of the ecliptic, and the position of Mars in relation to it, it only reaches 15 degrees of elevation when due south for mid-UK observers, still just before sunrise. In the period Mars grows in apparent size from 5.6 arc seconds to 8.4 and increases in brightness from +1.4 to +0.8. On the 24th of March Mars is at ‘Western Quadrature’; that is the angle between Mars and the Sun, as viewed from Earth, reaches exactly 90 degrees.
Jupiter rises before Mars, around 0220 ut in early February, 0035 mid-period and 2235 ut by the end of March. Over the same period it grows in apparent size from 36 to 42.5 arc-seconds and in steady air will show considerable detail. Its elevation as it transits due south will be a somewhat disappointing 20 degrees or so for most UK observers however an Atmospheric Dispersion Corrector will help tidy up the image both optically and photographically. Jupiter is the certainly best placed object for detailed observation in the period.
Saturn is elusive at the start of February; rising by 0600 ut it hardly has time to reach any appreciable altitude before sunrise. Things improve such that, by early March it rises at 0425 UT and passes 10 degrees of elevation by 0600 hours. Late in March it rises at 0235 ut and will be keeping very close company with Mars. The ring system of Saturn is tilted towards us by 25.5 degrees with the northern hemisphere of the planet prominently on view. On the 11th of March Saturn reaches its own Western Quadrature, so the shadow cast by the planet on its own rings is obviously elongated and worth observing.
As for the ice-giant planets; Uranus sets around 0125 UT in early February and by 2200 UT mid period. At magnitude 5.8,Uranus is just visible to the naked eye on a clear night away from light pollution and can easily be picked out from the background stars of Pisces with binoculars, looking like a distinctive green ’star’, and it has been argued that there are no truly green stars, so find one and it is almost certainly Uranus! A telescope reveals its small but obvious 3.5 arc-second diameter disc and, again, large aperture instruments can be used to see variations in shading from equator to pole and reveal cloud features photographically with appropriate filters. Both of the ‘Ice-Giants’ are best imaged with filters that pass light in the near infrared revealing important and scientifically useful atmospheric features. By late-March Uranus is setting before 2100 ut and will be below 20 degrees of elevation as darkness falls; best to observe it from early in the period.
Neptune is even worse placed, very low in the early evening sky. It sits close to Venus on the 21st of February and to Mercury on the 25th but observation will only really be possible from the extreme south-coast or the Channel Islands. For the rest of the UK Neptune is already lost until much later in the year.
Society for Popular Astronomy