The Planets in February and March

Mercury is an accessible planetary target in February but is best seen from as far south as possible. For northern observers it will be at low altitude in twilight. It reaches greatest eastern (evening) elongation from the Sun on the 10th of the month and for mid-UK latitudes will start to become visible around 20 minutes after sunset, low in the south-western sky, at an elevation a little above 10 degrees from the horizon on that date. It should be visible at lower altitude from around the first to the twentieth of February before moving into inferior conjunction, between the Earth and the Sun, on the 26th and into the morning sky in March. The following morning apparition of Mercury will be very poor for UK observers, despite the planet reaching its greatest elongation from the Sun for the year, some 28 degrees, on the 24th of March. UK observers would perhaps do best to follow the planet into daylight and, with care, view or image it at higher altitude.

Venus is in the evening sky throughout the period and, for once, is an object best seen from the northern-hemisphere. It reaches its greatest eastern elongation on the 24th of March, the same day that Mercury has its disappointing morning elongation. Venus however does not disappoint. It is the first ‘star’ to appear in the western sky following sunset and is so bright, around magnitude -4.3, that it is quite easy to find in full daylight, particularly if you have a go-to mount or can use setting-circles on the mount. If you wait until just after sunset Venus appears relatively high in the sky, at around 30 degrees up in the south-west in early February and steadily improving to above 35 degrees in early March; and 40 degrees, only a little south of due-west, on the 24th.

On that date Venus stretches 46 degrees from the Sun and visually should show approximately 50% illuminated phase. The exact moment when a 50% phase is seen varies somewhat from that of greatest elongation due to the slight angular inclination of Venus’s orbits with regard to Earth’s own orbit. In this case 50% phase (dichotomy) should occur very late on the 26th of March but varies by barely 1% from that seen on the 24th. In any case the exact moment of dichotomy, as determined by visual observation, is somewhat subjective due to the Schröter Effect.

Named after its discoverer, Johann Schröter, this effect describes the observation of dichotomy as arising, on average, a few days early for evening Venus elongations, and a few days late for morning ones and is possibly due to the way various frequencies of light are scattered near the terminator. Sequential images taken in red then blue light may show measurably different phases despite being taken within minutes of each other. This is because blue light is scattered more by particulates in the Venusian atmosphere thus altering the apparent phase in that colour; as two visual observers can easily differ in colour sensitivity they may disagree when 50% phase actually occurs. Why don’t you let me know your estimate of the time of dichotomy? Try using various colour filters and compare the results. Photographic observers should also compare filters.

Monochrome cameras using ultra-violet filters may also reveal the appearance and rotation of dark cloud features in the visible phase. For this period the illuminated phase is widest on February first (73%) and falls steadily to 47% by the end of March so UV imaging is perhaps best done early on however Venus remains an excellent target well after the end of March.

Mars, Jupiter and Saturn are all situated in the morning sky throughout this period. As the period progresses Mars suffer from the declining angle the ecliptic makes with the morning horizon as we move from late winter into early springtime. As a result it rises highest early in February rather than late in March as you might expect. At the start of February Mars rises around 0440UT and reaches around 15 degrees of elevation, nearly due south, by sunrise. Mid period Mars reaches 12 degrees by sunrise and does no better late in March despite rising around 0330 UT. The clustering of Mars, Jupiter and Saturn is very photogenic and their relative motions means that visually Mars passes very close to Jupiter on the 20th and 21st of March and to Saturn on the last day of the month. Mars grows from just below 5 arcseconds in apparent size in early February to just above 6 in March, meaning careful observers will start to discern surface features given good seeing conditions.

Jupiter appears considerably bigger and brighter than Mars; its apparent size grows from 32 to 37 arcseconds in this period so good seeing conditions will reveal considerable detail in its ever-changing atmosphere. Jupiter rises around 0630 UT in early February, reaching just 8 or 9 degrees of elevation by sunrise but may easily be followed into full daylight. Sadly, rather like the last apparition of Jupiter, the planet will culminate at a disappointingly low elevation as seen from the UK. At south transit it will reach barely 15 degrees of elevation in early February, and 17 degrees by late March.

Saturn fares only slightly better, initially rising 45 minutes after Jupiter; the pair grow slowly closer so that the separation is just 17 minutes by late March when Jupiter rises at 0320 UT. Saturn culminates slightly higher than Jupiter but the difference is small; its rings are tilted towards us by 21 degrees so they will be very obvious and worth seeking out despite the low elevation.

Of the ice-giants, only Uranus is worth finding. Neptune is in conjunction with the Sun on the 8th of March so is too close to it for effective observation. Uranus, however, sits high in the sky shortly after sunset. South transit times are initially in twilight and in full daylight by late March, so seek out this tiny green gem early in the period if possible. It sits close to the borderline between the tail of Pisces and the head of Cetus and may easily be swept up with binoculars because of its distinctive colour. Culminating at 50 degrees of elevation then slowly declining to the south-west, this often neglected planet is worth seeking out and, for photographic observers, particularly worth imaging in the infra-red where it occasionally shows surprising atmospheric detail.

Alan Clitherow.