Dundee Astro - Planetary Notes

Venus dominates the evening sky, being the brightest object, other than the Moon, to appear after sunset. Early in April it will appear due-west, some 15 degrees high from the UK, shining at magnitude -3.9 and it won’t set for around 90 minutes. It will appear as a near fully-illuminated disk (97%) some 11 arc seconds (11”) from pole to pole. As the month progresses it will gain slowly in elevation, on first appearance, as it stretches slowly from the Sun but is fighting the steadily lengthening hours of daylight. By the start of May it will be some 20 degrees high and to the right (north) of due-west; its angular size grown only slightly to 11.6” and illuminated phase fallen to 88%.

Later in the month the angle of the ecliptic to the western horizon starts to fall as day turns to night and the first appearance of Venus stagnates in altitude to a little more than 20 degrees despite it stretching away from the Sun. It will, however take nearly two and a half hours until it sets so is well worth observing. At more than 13” across and 80% illuminated, this will be a good period for deep-purple or ultraviolet observation of its cloud patterns.

In April, Mars (like Mercury, Saturn and also Neptune) is mired in brightening morning twilight, so is very hard to observe in the eastern sky. Initially it is perhaps 3 degrees high at sunrise and this improves only very slowly. On 13 April it sits just above Neptune and, as mentioned, alongside Saturn and Mercury around the 20th, but these events are only just visible at the very end of morning twilight and may be best followed into daylight to get a clearer view; as long as the usual safety precautions are used. Late in May, Mars rises around 02.30UT but the Sun is not far behind so Mars remains a challenging and low target.

Jupiter starts April by transiting (due-south) immediately after sunset, around 18.35UT. Then at 60 degrees high for most of the UK and is the most rewarding planetary target for observation at that time, not setting until after 02.00UT. It can therefore be followed at altitude through twilight into the night at a good apparent size of just under 39”. Make the most of Jupiter in early April as, by the end of the month it will be well around to the west when first becoming visible in early twilight, perhaps 45 degrees high and not until 19.45UT for mid-UK latitudes. Jupiter still won’t set for some 5 hours so detailed observation is possible. Its apparent size falls slightly to 35.5” but its elevation is falling noticeably, so steady seeing conditions will become more important.

By late May Jupiter won’t appear until around 20.45UT and will be around 20 degrees high and already a little north of due-west, sitting close to much more obvious Venus in twilight.

Saturn is, as mentioned, mired in morning twilight, near to Mars and Mercury in the second half of April but is very hard to spot. Saturn is most easily found late in May when it is rises around 02.00UT, however this is still nautical twilight in the UK so you won’t be able to follow it for long before it is washed out by the dawn. If you do follow it into daylight it will be obvious that the ring system is opening as seen from Earth, being tilted around 8 degrees towards us, as compared with virtually edge on at the beginning of the year.

Uranus is a difficult but viable target in Early April, but the viewing window will be short. It can be found some 4.5 degrees below and a little left of the Pleiades cluster, which should, itself, become visible around 30 degrees high around 19.45UT and sits due-west. At 3.5” across and at magnitude +5.8, this is more of a last opportunity to tick Uranus off the list than one for making detailed observation. By May it will be too low in the north-west to see when twilight falls.

Neptune is really a lost cause in this period. Even ‘though it sits close by other planets in morning twilight, it is really too faint to pick out.

For the northern hemisphere, Mercury rises only slightly ahead of the Sun in early April so is not observable in darkness or even morning twilight from the UK. It reaches its greatest western elongation from the Sun on 03 April and will be an excellent target for the southern hemisphere. Around 19 and 20 April, Mercury sits just below both Saturn and Mars but the sky will be too bright and the planets too low in the east for either conjunction to be clearly seen from the UK. After this Mercury slides back towards solar conjunction, which happens on 14 May; then it passes behind the Sun (superior conjunction) and into the evening sky.

In the last week of May, Mercury starts to appear low in the north-west shortly after sunset as the bottom of a descending line of three planets; Jupiter, Venus and Mercury; with Mercury making its first appearance perhaps 8 degrees high from mid-UK latitudes. At magnitude -0.55 it will be much less obvious than the other two but can be picked out and improves further into June.