Looking Forward to the Planets in June and July 2021

Mercury is in inferior conjunction, between the Earth and the Sun on 11 June then moves into the early morning sky. It puts on a very good show for observers near the equator and, especially, into the southern-hemisphere and can be followed there from the 1st to the 21st of July. Greatest western elongation from the Sun of 22 degrees is on the fourth of July. For observers from northern latitudes the view is not as good.


From the UK Mercury will rise around 0300 UT in early July. Its visibility is helped by the steady rise of the ecliptic as summer night turns into day; as a result Mercury is highest above the east-north-eastern horizon at sunrise on July 11 and it is best viewed for a few days either side of that date. Even then Mercury will be barely more than 10 degrees up at sunrise. It brightens steadily being magnitude +1 in late June and nearly -2 in late July and in the week 04 to 11 July shows around a 50% illuminated phase, steadily increasing as Mercury thereafter sinks back towards the Sun.


Venus is in the early evening sky in this period and is, yet again, best seen from equatorial and southern regions. From the UK and in early June it will first appear around 2030 UT about 10 degrees above the north-western horizon and, at a very brilliant magnitude -3.9, it will become increasingly obvious as the sky darkens. By early July look for Venus from around 2045 UT. It will have stretched slightly further from the Sun but will appear at no higher elevation as, from the UK, it is hindered by the sinking ecliptic as day turns to night, the opposite effect to that which helped the visibility of Mercury. By late July look for it due-west from 2015 UT, slightly brighter at magnitude -3.95 but no higher in the sky.


Mars sits close to Venus from the 8th to the 16th of July, moving steadily from left to right below the much brighter inner planet in that period. Closest approach is on July 13 when Mars will sit half a degree below Venus and this offers a good photo-opportunity. Otherwise Mars is at its farthest point from the Sun, Aphelion, on this same date and, from Earth is also distant and visually tiny, below 4 arcseconds in apparent size.


The opposite is true of both Jupiter and Saturn which appear large, bright and very obvious and sit close together in the late evening and early morning sky. Saturn rises first, around midnight UT in early June, with Jupiter following around 40 minutes later ‘though Jupiter is slowly stretching farther from Saturn. By mid-period Saturn rises at 2200 UT and in late July at 2000 UT, before full astronomical twilight; again Jupiter following somewhat afterwards. This means the pair can be followed for extended periods during the limited darker hours of a UK summer but at no very great altitude. South transit times for Saturn are at 0430 UT in early June, 0235 UT mid-period and 0025UT in late July. In all cases Saturn will then sit around 20 degrees above the horizon when due south. Jupiter will be some 5 degrees higher still when it transits at around 0140 UT in late July.


For northern observers this is a considerable improvement for both planets when compared with last year. Saturn will brighten slightly from +0.6 to +0.2 in the period and the ring system will be opening a little as we view them, from 16.6 to 17.9 degrees. Jupiter will be noticeably brighter at -2.5 improving to -2.8 as its apparent size grown from 41 to 48 arcseconds. Both planets reach opposition in August.


The outer ice-giants of Uranus and Neptune are challenging objects from the UK in this period as they suffer from the lack of true darkness in our summer skies; again they are much better seen from further south. Neptune rises first, before midnight by late June, but doesn’t transit in even limited twilight until the very end of July. At magnitude +7.9 it will be hard to pick out from background stars and will be better seen in the autumn. Uranus will be easier to find being brighter, at +5.9 but it rises later. By late July look for it in the east after midnight UT, 17 degrees west of the Pleiades, above the head of Cetus the Whale and below Aries, then follow its rise as the sky brightens.

Alan Clitherow