The Planets in August and September

Undeniably the best planetary targets for observation in this period, and probably for the rest of the year, will be Jupiter and Saturn. They will be big and bright throughout August and September and, compared with last year, reasonably high in the sky during the hours around midnight. Both planets reach ‘opposition’ in August. This is the point when an object is directly opposite the Sun in the sky as viewed from Earth making them due-south at midnight UT.

Saturn reaches opposition, and also at its smallest distance from Earth, on 02 August and will then be at its brightest in the night sky. You can follow it easily from evening twilight, low in the south-east, through south-transit at around 20 degrees of elevation (as seen from the UK) and on into the early hours before dawn. Its brightness will be an obvious magnitude +0.2, siting in the constellation of Capricorn where it will present, on opposition night, a planetary disc 18.6 arcseconds across with the rings stretching wider to 42.3 arcseconds. The northern hemisphere of Saturn will be tilted towards us by some 18 degrees so the rings themselves as well as northern polar cloud formations will be beautifully presented to us.

As the period progresses the tilt towards us increases very slightly but, as you would expect, the planet dims and shrinks a little in apparent size as we move past the opposition night. By early September south-transit is at 2200 UT with magnitude at +0.3 and in late September transit is at 2005 UT with magnitude at +0.47; by then the rings will be down to 41 arcseconds across. Elevation remains good at south-transit and the planet can now be more conveniently viewed in the mid-evening hours.

Jupiter follows Saturn across the sky reaching its own opposition on 19 August but at a greater elevation above the southern horizon; some 24 degrees from mid-UK latitudes. Jupiter moves slowly from Aquarius into Capricorn during the period and will be very obvious at magnitude -2.9, showing a huge planetary disc nearly 50 arcseconds across on opposition night itself. South transit will be 2320 UT in early September and at 2110 by month’s end. Even then the planet shines at magnitude -2.7 and is 46 arcseconds across at the equator.

Mercury, Venus and Mars will all be evening objects in this period but at very low altitudes for UK observers. Mercury is hidden behind the Sun on 01 August (superior conjunction) so is unlikely to be at all visible until late in the period. It does pass very close to Mars on 19 August but both planets are skirting the horizon at sunset so the appulse is unlikely to be seen from the UK. Mercury goes on to make its greatest eastern (evening) elongation from the Sun on 14 September and this will be well seen from the southern hemisphere but is so low from the UK it will probably be missed.

Venus is, initially at least, slightly better placed and will be found around 10 degrees up, due-west at 2000 UT in early August, shining at a brilliant magnitude -4.0 and it can then be followed until it sets more than an hour later. By early September Venus sets around 2000 UT so look for it around 1900 UT as the sky starts to darken after sunset but it will now be lower down and a little south of west. By the end of the month Venus will be around 5 degrees of elevation in the south-west at sunset so it’s best to catch it early in the period if you can.

Both of the outer ‘ice-giant’ planets are visible in this period. In fact Neptune is the third planet to reach opposition, this time on the 14th of September. Neptune follows Jupiter across the sky and rises around 2125 UT in early August, just as the sky is darkening for the short summer night. It reaches south transit around 0255 UT as the sky brightens again and is more than 30 degrees above the southern horizon. At magnitude +7.8 a detailed finder chart will be needed to pick it out from the background stars on the border between Aquarius and Pisces.
Mid-period, Neptune’s south-transit is around 0100 UT and on opposition night at 0000 UT when its pale blue disc will be just 2.4 arcseconds across. If you wish to seek it out Neptune can be found throughout September during the hours of darkness.

Uranus is best seen in late September. In early to mid-August it can be found in darkness but you will have to wait until after 0100 UT to catch it at significant altitude. By the start of September look for Uranus due east from 2300 UT sitting 12 degrees below the brightest star in Aries (Hamal) and 20 degrees above the horizon. Uranus will then rise to above 50 degrees of elevation by 0300 UT and will be beautifully placed for observation in the dark and steady morning skies of early autumn. By late September Uranus transits around 0215 UT, still above 50 degrees of elevation for the UK and is on the edge of naked eye visibility at magnitude +5.7; its visible disc will be some 3.7 arcseconds across.

Alan Clitherow