The Planets in August and September 2019

Mercury is often a difficult target for observers at UK latitudes but for a few days either side of the 9th of August it puts on a reasonable show in the early morning sky. On that date Mercury reaches greatest western (morning) elongation, stretching 19 degrees west of the Sun and visible above the north-eastern horizon from shortly before sunrise. From the 5th to the 16th of August look for Mercury rising around 0315 UT on a bearing of 060 degrees and reaching around 13 degrees of elevation by sunrise. In this period mercury brightens from +0.9 to +0.8 and can be followed at ever brighter magnitudes, if at lower and lower elevations, to the last week of August when it shines at nearly magnitude -1.7. Mercury moves behind the Sun, superior Conjunction, on the 4th of September then passes into the evening sky, putting on a good show for equatorial and southern-hemisphere observers but too low for most UK observers.

Venus is in superior conjunction on the 14th of August and remains too low for UK observation until at least early September ‘though, again, southern-hemisphere observers will fare much better. Venus lies very close to Mercury around the 13th of September but UK observers will need to find the pair in daylight, taking all precautions to protect eyesight and equipment from the Sun, in order to see them just 18 minutes of arc apart with bright Venus sitting directly above fainter Mercury.

Mars may be visible very low in the western sky, just after sunset at the start of August, but is very soon after lost to observation, reaching conjunction with the Sun on the 2nd of September. Thereafter Mars moves into the morning sky but remains too close to the Sun for observation. By late September Mars rises in twilight around 0515 UT, almost due east, but climbs barely 8 degrees above the horizon by sunrise.

Jupiter remains low as seen from the UK but is easily observable, transiting due south at around 15 degrees of elevation at sunset in early August. It shines at a brilliant -2.4 magnitude and shows a large disc some 42 arcseconds across. Jupiter sets at midnight UT in early August; 2200 UT mid period and 2025 UT by the end of September, by which time it has dimmed only a little to -2.0 magnitude and shrunk slightly to 36 arcseconds in size. Careful observation in good seeing conditions will reveal a wealth of detail although an atmospheric dispersion corrector (ADC) or observation using coloured filters is recommended to counter the rainbow dispersion effects of observing low altitude targets. On the 8th of September Jupiter is at East Quadrature when the Sun to Earth to Jupiter angle is exactly 90 degrees. Around this time the shadows cast by the Galilean moons stretch noticeably from the moons themselves and may have a more obviously oval appearance.

Saturn follows an almost identical path across the sky to that of Jupiter, just less than 2 hours behind, throughout this period. The planet is tilted towards us with the northern hemisphere on view and the rings, open to us by 25 degrees, are beautifully presented and should be observed whenever the seeing conditions are forecast to be good. Again, an ADC or colour filters will noticeably improve the view.

Of the outer ‘Ice-Giant’ planets Neptune rises first, around 2120 UT in early August, transiting due south at 0245 UT just as the sky starts to lighten. At more than 30 degrees of elevation its tiny 2.4 arcsecond blue-green disc sits one degree east and a little above Ф (Phi) Aquarii, shining at a distinctly telescopic +7.8 magnitude. On the 10th of September Neptune reaches opposition, due south at midnight UT, and around this date is extremely close to Ф Aquarii which can be used to locate the planet quickly and easily. Neptune is observable at a good elevation and in full darkness throughout the period.

Uranus is perhaps even better placed in that it rises later, around 2230UT in early August, but reaches reach 40 degrees of elevation in the south-east at the end of morning twilight. By early September Uranus transits in darkness more than 50 degrees up for mid-UK latitudes around 0340 UT and at 0150 UT by month’s end. Its 3.7 arcsecond disc will be on the edge of naked-eye visibility and it will easily be found with binoculars or a small telescope in an area of few bright stars 4 degrees above the head of Cetus the Whale and 9 degrees east of the tail of Pisces.

The Ice-Giants are increasingly becoming targets for well-equipped planetary imagers and respond well to filters passing only near infrared light. Videos many minutes long can be taken and the resulting video-frames stacked into surprisingly detailed images. These can reveal bright cloud patches as well as atmospheric shading, particularly with Uranus, an easier target due to its relative brightness and slightly larger apparent size when compared with Neptune.

Alan Clitherow