The Planets in August and September 2020

Mercury starts this period very well placed, just past the peak of the best morning apparition of the year for UK observers. Catch it in the first 5 days of August when it should be rising at or shortly after 0300 UT and will gain 10 degrees of elevation by sunrise for mid-UK observers. Look for it rising in the North-East shining at magnitude -1.2, its illuminated phase growing steadily from 70 to 83% with a disk some 6 arcseconds from pole to pole. After this brief observational window Mercury moves rapidly away from us, slipping behind the Sun into superior conjunction on the 17th of August. The subsequent evening apparition from early September and throughout the rest of the month is a poor one for the UK but is excellent for southern-hemisphere observers.

Venus rises ahead of Mercury and is very obvious in the morning sky throughout August and September. It reaches greatest western (morning) elongation from the Sun on the 13th of August and on that date will rise around 0100 UT in the north-east and gain more than 30 degrees of elevation by sunrise. It shines at magnitude -4.4 in early August and even by the end of September is still extremely bright at -4.1. Each morning the angle the ecliptic makes with the horizon is rising as the period progresses so Venus manages to improve its elevation at sunrise, reaching nearly 35 degrees by late September. Through the period the illuminated phase grows from 43% to 71% while its apparent size drops from 27 to 15.6 arcseconds. Dichotomy, or the point of 50% phase, should be seen on the 13th of August but many observers report this a few days late on morning apparitions. This is an excellent time to observe Venus and, once found, the planet can easily be followed into daylight and observed at even higher altitudes.

Mars is also putting on a good morning show. In early August it rises around 2230 UT, almost due-east, and reaches nearly 40 degrees of elevation in the south-east as morning twilight appears. It is sitting in the constellation of Pisces and at magnitude -1.1 is noticeably brighter than the background stars; its obvious red colouration will single it out. By early September it rises around 2040 UT and transits, due south, at nearly 45 degrees of elevation at 0315 UT, still in dark skies. By late September south-transit is at 0110 UT so Mars can be followed from mid-evening all the way until sunrise. This is a really good period for Mars observation from the UK with the apparent size of the disk growing from 14.6 to over 22 arcseconds. Its southern hemisphere will be tilted towards us but the southern ice-cap will be small or cloud covered as it will be high summer for the south of Mars. Assuming we don’t see planet-wide dust storms detail should be very good and the amount of sublimated ice in the atmosphere could lead to very obvious cloud formations including the famous ‘W’ shaped clouds forming over the shield volcanoes of the Tharsis bulge. Mars in this period is not to be missed!

Both Jupiter and Saturn were at opposition in July so are now well placed for evening observation, if at a rather low altitude. They will initially be seen close together in the south east mid-evening; the separation between them stays at 7 degrees throughout the period with Saturn following Jupiter across the sky. Because of their low elevation they are best seen around the time of south-transit. For Jupiter this is 2251 UT on the first of August, on which day Jupiter sits just above the Moon. Mid-period south-transit is at 2038 UT and in late September at 1845 UT. In all cases Saturn transits some 30 minutes after Jupiter. The elevation of these planets will be low at around 15 degrees and 16 degrees respectively but their large apparent size makes them worth seeking out and much can be seen if local seeing conditions are good. Even in late September Jupiter will show a bright disk, magnitude -2.3, more than 40 arcseconds across. Saturn is dimmer at Magnitude +0.5 but it too will be more than 40 arcseconds across the width of the ring system with the planet itself appearing some 17 arcseconds in diameter. The rings will be tilted towards us by 22.7 degrees with the northern hemisphere of the planet on view.

As for the ice-giants, Neptune rises first and comes to opposition in this period. In early August it rises around 2115 UT and can be followed above 30 degrees of elevation from 0130 UT until morning twilight; south transit is at 0250 UT. Mid period south transit is at 0049 UT and in late September at 2248 UT; opposition is on the 11th of September with a south transit at 0004 UT. Neptune is faint at magnitude +7.8 so you will need a telescope to pick it out against the background stars to the east of Aquarius. To locate it use a wide-field eyepiece to look 9 degrees East and a little above the star Hydor (λ Aqr) to find a distinctly blue-green ‘star’; centre it and add magnification to see its 2.35 arcsecond planetary disk.

Uranus rises 90 minutes after Neptune and gains more altitude. In early August it is best seen after 0200 UT when it reaches 30 degrees of elevation in the east, it can then be followed into morning twilight by which time it has climbed another 10 degrees or more. By early September Uranus transits in darkness around 0400 UT at more than 50 degrees of elevation and by late in the month transit is at 0200 UT so the planet can be followed for most of the hours of darkness. It can be found above the head of Cetus laying mid-way between Hamal in Aries and Menkar (αCet). At magnitude +5.7 it is on the edge of naked-eye visibility but is an easy binocular target. It looks like Neptune; a blue-green star requiring high magnification to show its disk. Both the ice giants are bland visually but infrared planetary photography can pick up cloud details in their frigid atmospheres.

Alan Clitherow