Mercury is at its greatest eastern (evening) elongation from the Sun on the 1st of October but is very poorly placed for observation from the UK and therefore well placed for equatorial and southern-hemisphere observers. For mid-UK latitudes it will set barely 30 minutes later than the Sun so is effectively unobservable. Solar conjunction is on the 25th of October, after which Mercury returns to the morning sky and puts on a very good show for northern latitudes. Best seen from around the 5th to the 22nd of November, Mercury reaches greatest western elongation, 19 degrees west of the Sun, on the 10th of November. On that date Mercury will attain around 15 degrees of elevation by sunrise (for mid-UK latitudes) and may show surface albedo features during steady seeing conditions. Mercury can be followed each morning to the end of November but at rapidly reducing altitudes.
Venus is past its greatest solar elongation but is still a brilliant beacon in the eastern sky before dawn. Shining at magnitude -4.0 it will rise around 0220UT in early October and reach over 30 degrees of elevation by sunrise. Its disc will be 72% illuminated and span 16 arcseconds pole-to-pole. By early November Venus has moved closer to the Sun, rising at 0345UT, due-east, and gains a height of 25 degrees by sunrise. Phase is now 82% illuminated and size has dropped slightly to 13 arcseconds. This trend continues such that by late November Venus rise is at 0515UT and it reaches some 20 degrees of elevation by sunrise; 88% illuminated and 12 arcseconds across. This is an excellent time to look for subtle cloud features using coloured filters or to image the planet in the near ultraviolet or infrared. Motion and shape of cloud detail is an active area of research so detailed observations from within this period would be most welcome.
Mars is the ‘standout’ target for observation at this time and the UK will see its best apparition of the red planet for far too many years. The last opposition in summer 2018 was a close one but appeared very low down for UK observers and was partially spoiled by extensive dust storms. Opposition of Mars this year happens on the 13th of October with Mars transiting due south close to midnight UTC (0100 BST) at some 43 degrees of elevation. It will be very obvious at magnitude -2.6 against the much fainter background stars of Pisces and its fully-illuminated disc will be nearly 23 arcseconds across. Throughout October and November Mars will be observable for most of the hours of darkness and even late in the period it will still be bright at magnitude -1.1 and 15 arcseconds in apparent size. South transit in late November is a more convenient 2030UT, still above 40 degrees of elevation.
The southern hemisphere of Mars will be tilting slowly towards us and the north polar ice cap may well be invisible as a result. The southern icecap will have shrunken to nearly its minimum size with most of its stored carbon-dioxide gas and water vapour sublimated into the atmosphere. As long as the dreaded global dust storms stay away then surface and cloud detail should be well presented and we have a rare opportunity for very detailed scrutiny of the red planet.
Jupiter and Saturn are still sailing close together in the early evening sky, if at a rather low altitude for UK observers. Recent amateur observations in the methane band (around 890 Nm in the near infrared) have shown the start of a significant outbreak of the North Temperate belt of Jupiter therefore continued observation is essential. In early October Jupiter transits, due south, in evening twilight at around 1840 UT with 15 degrees of elevation; Saturn follows close behind, transiting a degree higher at 1913UT. Jupiter shows a magnitude -2.3 disk some 41 arcseconds across the equator while Saturn is 17 arcseconds across its visible disc and 40 across the span of the rings, shining at magnitude +0.5. The rings will be tilted towards us by 22.7 degrees with the northern hemisphere of the planet on view. Jupiter dips below 10 degrees of elevation by 2045UT with Saturn following 50 minutes later. With this in mind the pair should be observed from as early as possible.
By early September Jupiter transits in fading daylight at 1655UT and Saturn at 1715UT and by the end of the month both transit a little before sunset at 1520 and 1530UT respectively. The pair are obvious and observable from twilight and this is worth doing as they draw ever closer to a rare and close conjunction in December.
The ice-giants are both well placed for observation in this period. They are small and distant and show little or no detail to the visual observer but they are worth seeking out at this time, if only to tick them off an observing list of the planets. Well-equipped imagers may like to try observing them in the near infrared as they show more atmospheric contrast and occasional highly reflective storm features.
Neptune transits due south around 2220UT in early October but the nearby Moon will make finding its faint magnitude+7.8 disc difficult. Better to wait a week or so to find it mid-month in darker skies. Look 7 degrees east and a little above the star Hydor in Aquarius for a distinct fuzzy blue or blue-green ‘star’ and increase magnification to isolate Neptune’s planetary disc from the background pin-point stars. From the UK Neptune will be around 32 degrees of elevation at south-transit and it can be followed above 30 degrees from 2100UT to 2335UT in the second week of October. By early September south transit is at 2040UT and late in the month at 1845UT, still above 30 degrees of elevation, so October and November offer an extensive period in which this distant planet can be studied.
Uranus is even better placed. It comes to opposition on the 31st of October so can be seen for most of the hours of darkness throughout this two month period. Look for its magnitude +5.7 disk above the head of Cetus the whale, midway between Menkar in Cetus and Sheratan in Aries. Its blue green disk should be visible in binoculars and small telescopes and large ones will show atmospheric shading from pole to equator. At over 50 degrees of elevation during south-transit Uranus is actually the highest planet visible from the UK in this period and should be sought out when the seeing is good. Transit times are 0156UT in early October, midnight UT on the 31st and 2150UT in late November.