Dundee Astro - Planetary Notes

By September Mercury will have swapped to the evening sky but, for northern-hemisphere observers, the path of the ecliptic is so low to the western horizon at sunset that Mercury is effectively invisible. Needless to say equatorial and southern regions will get excellent views into late September.

Venus is a bright beacon in the west after sunset in this period but, again, the low angle of the ecliptic means that from the UK it never appears much higher than 10 degrees up at sunset, and this falls through the period. Venus is at its Greatest Eastern (evening) Elongation from the Sun on 15 August and should show a 50% illuminated phase close to this date. Theoretical dichotomy (exactly 50% phase) should occur on the 18th but the well known Schröter Effect will affect observation. Visually, and to a lesser extent photographically, estimations of exactly 50% phase usually comes a few days earlier for evening elongations. Venus’s thick atmosphere bleeds light from the illuminated to the ‘dark’ side, making the slight indentation of the visible crescent seem fuller than it is.

By late August, and from the middle of the UK, Venus will first appear only 5 degrees high, midway between west and south-west; at magnitude -4.57 the planet will be very brilliant but also close to the horizon. This will be a period of more UFO sightings than normal! Venus can be followed into mid-September but will, by then, be resting just above the horizon when it appears.

Mars starts August by rising a little after midnight UT and then climbs slowly while moving from north-east to east; getting there by sunrise. It will be just below 5 arcseconds (5”) in apparent size until the end of the month, so detailed observation will be tricky, but even early in August Mars gains around 25 degrees of elevation before being washed out by morning twilight; views of it will therefore steadily improve. By mid-period Mars is rising just before midnight UT, in the middle of Gemini, and gains perhaps 40 degrees of elevation before being lost to twilight. Now just above 5” in size, the serious observation period of this planet begins!

By Late September Mars has slipped to below Castor and Pollux in Gemini and will be obvious as a +1 magnitude red ‘star’. It will then rise to nearly 50 degrees of elevation in the south-east towards the end of morning twilight. Mars has its equinox on 29 September, marking the start of spring in its northern hemisphere. This could trigger localized or even global dust storms, as well as significant cloud activity at low latitudes and over the Tharsis volcanic region.

Jupiter is, as mentioned, a morning object that slowly but steadily improves through the period. Too close to the Sun for sensible observation initially, start to look for it from mid-August onwards when it rises with Mercury. By early September Jupiter rises around 02.55UT and may make 20 degrees in the east before being washed out by dawn. Late in the period Jupiter rises around 01.30UT and follows Mars up the line of the ecliptic. By 05.40UT it will be more than 35 degrees high and can be followed telescopically into daylight. At magnitude -1.9 it will be very obvious and its 33” disk will show great detail in steady seeing.

Saturn is the best object of this period as it rises well before midnight UT (2 hours before in early August) and reaches decent elevation in the south before sunrise. By 04.00UT on the first, Saturn will sit 40 degrees high for mid-UK latitudes, much higher than in other recent apparitions. By early September Saturn rises around 20.00UT, almost due-east, and transits (due-south) at 02.13UT, still 40 degrees high for much of the UK. Late in the month, south-transit is at 00.15UT, with Saturn reaching opposition early in October. Therefore the planet can be viewed for most of the hours of darkness late in this period. The southern hemisphere of Saturn is on view with the rings initially tilted some 9 degrees towards us. The tilt closes slowly to 7.6 degrees by late September but the view should be spectacular. By that time, Saturn will shine at +0.35 magnitude and stretch 19.7” across with the rings spreading to 45.9”.

Uranus commences August, rising just before midnight UT. By the start of morning twilight Uranus will be some 25 degrees high in the east, midway between Aldebaran in Taurus and the Pleiades star cluster. Scan this region for a dim blue-green ‘star’ in binoculars, or with a telescope add power to see its 3.6” disk. By early September Uranus is rising around 21.40UT for mid UK latitudes and six hours later sits over 50 degrees high in the south-east, ‘though the Moon is nearby on the fourth making Uranus a difficult catch on that night. Late in that month Uranus will transit around 03.40UT, still between Aldebaran and the Pleiades and nearly 60 degrees high, making an excellent target, especially for imagers.

Neptune spends this period leading Saturn across the sky. In early August it sits 10 degrees ahead along the ecliptic in an otherwise barren area of obvious stars between Pisces and Cetus. At +7.7 magnitude and 2.4” in size, it will need good binoculars or a telescope to find, looking like an even fainter blue-green ‘star’ than Uranus. The Moon sits 3 degrees above it the third which will make it very hard to see. In early September Neptune passes due-south around 01.35UT, still 10 degrees ahead of Saturn and more than 35 degrees high. Neptune comes to opposition on 26 September so should be its at its best then, but the Moon is again close by, so it is better seen a couple of days earlier or later.