Dundee Astro - Planetary Notes

At GWE Mercury will shine at magnitude 0.0, be 41% illuminated and show a disk 7.5 arcseconds (7.5”) in apparent size. By early September Mercury will be significantly brighter, mag -1.4, and it will be 90% illuminated, but will have shrunk to just 5” in apparent size. Superior conjunction is on 13 September after which Mercury will move steadily towards its best evening apparition of the year for southern observers, but will barely skirt the western horizon as seen from the UK.

Venus is a brilliant ‘morning star’ in this period, rising in the north-east and, initially at least, leading Jupiter up and along the ecliptic before sunrise; however Venus is steadily sinking back towards the Sun and on 12 August the planets rise together, around 01.30UT, with Venus a degree below and right of Jupiter. They can easily be viewed together in a low power eyepiece for a day or so either side of the 12th, and they can then be followed until at least sunrise.

By early September, Venus rises well after Jupiter but will still make 25 degrees of elevation in the east before the Sun breaks the horizon; by late September this only reduces by 5 degrees, making Venus still an excellent morning target. Throughout the period its brightness declines only slightly from magnitude -4 to -3.9, while its illuminated phase grows from 75 to 91%; its apparent size also falls slightly from 14” to 11”.

Mars is a low to very-low altitude evening target throughout the period but is best seen as early in August as possible when UK observers may catch it 5 degrees high in the west a little after sunset.

Jupiter is a steadily improving morning target in this period, rising around 01.55UT early in August and reaching 20 degrees of elevation in the east by sunrise. At magnitude -1.9, Jupiter is bright but is spectacularly out-shone by nearby Venus. By early September Jupiter rises at 00.30UT; now well ahead of Venus, Jupiter will lift to more than 40 degrees high in the south-east by sunrise for mid-UK latitudes. Late in the month, rise will be at around 22.55UT and the elevation more than 55 degrees by sunrise. The planet will grow in apparent size from 33” to 37” over August and September, making it an easy and spectacular target for early morning observers.

Saturn doesn’t rise quite so high but is rapidly moving towards opposition, which occurs on 21st September. Around that date Saturn can be observed for all the available hours of darkness. South transit times, when Saturn can be seen highest in the sky, will be around 03.00UT in early August, 01.20UT mid-period and, obviously, around midnight UT on 21 September. In each case Saturn will sit around 35 degrees high when due-south. Around opposition we will see Saturn’s rings very nearly edge on, with the southern hemisphere tilted towards us by just 1.8 degrees; this means its bevy of orbiting moons will regularly transit the disk, as will the shadows they cast. Sadly, from the UK, the most obvious transits, involving the large Moon Titan, will either occur low in the south-west as Saturn is setting towards dawn or in daylight in this period.

Around opposition Saturn will show a disk 17.5” across the equator, with the thin strip of the rings stretching to 44.1”. Saturn will shine at magnitude +0.6 at the tail of Cetus the Whale and will be obvious as the brightest ‘star’ in the region.

Of the ice giants, it is easiest to cover Neptune first as it sits very close to Saturn throughout the period. In early August Neptune is one degree directly above Saturn when the ringed planet is due south. By early September Neptune lags Saturn a little, sitting 1.5 degrees above and left at transit, while late in September this stretches to 3 degrees. From this it is easy to see that Neptune has its own opposition only 2 days after Saturn and is therefore observable all night at that time. At magnitude +7.8 its tiny disk will be hard to find and at only 2.4” in size, no detail will be clearly visible, but nearby Saturn does make Neptune easy to find and its worth ticking it off the list!

Uranus rises near midnight in early August and can be found sitting 4 degrees below the beautiful Pleiades cluster, M45, and then followed into morning twilight. Its +5.6 magnitude disk is on the edge of naked eye visibility and is an easy binocular object, appearing as a de-focussed blue-green ‘star’. High power will show its 3.6” disk. Uranus will transit at dawn in early September and at 03.20UT late in the period.