Dundee Astro - Planetary Notes

Venus is also in the evening sky and likewise suffers from low altitude, but is better placed than Mercury for northern observers. In early October it will first appear around 5 degrees high in the south-west a little after sunset. At magnitude -4 it will catch the eye as a bright low-flying ‘object’ but it will set 50 minutes after the Sun and will not be high enough for sensible observation. As the period progresses Venus stretches steadily further from the Sun but its not until the second half of November that the planet appears much higher than 10 degrees, in the south-south-west, after sunset. By then Venus will be as bright as magnitude -4.2 and showing a 68% illuminated phase and, from then, detailed observation can start. In particular we need reports of atmospheric disturbances in the near infrared on this planet.

Mars is much better prospect from the start of October and improves steadily as an early morning object. Even in the first week of that month Mars rises before midnight UT and will reach more than 50 degrees of elevation in the south-east before the sky starts to brighten significantly. On 14 October the Sun to Earth to Mars angle forms 90 degrees (Quadrature) so the red planet will show a distinct phase, being some 88% illuminated. Mars is at its highest in the sky as it passes due south. This is shortly after sunrise in early October but, mid-period, transit is in twilight at 05.30UT and with nearly 60 degrees of elevation. Mars will then be around 9 arcseconds (9”) in apparent size and surface detail should be well on show. Late in November Mars transits in full darkness at 04.05UT and will have grown to 11.5” in size.

Jupiter rises before Mars so profits from fully-dark skies as it moves to the south, except perhaps for the very start of October when transit is at 04.45UT in early twilight. Like Mars, Jupiter approaches 60 degrees of elevation at south-transit and, in fact, Jupiter’s own orbit sits at its highest above the ecliptic in early October. At magnitude -2.5 Jupiter will be very obvious between the ‘horns’ of Taurus and above the head of Orion. At 43” in size, great detail can be teased out of its circling cloud belts. Mid period, transit is at 02.40UT and in late November at 00.35UT as Jupiter approaches opposition in early December; apparent size will have grown to over 48” so this period is outstanding for observation of Jupiter and its moons.

Saturn starts October a little after its own opposition so is slowly losing brightness and apparent size, however it is becoming more accessible in the mid to late evening sky making observation more convenient. South transit times for early October, mid-period and late October are 22.30UT, 20.20UT and 18.30UT respectively, always approaching 30 degrees of elevation for much of the UK population (26 degrees for me in southern Scotland). Brightness falls from +0.66 to +1.0 and visual size from 19” to 17.4 across the equator (44” to 40.6” across the span of the rings). These are not dramatic changes so Saturn is an excellent target throughout the period and should be the first object you turn to as you start an evening of observation. The rings are tilted some 5 degrees towards us, which is the widest they are going to be for the rest of this apparition and they will close completely in spring next year; appreciate them while you can.

Of the Ice Giant planets, Neptune was at opposition late in September and Uranus reaches opposition on 19 November, thus both planets are easily observed throughout this period. Taking outermost Neptune first, this visually small planet shines dimly, at magnitude +7.7, in a region devoid of bright stars in western Pisces, above Cetus and a little east of Aquarius. It is most easily found by following the line of the ecliptic back east from Saturn. Throughout this period Neptune transits around an hour after Saturn so once you have finished observing Saturn, swing your telescope back east from the ringed planet to find a dim blue-green star, then add power to show Neptune's 2.4” planetary disk. In early October Neptune sits 14 degrees east of Saturn and by late November this has stretched by less than a degree. It will take a large aperture to see any subtle cloud features but these are more visible to imagers using near infrared filters.

Uranus is brighter, on the edge of naked-eye visibility, and a little bigger; it leads Jupiter by 24 degrees. It sits 5 degrees south and a little west of the Pleiades star cluster so is best found by scanning down from there. South transit is 03.00UT in early October, 01.00UT mid period and at midnight UT at opposition. At more than 55 degrees high for most of the UK, when transiting, this is a good opportunity to see it both clear of low-level turbulence and atmospheric dispersion. Its disk will be just 3.8” across but some well equipped amateurs are now trying to image its very faint ring system; this stretches to 14.5” and requires longer exposures and some means of occulting the planetary disk.