Dundee Astro - Planetary Notes

Mercury starts October as a low altitude object in the east, rising before the Sun but, for mid-UK observers, gaining no more than around 15 degrees of elevation before the Sun breaks the horizon. At magnitude -1.08 it will be obvious in morning twilight and shows a waxing gibbous phase, 82% illuminated at the start of the month, but at a small angular size, just under 6 arcseconds (6”) from pole to pole. Thereafter Mercury declines in elevation with each sunrise and moves behind the Sun, superior conjunction, on 20 October.

Re-emerging into the evening sky, Mercury starts an excellent apparition for equatorial and southern-hemisphere observers, but is disappointingly low as seen from the UK. On 27-30 October Mercury and Mars will be very close but the pairing will be too low and close to the Sun for observation from the UK. By the end of November Mercury may be visible at perhaps 5 degrees of elevation in the south-west shortly after sunset but even at magnitude -0.4 it will be hard to pick out.

Venus is very obvious in the morning sky in early October, rising around 02.08UT and reaching 35 degrees of elevation in the east-south-east by sunrise, shining at a brilliant magnitude -4.6. The planet is moving towards its Greatest Western Elongation (GWE) from the Sun, which occurs on 23 October, and on that morning Venus should appear 50% illuminated, but the well known Schröter Effect will mean that most visual observers will estimate a 50% phase a few days later than this.

Thereafter Venus starts its slide back down towards the sun and its elevation slowly declines with each new sunrise. Venus, in its own orbit, passes above the plane of the ecliptic on 25 October which does help slow the decline, thus Venus is still well placed for observation throughout November. Late in that month it will reach some 27 degrees at sunrise for much of the UK, will shine at a still brilliant magnitude -4.2 and be 67% illuminated; an excellent period for imaging faint cloud features in the near ultraviolet or directly observing them with a deep-blue filter.

Mars is effectively lost to UK observers throughout this period. Initially an evening object it moves behind the Sun on 18 November and will not be obvious again in the morning sky until well into the New Year.

On the other hand, Jupiter is an outstanding object throughout this period. It is moving towards its opposition, on 03 November, so is observable for most of the night. South transit times for the UK are 02.19UT in early October, midnight UT at opposition and 21.50UT by late November. At opposition Jupiter will show a face an impressive 49.5” in apparent size and wont be smaller than 47.8” at any time in this period; on any steady and clear night considerable detail will be on view. Jupiter sits at the feet of Aries the Ram and at south-transit will at or above 50 degrees of elevation for most of England; slightly lower for Scotland. With Jupiter at its absolute best in this period, please do not neglect the chance to observe its dynamic face.

Saturn starts October a month past opposition so is becoming visible at a more convenient time in the evening sky; observe it before moving on to Jupiter. Best seen when due-south, the south-transit times will be at 21.43UT in early October, 19.37UT mid-period and 17.45UT at the end of November. In all cases Saturn will sit around 25 degrees up when due south for much of the UK, which is noticeably higher than during the last apparition, and it should be beautiful on a steady and clear autumn evening. The rings will be tilted towards us by 10.5 degrees; they will eventually close to appear edge-on but not until 2025. In the period Saturn is found at the belly of Aquarius, the water bearer, and will shine initially at magnitude +0.6, fading to +0.9 by late November; Likewise the visible apparent size across the span of the rings falls from 43.3” to a still impressive 39.4”. If available, an Atmospheric Dispersion Corrector will help with its lowish altitude.

Uranus is as well placed as Jupiter in this period and rises only slightly after that giant planet. It reaches its own opposition on 13 November, only 10 days after Jupiter, which acts as a marker to help find the much smaller and fainter ‘Ice-Giant’ in Aries. In early October Uranus trails Jupiter by 8.5 degrees back along the line of the ecliptic and transits around 02.45UT. Its brightness of magnitude +5.7 makes it faint, on the edge of naked-eye visibility, and it sits just below the midway point of a line drawn between Jupiter and the Pleiades cluster. Look for its tiny green planetary disc and add power to observe its 3.7” visible face. South transit is 00.50UT mid-period and 22.46UT by late November. By then, the gap between Uranus and Jupiter will have grown to nearly 13 degrees.

Neptune starts October just past opposition, so it is visible for much of the night. Its own south transit will be at 23.15UT early on, 21.08UT mid-period and 19.15UT late in November. At magnitude +7.7 and 2.3” in size, Neptune will be hard to find sitting in an area of faint stars below the circlet of five stars making up the most western ‘fish head’ of the constellation Pisces. Scan the area for a defocussed blue-green ‘star’ and add power to see its planetary disk. Sitting around 35 degrees high when due-south, this is a good opportunity to see the outermost ‘major’ planet in the solar system.