Planetary Notes
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Society for Popular Astronomy
Looking Forward; October & November. 2021.
Mercury is in Inferior Conjunction, passing between the Earth and the Sun, on the 9th of October, thereafter moving into the morning sky and giving an excellent apparition for observers in the northern hemisphere. Greatest western (morning) elongation from the Sun is on 25 October with Mercury then separated from the Sun by 18 degrees; look for it in the pre-dawn sky for a few days either side of this date rising in the East from around 0505UT. Mercury will reach nearly 15 degrees elevation above the horizon by sunrise and on the 25th Mercury will shine at magnitude -0.6 and show a phase just over 50% illuminated. After this Mercury will descend rapidly towards the Sun with each new dawn and will become unobservable by mid-November. On the 11th it will pass one degree from the planet Mars but the pair will be very low and just 11 degrees from the Sun so great care must be taken if you wish to observe this appulse
Venus is visible in the south-west from shortly after sunset at the start of this period, but at no great elevation for UK observers and is much better seen from further south. Greatest eastern (evening) elongation is on 29 October with Venus stretching 47 degrees from the Sun but even on that date the planet will first appear at below 10 degrees of elevation, its brilliant magnitude -4.7, 50% illuminated disk fighting the bright skies just after sunset. As we move into November the steady rise of the evening ecliptic in autumn means Venus first appears slightly higher each evening despite it sinking slowly towards the Sun. By 20November it should be seen by UK observers 10 degrees above the south-south-eastern horizon from 1615UT and even slightly higher by the end of the month.
Mars is in solar conjunction, hidden behind the Sun, on 08 October so is not observable until late in the period. You may just glimpse it in the dawn sky from mid-November, low in the south-east and rising around 0630UT.

Jupiter is an excellent and very obvious target for observation in the evening sky throughout this period. It passes due-south (transit) around 2105UT in early October at around 22 degrees of elevation, shining brightly (magnitude -2.7) against the faint stars in the eastern edge of Capricorn. Telescopically it will show a large disc, 46 arcseconds across, and cloud detail should be plentiful. 

Shadow transit events, when the shadow of one of Jupiter’s large orbiting moons is cast on the cloud tops of the planet, are relatively common. Double transit events involving the shadows of two moons visible at the same time, are rarer. On 04 October the shadows of both Ganymede and Callisto will be visible from around 1850 UT when Callisto’s shadow will sit on Jupiter’s central meridian (CM), just below the South Equatorial Belt (SEB) and Ganymede’s shadow will be entering the limb of the planet on the northern edge of that belt. At that time Jupiter will be around 18 degrees above the south-eastern horizon in twilight for the UK, so should be readily observable.

The progress of the double shadow transit can then be followed until 2120UT, or very shortly thereafter, when Callisto’s shadow leaves the disc; at the same time the Great Red Spot (GRS) will just be appearing on the opposite limb of Jupiter. Ganymede's shadow will be visible until 2225UT by which time the GRS will be half way to the CM; Jupiter itself sitting 19 degrees above the horizon in the south-south-west.
By early November Jupiter is due-south around 1900UT, still in full darkness and doesn’t set until 2345UT from the UK; it will have fallen only slightly in size and brightness but by late November it is down to magnitude -2.3 and 38 arcseconds in size. South transit will be in early twilight, around 1720UT, and the planet can still be observed for several hours, setting after 2200UT.

The story is similar for Saturn but the ringed planet leads Jupiter across the sky and both transits and sets around an hour earlier than its companion so, of the pair, it is perhaps best to observe Saturn first. It can be found at the head of Capricorn and at its highest will be some 18 degrees above the southern horizon for mid-UK observers. On the 11 October the ring system will be tilted towards the Earth by 19.3 degrees, the widest since early February, and it will close slowly from then on. At just under 40 arcseconds across, the rings will be beautifully presented and every opportunity should be taken to see Saturn.

Of the ice-giants, Uranus is at opposition at midnight Universal Time on 04 November, going into Bonfire Night, 05 November, which means it is a good target throughout this period. South-transit occurs at 0210UT in early October, midnight UT around opposition night and at 2210UT in late November. In each case the planet will be more than 50 degrees above the southern horizon when transiting. At magnitude +5.7 you might, just, find it with the naked eye if your skies are very dark but it is an easy binocular object and shows a pale green disk 3.76 arcseconds across in a telescope.  The nearest reasonably bright star is Hamal (+2.0) in Aries and Uranus is 11 degrees south-east of this star, midway between Hamal and Menkar (+2.5) in the head of Cetus the Whale. Scan this area for a green ‘star’ and you will have found Uranus. Imagers with large aperture telescopes should try filming in the near infrared to reveal any storm features in the planet’s atmosphere.

Neptune is also observable but is not as well placed as Uranus as it only gains a little more than 30 degrees of elevation when at its highest point in the sky. This potentially means we view it through a more turbulent atmosphere so look for it when seeing conditions are very good rather than average. Neptune transits, due-south, at 2255Ut in early October, 2050UT mid-period and at 1855UT in late November. It lies in a relatively empty region just east of Aquarius and below the head of Cetus. At magnitude +7.8 it is within the grasp of good binoculars but will need a telescope to show its tiny 2.3 arcsecond blue-green disc.