Looking Forward, Dec’17 and Jan’18

Apart from the outer ‘ice-giant’ planets of Uranus and Neptune, most observational opportunities take place in the pre-dawn sky during this period.

Mercury is in inferior conjunction, between the Earth and the Sun, on the 13th of December and reappears low in the south-east in the last week of the year; look for it at around 5 degrees of elevation at 0715 ut from around the 23rd of December. As we approach sunrise on the 1st of January we will see Mercury rising on its greatest western elongation from the Sun, some 23 degrees on that date. Shining at magnitude +1.48 it can be followed until perhaps 0800 UT before being lost in the brightening sky. Although very low, it can be observed for the 1st week of January and you may just catch it rising very close to Saturn on the 13th from around 0730 UT.

By early December Venus is very close to the Sun for observation in morning twilight and dangerously close for daylight observation and is therefore effectively lost until it moves into the evening skies of the New Year. Solar superior conjunction (behind the Sun) takes place on the 9th of January and Venus will become visible as an evening object from early February onwards.

Mars may be observed in the early morning, rising a little before 0400 ut in early December and shining at magnitude +1.7. Its apparent size is still very small at 4.3 arcseconds but this improves to over 5 arcseconds in the period and brightness increases to magnitude +1.2.  By late December Mars rises 20 minutes earlier and passes 20 degrees of elevation around 0645 UT in nearly full darkness. Throughout the second half of December Mars is close to Jupiter and will move on to have a spectacular close conjunction with it on the 6th to the 8th of January next year; on the 7thJupiter and Mars will be one-fifth of a degree apart. The pairing will be visible with a moderate-power eyepiece even in relatively large amateur telescopes. Mars shines at magnitude +1.4 on that date and grows above 5 arcseconds in size shortly afterwards so detail should start to become visible on its surface and it will therefore repay prolonged observation.  In the period the Areocentric longitude of the Sun from Mars increases from 92 degrees to 122 degrees. This means the northern hemisphere of the planet is passing from mid-summer towards autumn and the southern hemisphere from mid-winter towards spring.

The south polar ice-cap of Mars will be large but is rather tilted away from us, both by the season on Mars and by the respective orbital positions of the two planets. However the deep depression on Mars known as the Hellas Basin will, initially at least, be a bright feature, easily confused with the actual ice-cap of the South Pole. The floor of the basin is covered in a frost of frozen atmospheric carbon-dioxide and water-ice however this will already be starting to thaw and tends to disappear quite rapidly as Hellas slowly receives more sunlight.

Conversely the northern ice-cap should start the period tilted towards us and therefore be visible. It will be rather small in late summer with much of its ice already sublimated back into the thin atmosphere but there should be a dark ring visible north of the region known as Baltia and surrounding the pole itself. Known as Lowell bands, these rings are thought to be disturbed surface material and dust deposited by the force of the evaporating surface and sub-surface ice as the north polar ice melted in spring and early summer. The extent of these bands and the visible shape of the polar cap should be noted for changes as the Martian Autumn approaches.

With all this extra gas, including water vapour, in the atmosphere increased cloud activity should follow. Overnight on Mars low-pressure regions can spawn large spiral cloud features very like hurricanes and typhoons as seen from space over the oceans of earth. On Mars these features can become visible as they rotate from night into day and should be watched for close to the morning terminator of the northern hemisphere particularly as the Mare Acidalium or the Utopia regions move into daylight; such clouds may become visible towards the end of January.

As mentioned, Jupiter and Mars are rising close together, by late December Jupiter is rising around 0400 ut and reaching over 20 degrees of elevation, almost due south, by sunrise. Its apparent size will be over 32 arcseconds and much detail will be on view in steady air. By mid-January Jupiter rises at 0315 ut and shines at magnitude -1.8. It is bright enough to be followed into daylight if suitable safety precautions are taken to ensure the Sun comes nowhere near the telescope or viewfinder fields of view.  On the 12th of January at 0635 ut the shadows of both Europa and Ganymede can be found straddling the central meridian at high northern latitudes with the planet around 20 degrees up in the pre-dawn sky

Saturn is too low to observe effectively in the period, being in conjunction( behind the Sun) on the 21st of December, however it then moves into the morning sky and, as mentioned, rises close to Mercury in early January; its visibility improves significantly in the next reporting period.

The outer, ice-giant planets remain on display as the main evening objects of this period ‘though Neptune is only visible at increasingly low altitudes in the south-west and will require exceptionally steady skies if it is to be viewed photographically; it sets 2320 ut in early December.  At magnitude +7.8 it will take a telescope to find it against the background stars of Aquarius, close to the star Hydor (λ Aquarius), and its tiny blue-green disc will be only 2.3 arcseconds across however this will be sufficient for advanced imagers to search for bright clouds in its frigid atmosphere. By New Year’s Eve Neptune transits in bright twilight but can still be followed from 1830 UT at around 25 degrees up and setting at 2115 ut; by the end of the period it sets around 1925 ut.

Uranus is better placed. On the edge of naked eye visibility at magnitude +5.7 it transits more than 45 degrees up and is very well placed for observation. Transit time is around 2100 ut in early December and 1900 UT on New Year’s Eve and Uranus is therefore on show against the back ground stars of Aquarius for an extended period in the dark winter sky; even by late January Uranus doesn’t set until 0125 ut.

At magnitude 5.8, Uranus can easily be picked out from the background stars of Pisces with binoculars, looking like a distinctive green ’star’. A telescope reveals its small but obvious 3.5 arcsecond-diameter disc and, again, large aperture instruments can be used to see variations in shading from equator to pole and reveal cloud features photographically with appropriate filters. Both of the ‘Ice-Giants’ are best imaged with filters that pass light in the near infrared revealing important and scientifically useful atmospheric features.