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Mon, 28 Nov 2016 - Looking Forward; December '16 and January '17

Mercury reaches its greatest eastern (evening) elongation from the Sun on December the 11th when it lags the Sun by 21 degrees. This ought to be good news but the ecliptic is very low in the sky at this time of year so, sadly, Mercury is practically impossible to see from UK latitudes. For the first half of the month it is better placed for observers from the southern-hemisphere but Mercury passes into inferior conjunction, between the Earth and the Sun, on December 28th so viewing opportunities are limited. By early January Mercury has passed into the morning sky, stretching daily further to the west of the Sun until it reaches its greatest western (morning) elongation of 24 degrees on the 19th of the month. On that day Mercury will rise at around 0640UT for mid UK latitudes, shining at magnitude -0.2 and almost due south-west, lagging the brighter and more obvious Saturn which will be 10 degrees further south and at 6 degrees of elevation at that time. Mercury can be followed as the sky brightens but will itself be barely 6 degrees up before it is too faint to see. From the UK Mercury may best be observed from around the 10th of January up to a few days after elongation but this will still be at low elevation.

 

In contrast Venus is a magnificent object throughout the period; obvious in the early evening twilight to the south-south-west at around 10 degrees of elevation shortly after sunset in early December. At that time Venus shines at a brilliant magnitude of -4.2 showing a 67% illuminated phase and appearing some 17 arc-seconds from pole to pole. Initially Venus will appear low to the horizon but, through the period, it will rise steadily in declination, moving against the background stars of Capricorn and on into Aquarius. By New Year’s Eve Venus will be visible from around 1700UT on a compass bearing of 205 degrees at some 20 degrees of elevation or more, setting around 4 hours after sunset. By then it will have brightened only slightly to magnitude -4.3 and will show a 56% illuminated phase some 22 arc-seconds from pole to pole.

 

This is an excellent time to start observing Venus for subtle cloud patterns. It is well known that cloud features can be photographed in the near ultra-violet (UV) and deep-blue wavelengths but that not all the population are sensitive to these wavelengths causing some controversy as to whether the clouds can or cannot be seen; try using dark blue or purple filters, such as the Wratten 38A or 47 and let me know the results. One shot colour cameras do not tend to record cloud details but will show variations in brightness across the visible disc, particularly near the ‘cusps’, where the area of visible phase extends towards the poles. Monochrome cameras can be used with a UV-pass filter to capture dark patterns where light at these wavelengths is absorbed by material in the planet’s atmosphere. As yet scientists are unsure as to exactly the mechanism by which this light is being absorbed. It may also be worth imaging Venus in near infra-red wavelengths as very subtle detail may also appear on what will otherwise be a bland disc. IR clouds on Venus are an area of ongoing study and I would welcome clear images of this phenomenon.

 

On the 12th of January Venus reaches greatest eastern (evening) elongation from the Sun with a separation of 47 degrees; this means that it will not set for more than four and a half hours after sunset and may be found at around 25 degrees of elevation in the south-western sky around 1715UT. In theory Venus should show a 50% phase on this day but light scattering from the bright side into the dark side of the thick atmosphere of the planet along with optical illusions associated with contrast effects along the terminator means that the point of 50% phase is hard to determine. Most observers see it two or three days early for evening elongations and a similar period late for morning ones. This is known as the Schröter effect, first observed by Johann Schröter in 1793. By the end of January the phase of Venus will have shrunk to 40% illuminated and its apparent size grown to over 30 arc-seconds from pole to pole. On the 31st it can be found forming a tight triangle with the Moon and Mars, all three easily visible from around 1800UT onwards.

 

Mars itself is moving prograde (easterly) against the background stars from Capricorn, then into Aquarius and up to Pisces as the period progresses. In early December it can be found low in the south shortly after sunset shining at around magnitude +0.6 and setting around 2100UT. Its period of visibility actually improves through December and into the New Year as it is rising with the ecliptic so can be seen slightly higher each night. To balance this it is moving further away from us so its brightness falls along with its apparent size as night follows night. On New Year’s Day it can be seen from around 1700UT some 27 degrees up on a bearing of 195 degrees, shining at magnitude +0.9 showing a 90% illuminated phase some 6 arc-seconds from pole to pole. By mid-month it appears nearly 30 degrees up from around 1730UT and, as already mentioned, it is close by Venus on the 31st, on a bearing of 225 degrees, still 30 degrees high from around 1800UT. On that day its brightness will have slipped to magnitude +1.1 and its size to a little over 5 arc-seconds. Due to its size detailed observation will be difficult but major surface features should still be on show and I would welcome any reports or images.

 

Jupiter dominates the morning sky; rising just after 0300ut in early December this giant planet is more than 30 degrees high, in the south, by sunrise. By Christmas it rises around 0145UT and by the 23rd of January it rises around midnight UT. It brightens from magnitude -1.8 to -2.1 through the period and grows in apparent size from 34 arc-seconds in December to 39 in late January. Jupiter rotates on its own access very rapidly causing a succession of features to come into view as an observing session progresses and its four most obvious moons, Io, Europa, Ganymede and Callisto follow a readily predictable dance around it, sometimes passing in front of Jupiter’s face and casting shadows on the top of its clouds. The Juno probe will be in orbit around Jupiter throughout this period (it is not planned to end its mission until February 2019) and scientists involved with the project have asked for good quality current amateur images of Jupiter to be sent to them to help them plan high-resolution imaging runs by the probe. Details of how to do this can be found at:-

 

http://www.popastro.com/planetary/reference/downloads/index.php

 

Saturn is in conjunction with the Sun on the 10th of December and therefore not visible around that time. By January the first it is just 20 degrees west of the Sun in the early morning sky and hard to follow in morning twilight; its visibility will improve as the New Year progresses; look for it close to Mercury very low to the south-east in the third week of January.

 

Uranus will be well placed for evening observation for those with settled seeing conditions and a powerful telescope. It transits due south at around 45 degrees of elevation, for mid-UK latitudes, at 2040UT on the first of December, four degrees east of the star ɛ Pisces, and can be followed until at least 0200UT the next day. On December 31st Uranus transits around 1830UT and from then on can be found declining slowly to the west as full darkness falls. Even by the end of January it can be found more than 30 degrees up in the south-west from 1900UT. Its tiny blue-green disc will be on the edge of naked eye visibility at a magnitude of +5.8 but it can be readily found with binoculars or a small telescope. To satisfactorily see this disc as much more than what appears to be a de-focused star you will need high magnification, around 200 times or more, preferably with a larger aperture telescope. 

 

The same is true of Neptune, which may be observed, especially early in the period, during the first part of the evening, sitting in the heart of Aquarius. Around 25 degrees up in the south-west as full darkness falls Neptune holds its position against the background stars and slowly subsides to the west as night follows night. On the 6th of December it is one-and-a-half degrees north-east of the Moon at 2000UT and on New Years Eve it is a tiny 22 arc-minutes above and to the east of Mars, a conjunction observable from 1800UT on until the pair set around 2125UT; given the close proximity, this is an interesting observing and imaging challenge. On the 12th of January Neptune can be found almost as close to Venus, just 24 arc-minutes to the east and a little south; given the massive contrast in brightnesses, magnitude -4.4 for Venus and +7.9 for Neptune you will do well to see both of these planets in the same eyepiece.

 

All times, position angles and elevations in this article are predicted for an observer in the heart of the United Kingdom, around the latitude of Birmingham.

 

Added by: Alan Clitherow