2020 December 3

Observer’s Challenge – Jupiter/Saturn Conjunction

Mike Foulkes

This month’s observers’ challenge is to observe the close conjunction between Jupiter and Saturn.

Jupiter and Saturn have appeared close to each other in the sky for much of this year but over the last couple of months, their angular separation has been decreasing. This separation will reach a minimum on December 21 at 17:00UT, when the two planets will be in a close conjunction. This will be of interest for two reasons.

Firstly the minimum separation will only be about 6 arc minutes, which is approximately a fifth of the angular diameter of the Moon.

Secondly, this will be the closest conjunction of these planets since 1623.

Such conjunctions occur at a frequency of approximately 19.8 years which is the synodic period between two planets. The last such conjunction was in 2000 and the next will be in 2040.

This conjunction will provide an interesting sight for observing with the naked eye, binoculars or telescopes or taking images with camera lenses or a telescope. At the time of the conjunction, Jupiter will be to the south of Saturn.

The position of these planets in the sky provides the challenge in observing this event. As both planets will be in conjunction with the Sun in 2021 January, they will only be visible for a short period after sunset at a low altitude.

The positions of these two planets at the time of their mutual conjunction for any geographical location can be found by the use of software such as Stellarium. Figure 1, shows the sky at the time of the conjunction from a representative location in the UK, the city of Birmingham. At 17:00 UT, the two planets will be in the South West at an altitude of just under 9° above the horizon. Further north, from the city of Edinburgh, they will only be just over 6°above the horizon. Consequently a good unobstructed horizon looking to the south west will be required to see this event.

Sunset occurs approximately an hour before the time of the conjunction. It may be possible to detect both planets at a higher altitude in the twilight using binoculars for example. However for safety reasons, this should be done after the sun has set.

Figure 2 shows the simulated view of the conjunction centred on Saturn as seen through a representative telescope (a 127mm aperture F10 telescope with a magnification of x125.)

The low altitude may result in poor seeing which may make the detection of detail on the disks of either planet difficult. Further, the increased thickness of the atmosphere at low altitude will reduce the brightness of these objects which also make even the brighter satellites of Saturn more difficult to observe.

It will be possible to image the event with a variety of camera focal lengths or through a telescope. The correct exposure for any lens or telescope/lens focal length combination can be found by practising imaging both planets during the couple of weeks leading up to the conjunction.

If you are unable to observe on the night of the conjunction, it will still be possible to observe both planets in the same telescopic field of view for a few days either side of the conjunction as illustrated in Figure 3.

Alternatively, some observatories will be providing live streaming of the event, if not clouded out, or a recent recording of a telescope view.

The Physics and Astronomy Department at the University of Exeter and the Exeter Science Centre are planning to live-stream a view from a telescope one evening in the period from the 16th and the 24th of December, dependent on the weather. You can sign-up to receive email notification of which night they will try to live-stream on at the website: http://jupitersaturn2020.org/ They also provide videos on the website that give more information about the Great Conjunction, how to observe it, and about Jupiter and Saturn themselves, including one video aimed at primary school children.

The University of Keele is also producing a 2-hour broadcast from 4pm on December 21st that will be broadcast on AstroRadio and YouTube. More details can be found at: https://www.keele.ac.uk/scps/newsandevents/news/2020/november/thegreatconjunction/the-great-conjunction-of-2020.php

Please post your observations on the BAA members’ pages.

Mike Foulkes