See bright Comet 21P’s close brush with conspicuous Capella

By Ade Ashford

Comet 21P/Giacobini-Zinner lay among the stars of western Cassiopeia when Russian astrophotographer Alexander Vasenin captured this image close to 21 UT on 17 August 2018 using a Celestron AVX 8-inch SCT, focal reducer and Canon 600D DSLR. Image credit: Alexander Vasenin, CC BY-SA 4.0
By their very nature comets can be faint and unpredictable bodies, plus they often take some effort to track down. However, this is not the case with Comet 21P/Giacobini-Zinner, as it’s steadily brightening, approaching magnitude +7 (hence a viable binocular target under dark, moonless skies) and following a path that takes it tantalisingly close to Capella in the constellation of Auriga – the 6th brightest star in the night sky – in early September.

Comet 21P was discovered by French astronomer Michel Giacobini (1873–1938) in the constellation of Aquarius at the Nice Observatory on 20 December 1900. Two orbits later, the comet was recovered by the German astronomer and historian Ernst Zinner (1886–1970) on 23 October 1913, hence the joint name.

The ‘P’ in the body’s name immediately informs us that 21P/Giacobini-Zinner is a periodic comet, one which we now know orbits the Sun once every 6.62 years in a path that carries it to within 1.038 astronomical units (AU) of our parent star at perihelion and out beyond the orbit of Jupiter to 6.014 AU at aphelion.

Comet 21P’s orbit is inclined by almost 32 degrees to that of the Earth, so the closest that the pair can get is 0.035 AU, or 5.2 million kilometres. The comet next passes through perihelion on 10 September this year – coincidentally the same date that we are closest to it, some 0.392 AU or 58.6 million kilometres away at 14:14 UT.

This extract of a high-resolution PDF finder chart suitable for printing that you can obtain by clicking on this graphic shows the track of Comet 21P/Giacobini-Zinner up until its perihelion passage on 10 September. The comet passes less than a degree from prominent star Capella on the UK night of 2–3 September 2018. AN graphic by Ade Ashford.
Tracking down Comet 21P
As mentioned above, the comet’s track carries it close bright star Capella in Auriga, passing just 0.9 degree (55 arcminutes, to be precise) southwest of the star around 20:30 UT (9:30pm BST) on Sunday, 2 September 2018. For observers in the British Isles, Capella is the unmistakable magnitude +0.08 star currently found about a span of an outstretched hand at arm’s length above the northwest horizon at midnight.

If Comet 21P brightens according to predictions then it could be magnitude +7 and easily a binocular object in the same field of view as Capella. In fact, the comet passes so close to the star on the UK night of 2–3 September that backyard telescopes magnifying less than about 50× will squeeze both the star and comet in the same field of view.

You can obtain a high-resolution PDF chart suitable for printing and use outside by clicking on the accompanying star chart graphic, or by clicking here.

When to view Comet 21P
21P/Giacobini-Zinner is presently circumpolar for observers in the UK, meaning that it lies above the horizon day and night, never actually setting. To see the comet highest in the sky, your best window of opportunity is between local midnight and the first glimmer of morning astronomical twilight (tip: you can obtain all of this information worldwide from our interactive online Almanac).

With a waning gibbous Moon currently in the late evening sky of Western Europe, midnight observations of the comet devoid of moonlight start around 2 September. Of course, you can look for the comet at any time after dusk, but it will be low in the north-northeast sky of early evening.

Note that Comet 21P continues to be visible post-perihelion. For another binocular and astrophotographic treat, it’s visible about a degree either side of magnitude +5.6 open cluster Messier 37 at the first glimmer of nautical twilight in the UK on the mornings of 10 and 11 September. And around 5am BST on Saturday, 15 September you can see and photograph the comet nestled close to the northern edge of magnitude +5 open cluster Messier 35 in Gemini.

For the best views of Comet 21P/Giacobini-Zinner, try to seek an observing location offering an unobscured northeast horizon as free of light pollution as you can. If using binoculars or a telescope, allow at least 20 minutes for your eyes to become fully dark-adapted too.

Ephemeris for Comet 21P/Giacobini-Zinner

This topocentric ephemeris of Comet 21P/Giacobini-Zinner is computed for the centre of the British Isles (54°N, 2.5°W) at 0h UT (1am BST) on the dates shown. The comet’s right ascension (R.A.) and declination (Dec.) are for the J2000.0 epoch; Delta is the distance from Earth in astronomical units; Mag. is the predicted visual magnitude; °/day is the comet’s daily motion in degrees. AN computation and graphic by Ade Ashford/Minor Planet Ephemeris Service.