Comet 2020 F3 (NEOWISE) is proving to be a smash hit, provoking an incredible level of interest as it captivates astronomers across the Northern Hemisphere, who have been crying out for a comet that’s visible to the naked-eye and also looks like a ‘proper’ comet. NEOWISE shows a stellar-like nucleus with a marvellous fan-shaped tail that most binocular viewers can trace to at least four to five degrees in length. A plethora of fantastic images are revealing a tail that extends to some 15 degrees and splits into a stunning combination of a primary dust tail contrasting nicely with a blue gas (ion) tail.
The astronomical gods have smiled down upon us for sure this time, as we probably have to go back as far as iconic Hale-Bopp (1995 O1), which wowed with its brilliance in 1997, to recall a comet with such a visual presence from UK shores (2006 P1 (McNaught), ‘the great comet of 2007’, was primarily a southern hemisphere object).
Comet NEOWISE emerged in fine fettle from perihelion (closest point to the Sun) in the northern sky in early July, peaking in brightness at around magnitude +0.5 to +1 soon after it was recovered in the morning sky. The comet has been putting on a marvellous showing while lying low in the pre-dawn sky, but it has now moved into to the evening sky and will steadily gain in altitude each night right until the end of July.
Find NEOWISE in Ursa Major
As night falls on 17/8 July, the comet can be found above the north-north-western horizon, lying in the far south-western corner of Ursa Major, close to its boundary with Lynx. Over the course of the next 10 nights or so the comet tracks rapidly eastwards across southern Ursa Major. By 25 July, it directly below the Great Bear’s iconic Plough asterism of seven bright stars.
We can use the four bright stars that form the ‘bowl’, or ‘body’ of the Plough as a useful celestial signpost to the comet. On 17/18 July, locate Megrez (delta UMa, magnitude +3.3) and Merak (beta UMa, magnitude +2.3), the stars marking the north-east and south-west corners of the bowl, respectively. If you draw an imaginary line from Megrez to Merak (incidentally, a distance of 10 degrees) and extend it twice as far again from Merak in the same direction, you should notice two third-magnitude stars lying close together (just over a degree apart). The brighter and more westerly lying of the two is Talitha (iota UMa, magnitude +3.1); comet NEOWISE lies about 3.5 degrees to its south-west. On the following night, 18/19 July, the comet skirts under a quarter of a degree north of Talitha.
By the end of nautical twilight (when the Sun lies between six and twelve degrees below the horizon), occurring at about 11pm BST from London (from Edinburgh, at about 12.20am BST), NEOWISE lies around 17 degrees high in altitude (hold out your fist at arms length and about 10 degrees is covered across your knuckles). Although it’s now improving in altitude, you’ll still need to find an observing site with an unobstructed horizon from the north-west around to the north.
The comet is circumpolar across the UK until 25/26 July. This means it doesn’t set, so you can choose to follow it all night as it descends to around 10 degrees from London when it scraps the horizon due north at about 2am BST (it remains a few degrees higher from Edinburgh).
What to expect
In the gathering darkness at a dark, countryside site, the comet should be readily visible to the naked eye if the sky shows good transparency, especially as the Moon is out of the way as just a very thin crescent that rises at about 2am (it’s new on 20 July). If you live in a suburban location then NEOWISE should still be seen, though try to shield your eyes from any nearby sources of light pollution and become well dark-adapted.
The comet has faded to shine around magnitude +2, but it’s bright head should be easy to spot along with its main dust tail. Sweep for it in a pair of binoculars if you’re unsure you’ve found it and once located try for it again without optical aid. I found the neat trick of averted vision (looking off to one side of your target) came in very handy when I spotted the comet in nautical twilight on the evening of 12 July.
Observers are reporting the comet’s coma currently spans between three to five arcminutes, with a concentrated, almost stellar-like central condensation. The dust tail is estimated to extend to at least five degrees in length and perhaps as long as ten degrees.
Images have be flooding into Astronomy Now headquarters, taken from around the world. Many are simple but spectacular shots taken on a fixed tripod from picturesque settings. The comet’s appearance in the northern sky has fortuitously coincided with splendid displays of noctilucent clouds and/or the Northern Lights (aurora borealis), which feature in many images.
Through a telescope the comet yields visible structure, especially in the dust tail, with deep telescopic images showing incredible detail in both tails. Look out for a special Comet NEOWISE image gallery in the September issue of Astronomy Now, which is scheduled to go on sale on 20 August.
NEOWISE’s fortunes for rest of July
Presently, Comet NEOWISE isn’t springing any surprises in brightness; magnitude estimates are hugging the light-curve of its projected downward path as it fades. Over the course of the next 11 nights, the comet zooms across southern Ursa Major, climbing in altitude every night. This means the comet can be seen in a darker sky, which will to some degree counter its fading brightness. The comet enters the north-western corner of Coma Berenices on the night of 29/30 July; by this time it will lie around 26 degrees above the west-north-western horizon at the end of nautical twilight, which occurs at around 10.30pm BST from London (11.30pm BST in Edinburgh).
Comet NEOWISE is travelling across the sky at around nine arcminutes per hour with respect to the background stars (use the finder chart here to keep tabs the comet). This rapid motion is owing to the fact that the comet is heading towards closest approach to Earth. This occurs on 23 July, when it passes us at a distance of 0.69 astronomical units or 103 million kilometres.
By the end of July, Comet NEOWISE will probably have faded in brightness to below naked eye detection, but it will surely remain brilliant to observe through binoculars or a telescope.
How will we remember NEOWISE?
In an earlier piece for this website, written soon after the comet emerged after perihelion, I remarked it could not yet be classed as a great comet alongside the like of Hale-Bopp or 2006 P1 (McNaught). I suppose it depends on your definition of a ‘great comet’, but in the light of the happenings of the last week or so, and what’s hopefully yet to come, astronomers may well be minded to include 2020 F3 (NEOWISE) in the pantheon of great comets. However it’s eventually ranked, one can be sure that this is a comet to be remembered.