Follow Comet Wirtanen


Comet Wirtanen / 46P on December 6, 2018

A well-known comet is making its closest approach to Earth in many years and promises a good show. 

Comet Wirtanen is now climbing up the late autumn and winter sky for northern hemisphere viewers, and is already a fine binocular comet. By mid-December it might be bright enough to be visible to the naked eye, but only from a dark rural site.

Discovered in 1948 by Carl Wirtanen at the Lick Observatory, his namesake comet orbits the Sun every 5.4 years. So unlike other recent bright comets that have visited us for the first time, Comet Wirtanen (aka 46P) is well known. It is one of many “Jupiter-family” comets whose orbits have been shaped by the gravity of Jupiter and orbit the Sun about every 6 years.

So since it was discovered, Comet 46P (the 46th comet in the catalog of periodic comets) has been well observed. It isn’t better known because at most returns it never gets bright, and that’s because it never gets closer to the Sun than a little more than the distance from the Earth to the Sun. (Its perihelion distance is 1.06 AU, with 1 AU, or Astronomical Unit, being the average distance from Earth to the Sun.)

However, despite this, we’re expecting – indeed already enjoying – a good show at this return.

Due to the quirk of orbital clockwork, on this return the comet reaches its closest point to the Sun just before it is also closest to Earth.

That puts the comet “just” 11,680,000 kilometres from us at its closest approach to Earth on December 16, four days after perihelion, the point when the comet is closest to the Sun.

Comet Wirtanen from Space
The relative position of the Sun, Earth and Comet Wirtanen on December 16, 2018.

Comet Wirtanen will be relatively bright simply by virtue of its proximity.

But it is also an active comet, emitting a lot of gas and dust into a large “coma,” and that’s what we see, not the 1-kilometre-wide icy nucleus itself which is too small and shrouded by the coma. (As a footnote, Comet Wirtanen was to have been the comet that the European Rosetta probe was to visit, but launch delays forced ESA to switch cometary targets.)

Comet Wirtanen is glowing at magnitude 5 to 6, technically making it visible to the naked eye. However, because it is large and diffuse, in practice you need binoculars to see it – now.

But as it approaches Earth and the Sun, Wirtanen will brighten, perhaps to magnitude 3 (the brightest stars are magnitude 0 to 1), making it easier to see with the unaided eye from a dark site.

The one catch is that as it heads toward its brightest in mid-December the waxing Moon also begins to enter the sky and wash out the comet with moonlight.


The first two weeks of December will be prime time for Wirtanen


Comet Wirtanen Path
The path of Comet Wirtanen across the sky in December 2018. The yellow dots mark the position of the comet at nightly intervals for late evening (10 p.m.) for North America. While comet will be in the sky most of the night, it will be highest in late evening about 10 p.m. local time when the sky will look as depicted, with the comet high in the south to southeast. Click or tap to download a full-sized version.

The first two weeks of December will be prime time for Wirtanen, with a particularly good opportunity coming on the evenings of December 15 and 16 when it shines below the Pleiades star cluster. The gibbous Moon will set about 1 to 2 a.m. with the comet still high enough for a dark sky view and photos.

Those will be great nights to shoot the comet and the cluster with a telephoto lens, provided the camera is on a tracker for untrailed exposures of 1 to 4 minutes. A 135mm to 300mm lens will frame the pair well.

Winter Green Comet with Orion
Comet Wirtanen as a green glow at upper right here in Eridanus. and well to the west of Orion, rising here at left, on the evening of December 6, 2018. I shot this with a wide-angle 35mm lens in a blend of tracked and untracked 1-minute exposures.

After that, through late December, the bright Moon will interfere with  the view. For example, a close approach of the comet near the star Capella on December 23 happens with the nearly Full Moon not far away.

Comet Wirtanen / 46P on December 6, 2018
Comet Wirtanen in a close-up through a telescope on December 6, 2018 in a stack of short and long exposures.

I took the above close-up photo of Comet Wirtanen on December 6. It is a long-exposure telescopic view, but the comet is easy to see with binoculars. It appears visually and photographically as a diffuse fuzzball, with the camera recording a vivid cyan colour from glowing cyanogen and diatomic carbon molecules. You won’t see that colour with your eyes, even in a telescope.

Comet Wirtanen Path Dec 8 to 16
The path of Comet Wirtanen Dec 8 to 16 superimposed on an actual sky image with the comet taken December 8. The circle indicates the field of view of typical binoculars. On Dec 15 and 16 the comet will be in the same binocular field as the Pleiades star cluster. The positions are for about 10 pm Mountain Standard Time for each of those dates.

Even at the comet’s best in mid-December any tail might be hard to see and even photograph (it appears faintly above) as it will be both faint and pointed directly away from us because, as comet tails do, it will also be pointed away from the Sun.

Look for a large glow which will be grey to the eye but green to the camera.

While you can just take pictures for yourself, astronomers are asking amateur astrophotographers to participate in a worldwide observing campaign to monitor Comet Wirtanen. More details are available here at wirtanen.astro.umd.edu and at http://aop.astro.umd.edu/

Clear skies and happy comet hunting!

— Alan, November 30, 2018 (Revised December 6) / © 2018 Alan Dyer / AmazingSky.com 

Heads Up! Sight the Thin Moon by Mercury & Venus


Jan 21 Planets

On Wednesday, January 21 look low in the southwest for a conjunction of the Moon and inner planets.

Mercury is ending its brief evening appearance and proximity to Venus. But this week you can still spot it a binocular field or so below Venus as it descends back toward the Sun.

On Wednesday, January 21, look low in the southwest to sight the thin waxing crescent Moon sitting near Venus and Mercury, forming a wide triangle of inner rocky worlds.

The other rocky planet in the inner solar system, Mars, shines higher up in the evening twilight as a moderate brightness reddish star. The next night, January 22, the waxing Moon will sit beside Mars in a wide conjunction.

Catch the Moon-Mercury-Venus trio early, as they will set an hour or so after local sunset.

Clear skies!

– Alan, January 19, 2015 / © 2015 Alan Dyer / www.amazingsky.com

Relaunching Stargazing in Barbados


Observatory Viewing in Barbados (Nov 16, 2013) #1

Barbados is soon to have a new state-of-the-art public observatory for promoting astronomy.

On Saturday night, November 16, I was fortunate and privileged to be the guest speaker at the first event at the newly refurbished Harry Bayley Observatory in Bridgetown, Barbados. A grant from an educational foundation in the UK has allowed the Barbados Astronomical Society to renew the aging 50-year-old facility with a fresh new interior, and all the high-tech fittings of a modern public observatory.

A new dome was lifted into place on top of the 3-storey structure earlier in the week, and the painting and interior finishing was completed just a day or two before my talk, in time for a public RSVP event Saturday night.

Observatory Interior Panorama #1

I gave a talk on The Amazing Sky, showing images and movies from the November 3 total eclipse, among many other photos of the sights anyone can see in the day and night sky. I gave the same talk twice, to two packed houses of 40 people per session in the main floor meeting room/lecture hall. A wonderful spread of local food and drink was served upstairs.

Lots of work remains to complete the refurbishment but the facility was in good enough shape to host a public event. The official opening is in January.

Observatory Viewing in Barbados (Nov 16, 2013) #3

A new Meade 16-inch telescope on a Software Bisque MX2 mount is on its way for installation later this year, equipped with the latest robotic control and digital cameras for public viewing. A hydrogen-alpha solar telescope will also be part of the arsenal of equipment.

This night, members set up a portable Celestron 8-inch telescope outside for viewing the Moon and Jupiter. In contrast to viewing at home at this time of year, observing from 13° North latitude was in shorts and shirt-sleeves.

It was a terrific evening and I’m pleased to have been part of the relaunching of the Observatory and astronomy activities on the island. Many thanks go to my host on the island, Greg Merrick, for making the evening – and my stay this week – possible.

– Alan, November 17, 2013 / © 2013 Alan Dyer