‘Christmas comet’ 46P/Wirtanen and the Geminids meteor shower herald a cracker week in the sky – Science News
Grab your cameras, get away from city lights and you’ll have front-row seat for a beautiful sky show this week.
The so-called “Christmas comet” — officially known as 46P/Wirtanen — is the brightest comet of the year. It will make its closest approach to Earth in the late evening of December 16 (or early morning of December 17 if you are in the AEDT time zone).
The comet is already visible if you have binoculars, and will be at its brightest between December 14 and December 18, according to amateur astronomer Ian Musgrave.
A day or so earlier, if you are keen to get up in the wee hours, you’ll also catch the Geminid meteor shower, which peaks on the morning of December 15.
Flying inwards from a point near Jupiter, 46P/Wirtanen swings by the Sun every five years or so, but it’s usually too far away for us to see.
This year it will zoom past about 11.5 million kilometres away from us — or 30 times the distance to the moon. It won’t come this close again for another 20 years.
The comet, including its diffuse halo of light, is about the size of the full moon. At its brightest it will be about the equivalent of the two dimmest stars we can see in the Southern Cross.
But unlike the stars in the Southern Cross, it “looks like a fuzzy patch” so it’s unlikely you’ll be able to see it with your naked eye, Dr Musgrave said.
The good news is, though, you should be able to see it through binoculars or using a standard DSLR camera if you are away from bright city lights.
“People are getting good views in binoculars and it should be potentially visible all the way out until the beginning of January,” Dr Musgrave said.
How to spot the Christmas comet
You can spot the comet from anywhere in Australia in the north-eastern sky from about an hour and a half after sunset until early morning when it sinks below the horizon.
“[Later in the week] it is better starting a little bit after midnight; by then the waxing moon is setting and is out of the way,” Dr Musgrave explained.
The tricky thing with comets is that they move each night — by about a handspan — so you’ll need to use the stars as landmarks.
Early in the week, draw an imaginary line between Sirius the brightest star in the sky and Rigel, the bright blue star in the constellation of Orion above the eastern horizon, and extend that out to the next brightest star — then back off by about a handspan.
By mid-week the comet will be near the constellation of Taurus. Find the V-shape of the bull’s head and look upwards towards two bright stars at the top of the constellation.
And by December 16 the comet will be sitting between the red star Aldebaran and the Pleiades star cluster.
If you have a good view to the horizon, then you could also glimpse the annual Geminids meteor shower as it rises in the early morning.
Geminids meteor shower
The Geminds meteor shower is the most reliable meteor shower in the southern hemisphere.
Most meteor showers are caused by the dust and debris left over from passing comets, but the Geminids is the result of dust and debris from an asteroid called 3200 Phaethon that is five kilometres wide.
You can see the shower from anywhere from Australia between December 13-16, peaking on the morning of December 15.
To see it you need to look for the constellation of Gemini, about two handspans above the horizon, below and to the right of Orion (or the saucepan).
“If you look north, the two brightest stars you see immediately above the horizon are Castor and Pollux and the Geminids radiant is immediately below that,” Dr Musgrave said.
Instead of looking directly at the radiant, the origin point for all the meteors in the shower, scan the sky.
“If you stare at the radiant nothing much is going to happen because the meteors start their burn just to the side of where the radiant is,” he said.
The best time to look is between 2:00 and 3:00am AEDT, after the first-quarter moon has set.
And the further north you live the better, with the potential for a meteor every one to two minutes predicted under a dark sky.
“In the suburbs you’ll see a bit less, but you should be able to see a decent number of meteors,” Dr Musgrave said.
How to get good photographs
Astrophotographer Dylan O’Donnell took photos of 46P/Wirtanen (see above) earlier this month, from his backyard in Byron Bay.
“The coma is particularly bright even now,” Mr O’Donnell said.
He has also started to pick up a dust tail as it is blown off the comet on its approach to the Sun.
Other astrophotographers have also started to pick up a second tail, made up of gas particles ionised by the Sun’s UV radiation.
Mr O’Donnell used a specialised high-end photographic telescope to track-capture the image over three minutes.
But, he said, you don’t need high-end kit to take a photo of the comet.
All you need is a camera that can take a 15-second exposure with a wide lens and a tripod.
“Anyone with a DSLR camera and a wide lens — 11mm, 16mm, even down to a fish-eye — can just point the camera in the general direction of the comet.
“Set it up for a 15 second exposure, with the aperture down as low as it can go.
“And then set the ISO to 1600 or 3200 so it is nice and high, and you will see a green blob in your images.”
He said photographing comets can be tricky because they move.
“You take one photo and then another photo and another photo so it’s moving through the frame,” Mr O’Donnell said.
“That allows you to make an animation if you take successive photos.”
You’ll catch the best images earlier in the evening when the comet is higher, away from the horizon.
But, if you stay up later to catch the Geminids, leave the exposure going so it takes a succession of images.
“Then you can animate them later and watch to see any meteors flying through the frame and pick out any good Geminids in those frames,” Mr O’Donnell said.
A widefield view could even catch another star called Mira at its brightest, further left of the comet in the constellation of Cetus, Dr Musgrave added.
“It should be a very good night,” he said.