Venus and the Moon guide you to Comet Catalina in the pre-dawn sky

By Ade Ashford

At the onset of astronomical twilight (~6am GMT for the centre of the British Isles) on the mornings of 7 and 8 December 2015, dazzling planet Venus lies within 4.5 degrees of Comet Catalina (C/2013 US10) low to the southeast horizon. The comet is currently around magnitude +6, so it is an easy binocular object if you ensure Venus is out of the field of view and observe before dawn twlight grows too bright. The 26-day-old waning crescent Moon makes it a triple conjunction on the morning of Tuesday, 8 December. Comet Catalina is rapidly heading north on consecutive mornings and its magnitude should hold steady throughout the month. AN graphic by Ade Ashford.
At the onset of astronomical twilight (~6am GMT for the centre of the British Isles) on the mornings of 7 and 8 December 2015, dazzling planet Venus lies within 4.5 degrees of Comet Catalina (C/2013 US10) low to the southeast horizon. The comet is currently around magnitude +6, so it is an easy binocular object if you ensure Venus is out of the field of view and observe before dawn twlight grows too bright. The 26-day-old waning crescent Moon (slightly enlarged here for clarity) makes it a triple conjunction on the morning of Tuesday, 8 December. Comet Catalina is rapidly heading north on consecutive mornings and its magnitude should hold steady throughout the month. AN graphic by Ade Ashford.
Discovered on 31 October 2013 by the Catalina Sky Survey, C/2013 US10 is an Oort Cloud comet making its first foray into the inner solar system. For most of the intervening two years Comet Catalina has been a faint object of the far southern sky, but around September 2015 it started heading northwest, picking up speed through the constellations Lupus, Centaurus and Hydra, before reaching perihelion on 15 November in Libra. Since then, C/2013 US10 has been in eastern Virgo where its motion has turned almost due north in declination, greatly improving its visibility from the British Isles.

Although Comet Catalina hasn’t quite lived up to the peak magnitude predicted for it, C/2013 US10 is hovering around naked eye visibility and is likely to remain so for the following month. Although it is now receding from the Sun, Comet Catalina’s distance from Earth is still decreasing and we make our closest approach on the morning of Sunday, 17 January 2016 at a distance of 67.3 million miles (108.4 million kilometres). At that time it will be situated between M101 and Alcor in Ursa Major and therefore a circumpolar object for UK observers.

Following its conjunction with Venus on 7—8 December 2015, Comet Catalina (C/2013 US10) rapidly heads north through the constellation Virgo, crossing the border into Boötes on Christmas Eve. It passes within ½ degree of the brightest star in the northern celestial hemisphere — magnitude +0.1 Arcturus — on the morning of New Year's Day 2016. Click the graphic for a greyscale version suitable for printing and use outside. AN Graphic by Ade Ashford.
Following its 4.4-degree separation from Venus at 6am GMT on Tuesday, 8 December 2015, Comet Catalina (C/2013 US10) rapidly heads north through the constellation Virgo, crossing the border into Boötes on Christmas Eve. It passes within ½ degree of the brightest star in the northern celestial hemisphere — magnitude +0.1 Arcturus — on the morning of New Year’s Day 2016. The positions shown are for 06h UT on the stated date. Click the graphic for a greyscale version suitable for printing and use outside. AN Graphic by Ade Ashford.
Comet Catalina, Venus and the Moon
At 6am GMT on the morning of Monday, 7 December — the onset of astronomical twilight in the centre of the British Isles — planet Venus and Comet Catalina will lie just 4.5 degrees apart low to the southeast horizon in a dark sky. At the same time the following morning (Tuesday, 8 December), the brightest planet and C/2013 US10 will be fractionally closer at 4.4 degrees separation.

This means that on both 7 and 8 December it’s theoretically possible to see C/2013 US10 and Venus in the same field of view of a low-power binocular, though in practice the dazzling glare of the brightest planet will render the comet invisible. However, one need only displace Venus just outside the lower right-hand side of the field of view for the comet to be revealed (weather and local light-pollution permitting, naturally).

Cameras at the ready
Astrophotographers are in for a real treat on the morning of Tuesday, 8 December, not only due the comet’s close proximity to Venus mentioned above, but because the 26-day-old waning crescent Moon also joins the show 6.4 degrees to the lower left of the brightest planet. A typical APS-C sensor DSLR camera with a lens of 135mm focal length or less will neatly encompass the trio, though bear in mind that you need a totally unobstructed view low to the southeast. If you are close to the heart of the UK you might be able to image up until 6:30am GMT on 8 December before dawn twilight gets too bright.

Comet Catalina dates for your diary
The comet’s track is currently directed towards the north celestial pole, so its motion through eastern Virgo is largely in declination. C/2013 US10 crosses the constellation border into Boötes on Christmas Eve and it passes within ½ degree of the brightest star in the northern celestial hemisphere — magnitude +0.1 Arcturus — on the morning of New Year’s Day 2016.

The following J2000.0 topocentric coordinates of Comet Catalina for the centre of the British Isles are for 06h UT on the dates shown. They include optimistic predicted magnitudes (Mag.) with distances in astronomical units (1 AU = 92,955,807.3 miles or 149,597,870.7 kilometres) from both the Sun (r) and the Earth (Delta).
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Inside the magazine

You can find out more about Comet Catalina and this month’s planetary events in the December edition of Astronomy Now in addition to a full guide to the night sky.

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