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Although Mercury passed inferior conjunction on December 28th it's not long before the fast moving planet re-appears in the pre-dawn sky. From southern latitudes it rises about 30 minutes before the Sun during the first few days of the New Year, although against the bright twilight sky it will be very difficult to spot. The visibility gradually improves until peaking when greatest western elongation (GWE) is reached on January 19th. On this morning from latitude 35S (approx. equal to Sydney, Cape Town and Santiago) the planet shines at mag. -0.2 and appears 11 degrees above the eastern horizon 45 minutes before sunrise. A further 10 degrees higher in the sky and slightly more to the east is fainter Saturn (mag. +0.6).
Mercury remains visible for the remainder of the month but gradually sinks back towards the horizon. From northern temperate latitudes the apparition is not as favourable with the planet always hovering low down. For example from London, England (51.5 N) at GWE it appears only 5 degrees above the southeastern horizon, 45 minutes before sunrise.
The thin waning crescent Moon passes 4 degrees north of Mercury on January 26th.
The first annual meteor shower of the year is always the Quadrantids and it can be a good one with up to 100 meteors per hour visible at peak time. This rate is comparable to the superb Perseids and Geminids, but unlike these well-observed showers the Quadrantids are elusive since they have a narrow peak activity window that lasts just a few hours.
Radiant and History
Meteor showers are named after the constellation (or nearby star) where the radiant is located but you will have difficulty figuring out the associated constellation for the Quadrantids, the reason is it no longer exists.
Today the Quadrantids radiant is located in the northern constellation of Boötes, not far from the tail of Ursa Major. When the shower was discovered by Adolphe Quetelet of the Brussels Observatory in the 1830s the radiant was located in the now obsolete constellation of Quadrans Muralis, hence the name "The Quadrantids".
The New Year begins with the closest planetary-planetary conjunction of 2017 as Mars passes just 1 arc minute south of Neptune, which corresponds to an apparent separation of just 1/30 the diameter of the full Moon. The planets will be visible in the evening sky just after sunset towards the west. At mag. +0.9, Mars can be easily spotted with the naked eye while Neptune (mag. +7.9) requires at least a pair of binoculars or a small telescope to be seen. Also located in the same region, providing nice viewing, is much more brilliant Venus (mag. -4.4) and the crescent Moon.
Vesta the brightest asteroid or minor planet is now visible with binoculars and small telescopes and well placed for observation as it heads towards opposition on January 17th. Although it won't quite reach naked eye brightness this time, the asteroid remains at least as bright as seventh magnitude from now until the end of March. At peak, Vesta hits magnitude +6.2 in January and throughout the next few months will appear bright, easy to spot and follow with either a pair of quality binoculars or a small scope.
The "Winter Circle" or "Winter Hexagon" is a bright asterism that's best seen during the months from December to March. The group consists of a large circle of stars or more actually a hexagon shape loosely centred on red supergiant Betelgeuse (mag. +0.42). All stars are bright. Starting from the most northerly point and moving in a clockwise direction they are as follows: Capella (mag. +0.1), Aldebaran (mag. +0.9), Rigel (mag. +0.1), Sirius (mag. -1.4), Procyon (mag. +0.3), and Pollux (mag. +1.1). It's possibly the largest of the well-known asterisms.
What is an asterism?
An asterism is a recognised pattern of stars in the night sky. It may be part of one of the 88 official constellations or it may be composed of stars from many constellations. Anyone can make his or her own asterism and if many people use it often enough it can find its way into Astronomy vocabulary. In the case of the Winter Circle it's no accident it's so large, the six stars come from six different constellations (Auriga, Taurus, Orion, Canis Major, Canis Minor and Gemini).
NGC 559 is a magnitude +9.5 open cluster in Cassiopeia that's faintly visible with binoculars, but better seen with telescopes. The surrounding area of sky is full of bright clusters including NGC 457, M 52, M 103, NGC 129, NGC 7789, NGC 654, NGC 663 and IC 1848. As a result, tenth magnitude NGC 559 is not a standout object. However, it's easy to find and a nice target for small and medium size scopes.
The major meteor event in December is the Geminids but there's another shower later in the month that doesn't quite make as many headlines but on occasions can be quite good. It's the Ursids. With the radiant located close to Polaris the North Pole Star, the Ursids are a Northern Hemisphere shower. They are much less dramatic than the Geminids with only about 10 meteors per hour visible but on past occasions have shown significant bursts of activity and a re-occurrence may occur anytime.
This year's Ursid peak occurs on the night of December 22/23. The last quarter Moon rises around 3am and won't significantly interfere.
The comet that sources and is therefore responsible for the shower is 8P/Tuttle (also known as Tuttle's Comet or Comet Tuttle). It has a period of 13.6 years and during the last perihelion on January 27, 2008 was visible telescopically. On January 1, 2008 it passed Earth at a distance of 0.25282 AU (37,821,000 km or 23,501,000 miles) and anticipation was high that the 2007 and 2008 showers may produce much increased activity but that was not to be. In the end only a small increase was noted.
The Geminids or "Winter Fireworks" is one of the finest annual meteor showers with this year's shower peaking on the night of December 13/14. During peak activity up to 120 meteors per hour (Zenithal Hourly Rate) can be seen under perfect conditions. Of the other annual showers only the Perseids in August comes close to attaining such highs. Unfortunately, the full Moon in neighbouring Taurus will significantly interfere during peak time and wash out many of the meteors.
Algol (β Per) is a bright eclipsing binary system located in the northern constellation of Perseus and one of the best-known variable stars in the sky. Often referred to as the "Demon Star" most of the time it shines at mag. +2.1 but every two days, 20 hours and 49 minutes the star suddenly dips in brightness to mag. +3.4, remaining dim for about 10 hours before returning to its original state.
Why the change in brightness? The Algol system consists of at least three-stars (β Per A, β Per B and β Per C) with the orbital plane of Algol A and B directly in line with the Earth. The regular dips in brightness occur when the dimmer B star moves in front of and eclipses the brighter A star. There is also an extra dimension in that a secondary eclipse occurs when the brighter star occults the fainter secondary resulting in a very small dip in brightness that can be detected with photo-electrical equipment.