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The major meteor event in December is the Geminids but there's another shower later in the month, the Ursids, that doesn't make as many headlines but on occasions can be quite good. With the radiant located close to the north pole star, the Ursids are a northern shower. They are nowhere near as spectacular as the Geminids, but on occasions have shown significant bursts of activity and a re-occurrence may occur any year.
This year, the shower peak occurs on the night of December 22 / 23 and the four day old waxing crescent Moon won't significantly interfere.
The comet that sources and therefore is 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 for the 2007 and 2008 showers, but in the end only a small increase in activity was observed.
The Geminids or "Winter Fireworks" is widely regarded as the richest and most active of the annual meteor showers with this year's event peaking on night of December 13th/14th. During peak activity up to 120 meteors per hour can be seen under perfect conditions, including many bright ones. Of all annual showers, only the Perseids comes close to attaining such highs. This year's event promises to be a good one as the waning crescent Moon won't interfere. It may even be extra special as the parent asteroid, Phaethon, passes close by the Earth during the activity period.
The Geminids are unusual in that the source object is an asteroid, 3200 Phaethon. Together with the Quadrantids, they are the only major meteor showers not originating from a comet. Phaethon has an orbit that brings it closer to the Sun than any other named asteroid, although there are several unnamed asteroids that do approach closer. At perihelion - the point of closest approach to the Sun - it's only 0.14 AU distant and much closer than the innermost planet, Mercury. However, at aphelion (the point of furthest distance from the Sun) the asteroid moves out to 2.4 AU, and therefore beyond the orbit of Mars.
Mercury starts the month as a magnitude 0.0 evening object, visible from southern and equatorial latitudes and setting up to 2 hours after sunset. The fast moving planet then draws rapidly back to the Sun until inferior conjunction is reached on December 13th. On the previous day, Mercury reaches perihelion at 0.307 AU (approx. 45.9 million kilometres or 28.5 million miles).
During the 3rd week of the month, Mercury re-appears in the pre-dawn sky for observers at northern locations. The planet brightens quickly and by months end will have reached magnitude -0.3. It will be well placed in the sky, just before sunrise.
The long morning apparition of Venus finally comes to an end in December. At the start of the month the planet remains a brilliant magnitude -3.9 object, low down above the eastern horizon just before dawn. However, it's not long before Venus is lost to the glare of the Sun. For those at southern latitudes, the planet will only be visible at the very beginning of December although observers at northern temperate latitudes should be able to spot it for a few days more.
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 it 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.
The Leonids is a famous meteor shower that's peaks on the night of Friday, November 17, and early the following morning. What adds to the mystique of this shower is its fantastic history. Over the years it has produced some of the most spectacular storms ever seen, including a famous outburst of incredible proportions in 1833. Even though no super storm is predicted this year, the event is very favourable and promises to be a good one as the new Moon won't interfere.
Parent comet and great storms
The source of the Leonids is comet 55P/Tempel-Tuttle, or as it's more commonly known Tempel-Tuttle. It was discovered by Ernst Tempel on December 19, 1865 and independently by Horace Parnell Tuttle on January 6, 1866. With an orbital period of 33 years, Tempel-Tuttle can pass close to the Earth and when this happens the chance of a witnessing a super storm is greatly increased.
The northern part of the Taurid meteor shower peaks this year on November 12th. Although the number of meteors per hour is low, the Northern Taurids often produces spectacular fireballs. As was the case in 2015, when more fireballs were seen than usual. When bright Taurids arrive, authorities are usually in for a busy night with a flurry of UFO reports. The event this year promises to be reasonably good as the 35% illuminated waning crescent Moon won't significantly interfere.
Parent Comet and Radiant
The Taurids have long been identified as an old meteor stream with the first recorded observations made in 1869. Despite frequently seen during the remainder of the 19th century, it wasn't until 1918 when realised a new shower had been found.
Although originating from the same parent comet, the Taurids have now spread out to create two individual showers (Northern Taurids (NTA) and Southern Taurids (STA)). The northern stream is the slightly better shower although both have low activity rates. The southern stream peaks on November 5th, but this year the almost full Moon will effectively wash out the shower. All Taurids meteors are associated with periodic comet Encke (2P/Encke), which orbits the Sun once every 3.3 years. This is the shortest period of any known comet. Comet Encke and the Taurids are believed to be remnants of a much larger comet, which disintegrated sometime over the past 20,000 to 30,000 years.
Gemini is a northern zodiac constellation and one of the 48 constellations described by second century astronomer Ptolemy. Its name is Latin for the twins and it's one of the few constellations in the sky that actually looks like what it suppose to represent. This bright grouping contains two-standout stars, Castor (α Gem) and Pollux (β Gem). At mag. +1.16, Pollux is the brighter while multiple system Castor shines at mag. +1.58. Surprisingly, Pollux was assigned beta Geminorum by Johann Bayer - the German astronomer who labelled the stars with Greek letters in 1603 - even though it easily outshines Castor. Some astronomers have suggested that maybe Pollux has since brightened or Castor faded, but both possibilities seem extremely improbable. The likelihood is that Bayer simply made a mistake and didn't carefully distinguish which was the brighter star.
In Greek mythology, Castor and Pollux were twin brothers whose mother was Queen Leda although Castor was the mortal son of King Tyndareus and Pollux the divine son of Zeus. Together the twins were known as the Dioscuri, which means the sons of Zeus. However, in most versions of the myth only Pollux was Zeus's child. The twins were the patron saints of mariners, appearing in ships rigging as the St Elmo's fire phenomena. When Castor died, Pollux begged Zeus to give Castor immortality, which he did, thereby reuniting the twins together in the heavens forever.
NGC 2362 is a small compact young open cluster in Canis Major that surrounds fourth magnitude star, tau Canis Majoris (τ CMa - mag. +4.37). This attractive grouping of 60 stars is packed into an area spanning just 6 arc minutes of apparent sky. The apparent magnitude of the cluster is quoted as +4.1, however this value is misleading as its significantly skewed because of the brilliance of τ CMa. The remaining members of NGC 2362 are much fainter, the brightest being of 7th magnitude.
Finding NGC 2362 is not difficult. It's located 2.75 degrees northeast of Wezen (δ CMa - mag. +1.83), the third brightest star in Canis Major. Sirius (α CMa), the brightest star in the night sky (mag. -1.46), is positioned 11 degrees northwest of NGC 2362. Tau CMa, a spectroscopic multiple system, shines with a combined light of tens of thousands times that of the Sun and is the stand out cluster member. It's bright enough to be seen with the naked eye. For comparison, the Sun at the same distance would shine at a feeble magnitude of +15.
NGC 2362 was discovered by Giovanni Batista Hodierna sometime before 1654 and then re-discovered by William Herschel on March 4, 1783. It's best seen from southern latitudes during the months of December, January and February.
NGC 2392 is a 9th magnitude bipolar double shell planetary nebula located in the constellation of Gemini. Resembling a person's head surrounded by a parka hood, it's commonly known as the "Eskimo Nebula" or "Clown Face Nebula". William Herschel discovered this planetary nebula from his observatory in Slough on January 17, 1787, describing it as a 9th magnitude star with a bright centre surrounded by equally dispersed nebulosity.
Locating the Eskimo Nebula is relatively easy since it's positioned just east of constellation centre and close to Wasat (δ Gem - mag. +3.5). The easiest way to find Gemini is by identifying its two brightest stars Castor (α Gem - mag. +1.58) and Pollux (β Gem - mag. +1.16). They are positioned east of the Hyades open cluster in Taurus and northeast of the bright prominent constellation of Orion.
Imagine a line extending from Pollux - the brighter of the twins - in a southwesterly direction towards Orion's belt. Positioned just over 8 degrees along this line is Wasat and 2.3 degrees southeast of Wasat is NGC 2392. The planetary nebula is positioned next to a mag. +8.2 yellow-white star. Through a telescope the pair appears as a wide double star, separated by about 100 arc seconds.