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Mercury reaches superior conjunction on the far side of the Sun on October 8th. From northern locations, the planet is not visible this month but may be seen just after sunset towards the end of month from southern latitudes, close to the west-northwestern horizon.
Venus, mag. -3.9, continues to move back towards the Sun but remains a morning object for most of October. From northern temperate locations, the brilliant planet can be seen low down above the eastern horizon just before sunrise, although by month's end observers may have difficulty spotting it against the bright twilight. Venus can also be seen from southern locations, but with a reduced visibility period.
On October 3rd, Venus reaches perihelion when it's 0.718 AU (approx. 107 million kilometres or 66.7 million miles) from the Sun. A close conjunction occurs on October 5th when Venus and Mars are separated by only 1/5 of a degree. At mag. +1.8, Mars appears almost 200x fainter than Venus and binoculars will help to spot it. On the mornings of October 17th and 18th, the thin waning crescent Moon appears near the two planets creating pleasant viewing.
M92, mag. +6.4, is a bright globular cluster located in the northern part of the constellation of Hercules. Despite being almost visible to the naked eye, it's often overlooked due to its close proximity to more spectacular, M13.
M92 is one of the original discoveries of Johann Elert Bode who found it on December 27, 1777. He described it as "a nebula that's more or less round with a pale glow". Charles Messier independently rediscovered it and catalogued it on March 18, 1781. Incidentally, this proved to be Messier's most productive night, during which he discovered another 8 objects all of them Virgo Cluster galaxies (M84 to M91). As with many globulars, it was William Herschel who first resolved it into stars. To date, only about 16 variables have been discovered in M92 of which 14 are of the RR Lyrae type, and one is a rare globular eclipsing W Ursae Majoris type binary.
NGC 40, mag. +10.7, is a planetary nebula located in the northern constellation of Cepheus. It was discovered by William Herschel on November 25, 1788. He described it as "a 9th magnitude star, surrounded with milky nebulosity". Herschel used his 475mm (18.7-inch) telescope to make the discovery, but today's amateur astronomers don't require such a large instrument and it can be glimpsed with just a 100mm (4-inch) refractor. NGC 40 is also known as the Bow Tie nebula, a nickname it shares with another planetary nebula, NGC 2440 in Puppis. It's listed as number 2 in the Caldwell catalogue.
NGC 40 is located just over 17 degrees from the North Celestial Pole and is therefore circumpolar from most northern latitudes. It's one of the finest examples of its type in the far northern section of sky. The best time to look for the nebula is during October, November and December when it appears high in the sky during early evening. The Bow Tie nebula is also visible, although lower down, from most tropical latitudes. However, from southern temperate locations it never rises above the horizon.
Finding NGC 40 can be tricky as it's positioned in a star poor region of eastern Cepheus. One method is to imagine a line connecting Errai (γ Cep - mag. +3.21) with γ Cassiopeiae (mag. +2.15). The planetary lies approximately one-third of the way along this line.
Vega is a brilliant magnitude +0.03 blue-tinged white main sequence star located in the constellation of Lyra. It's marginally brighter than Capella and slightly fainter than Arcturus, making it the second brightest star in the northern section of sky. With a declination of 38 degrees north, Vega appears high in the sky and sometimes overhead from many northern temperate locations. This star can be seen anywhere north of 51 degrees south and therefore is visible from the majority of the Southern Hemisphere, including all of Australia, New Zealand, South Africa and most of Argentina and Chile.
Vega is a dazzling beacon of light in the relatively faint, but prominent constellation of Lyra. It was originally named Wega from a derivative of the Arabic phrase "Al Nasr al Waqi" or Swooping Eagle. Around 12,000 BC, Vega was the North Pole star and it will once again return there around 13,700 AD. It's probably unsurprising, given its brilliance and prominent northern declination, that Vega is one of the most investigated stars. It was one of the first stars to have its distance determined by parallax, the first star other than the Sun to be photographed and the first to have its spectrum evaluated. Vega, along with Altair (α Aqr - mag. +0.77) and Deneb (α Cyg - mag +1.25), is a member of the well-known Summer Triangle. This bright asterism was popularised, in the 1950's, by American author H.A. Rey and British astronomer Sir Patrick Moore.
M11, also known as the Wild Duck Cluster, is a famous open cluster located in the constellation of Scutum. It's just beyond naked eye visibility but easily visible with binoculars and an outstanding telescope object. The brightest stars form a triangle that has been likened to a flock of flying ducks, hence the name Wild Duck Cluster. Of all know open clusters, M11 is one of the richest and most compact with about 2,900 members spread over a diameter of 25 light-years.
M11 was discovered by German astronomer Gottfried Kirch of the Berlin observatory in 1681. English clergyman William Derham is believed to have been the first person to resolve it into stars (around 1733), with Charles Messier adding it to his catalogue on May 30, 1764. The name the Wild Duck Cluster was provided by British Admiral William Smyth, who imagined the distinct V shape of the cluster as a flock of flying ducks.
The cluster is an easy target to find despite been located in the small and dim constellation of Scutum, whose brightest stars are of only 4th magnitude. The starting point on the way to the Wild Duck Cluster is to locate Altair (α Aql - mag. 0.8), the brightest star in Aquila and 12th brightest in the night sky. Altair forms the southern corner of the famous Summer triangle along with first magnitude stars, Vega in Lyra and Deneb in Cygnus.
M2 is a rich compact globular cluster at the edge of naked eye visibility that's located in the constellation of Aquarius. It's an easy binocular object (mag. +6.3) but requires a dark site with extremely good seeing conditions to be spotted with the naked eye.
M2 was discovered on September 11, 1746 by French astronomer Jean-Dominique Maraldi while observing a comet with Jacques Cassini, the son of legendary Italian astronomer Giovanni Cassini. Charles Messier then rediscovered it exactly 14 years later. He described the cluster, like many of his other objects, as "a nebula without any stars associated with it". William Herschel was the first to resolve individual stars in 1783.
Finding M2 can be initially challenging as it's located in a relatively sparsely populated area of sky. The key is to locate the two brightest stars in Aquarius, Sadalsuud (β Aqr - mag. +2.90) and Sadalmelik (α Aqr - mag. +2.95). Sadalsuud is positioned 10 degrees southwest of Sadalmelik and located 5 degrees north of Sadalsuud is M2. Together the two stars and the globular cluster form a large right-angled triangle.
M2 straddles the celestial equator and is therefore visible from all over the World, although it appears higher in the sky when viewed from tropical latitudes. It's best seen between July and October.
Capella is a bright yellow star located in the northern section of sky. With an apparent magnitude of +0.08, it's marginally fainter than another bright northern star, Vega. Capella is the standout star of the relatively large constellation of Auriga, the Charioteer. The Milky Way passes through the heart of Auriga and as a result it contains numerous bright open clusters, nebulae and interesting stars.
Capella is the northernmost first magnitude star and is circumpolar from latitudes greater than +44 degrees. The star can still be spotted from most southerly regions including Australia, New Zealand and Argentina. However, it's not visible from the Falkland Islands or Antarctica.
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.
NGC 869 and NGC 884 are two bright open clusters in the constellation of Perseus, that are separated by only half a degree of apparent sky. Together they are commonly known as the "Double Cluster" and form a famous showpiece object, that's easily visible to the naked eye and a wonderful sight in binoculars and telescopes. Both clusters have been known since antiquity and probably pre-historically. Greek astronomer Hipparchus first catalogued them around 130 B.C, with early celestial cartographers naming them as "h Persei" (NGC 869) and "χ Persei" (NGC 884).
The Double Cluster is located in the far northwestern part of Perseus, close to the border with Cassiopeia. With a declination of 57N, it's circumpolar from many northern locations and therefore never sets. To locate the object, draw an imaginary line from Mirfak (α Per - mag +1.8) in a northwesterly direction towards the centre of the "W" of Cassiopeia. The Double Cluster lies just over halfway along this line.
It's listed as number 14 in the Caldwell catalogue.