Northern Hemisphere

Ian Morison tells us what we can see in the northern hemisphere night sky during September 2010.

The nights are drawing in. Overhead in the south after sunset are Cygnus the Swan, Lyra the Lyre and Aquila the Eagle. Their respective brightest stars, Deneb, Vega and Altair make up the Summer Triangle. A third of the way up from Altair to Vega, the dark patch of sky known as the Cygnus Rift can be seen through binoculars. It is a dust cloud obscuring the starlight beyond, and contains the asterism Brocchi’s Cluster, often called the Coathanger. The constellatiof Pegasus, the Winged Horse, is low and inverted in the south-east, near to our neighbouring giant galaxy, M31, located in the Andromeda constellation and bearing the same name. The galaxy can be found by curving two stars up and left of the top left corner of the Square of Pegasus, which is the star Alpha Andromedae, then moving two stars to the right. It appears as a hazy glow in binoculars or, in a dark sky, to the unaided eye. The Andromeda galaxy is 2.5 million light years away, and may be around 20% more massive than our Milky Way. Andromeda and the Milky Way are the two largest galaxies in the Local Group. The constellations of Cassiopeia and Perseus rise in the east, beneath the band of the Milky Way. The Perseus Double Cluster lies between them, visible to the naked eye, distinguishable with binoculars and full of stars through a telescope.

The Planets

• Jupiter, at magnitude -2.9, rises in the east around 21:00 British Summer Time (BST; one hour ahead of Universal Time) and remains visible for much of the night. It rises earlier each night, appearing around 19:00 BST by the end of the month. It reaches opposition on the 21st, appearing due south around 01:00 BST on the 22nd. It still lacks its south equatorial belt, one of the dark bands normally visible on either side of its equator. Meanwhile, the Great Red Spot is darker than usual.
• Saturn can only just be seen in the first week of the month, low in the west and to the right of Venus. After this, it disappears for two months. It passes behind the Sun on the 30th.
• Mercury passes behind the Sun on the 3rd, a location called inferior conjunction. It becomes visible again high in the sky later in the month, the best time to see it before dawn this year.
• Mars is at magnitude +1.5, very low in the south-west after sunset. Its angular size is about 4”, making it unresolvable with a normal telescope.
• Venus is very close to Mars in the sky.

Highlights

• Mercury presents a good pre-dawn apparition on the 19th, at magnitude -0.3. Never more than 18° away from the Sun, it is often lost in sunlight. It is best seen at greatest elongation, when the ecliptic is at its greatest angle to the horizon, which occurs around dawn in autumn and sunset in spring. This month, it sits about 15° above the horizon for some time each morning.
• M15 is a globular cluster in Pegasus, imaged by schools with the Faulkes Telescope. Following the line of the last two stars of the neck and head of Pegasus up and to the left of the constellation Delphinus, it can be seen as a fuzzy glow with binoculars. A telescope reveals the individual stars, the brightest of which is at magnitude +12.6. Over 13 billion years old, it is one of the most ancient globular clusters known, and lies 33,000 light years from Earth, within our galaxy. It has a total magnitude of +6.4, corresponding to a luminosity of 360,000 times that of the Sun, and may contain a black hole.
• Mars and Venus come within a few degrees of each other in early September, either side of the star Spica in Virgo, low in the west after sunset. The thin crescent Moon is just below Spica on the 10th, and above and left of Venus on the 11th.
• , the Bay of Rainbows, is a curved inlet of Oceanus Procellarum on the Moon. It is best seen when the terminator crosses the bay, as the tops of the mountains are in sunlight and resemble a dinosaur’s backbone. Virtual Moon Atlas states that this will happen on the evening of the 18th.
• Jupiter comes within a degree of Uranus as both reach opposition on the 21st. Jupiter is at its largest angular size since 1963, and will not appear bigger until 2022. This is because it is at its closest to the Sun, while Earth is at it furthest from the Sun, so the two planets are near their minimum separation. Now is the best time to see the details of the Jovian system.

The Southern Night Sky

John Field from the Carter Observatory in New Zealand speaks about the southern night sky during September 2010.

Jupiter returns to the evening sky this month, rising in the east after sunset. Named after the King of the Greek gods, the largest of the Solar System’s planets takes 12 years to orbit the Sun, passing through one zodiacal house roughly every Earth year in our sky. In Māori, it is called Pareārau or Kōpū-nui. Galileo observed Jupiter’s disc and four largest moons in the 17th century, the moons ranging from 3000 to 5000 km in diameter. Io is the nearest of these to Jupiter, and is the most volcanically active body in the Solar System due to gravitational friction from the objects around it. Europa is the smoothest object in the Solar System, probably containing water under an ice layer many kilometres thick, and may be capable of supporting life. Ganymede and Callisto are the outermost of the four main moons, which are among a total of 63 known to orbit Jupiter. Galileo also observed bands of cloud on the planet, one of which contains the Great Red Spot, a storm 2.5 times the diameter of the Earth that has be seen continuously for 200 years. Jupiter, at 318 times the mass of the Earth, outweighs all the other planets in the Solar System together. Venus, the Evening Star, appears in the west after sunset. Mars, fainter, sits below. The star Vega shines on the northern horizon, while the Milky Way spans the sky from north to south. The orange star Antares, the heart of the constellation Scorpius, is overhead to the west. The Scorpion’s tail, or hook of Māui to the Māori, curls towards the zenith, while the Southern Cross and its pointers lie in the south-west. Beyond Scorpius’ tail is Sagittarius, often named the Teapot after the shape of its brightest stars. Sagittarius, the Archer, is said to be firing an arrow at Scorpius in revenge for its killing of Orion the Hunter. Aquila, the Eagle, is north along the Milky Way. Its brightest star, Altair, referred to as Poutū-te-rangi by Māori astronomers, is the twelfth-brightest in our sky and one of the closest at 16 light years distant. Imaging reveals that this star spins rapidly enough to make it noticeably oblate. Altair, Vega in Lyra and Deneb in Cygnus form what is known in the southern hemisphere as the Winter Triangle, which is the Summer Triangle to those in the northern hemisphere. Canopus, the second-brightest star in the sky, is low in the south. The navigator of Spartan King Menelaus in Greek mythology, to the Māori it is Atutahi, chief of the heavens. It appears as a circumpolar star from New Zealand, and was once called Alpha Argos, part of the constellation Argo, the great ship of Jason in Greek mythology. This constellation has since been divided into three, and Canopus is known as Eta Carinae, the brightest star in Carina, the ship’s keel. The Hipparcos satellite measured Canopus to be 310 light years from Earth, with a mass 8.5 times that of our Sun and outshining it by a factor of 15,000. Carina contains a number of star clusters. One of these, IC 2602, known as the Southern Pleiades, is a degree across and surrounds the 3rd magnitude star Theta Carinae. Binoculars reveal its many stars. Nearby, NGC 3532 is visible to the naked eye as a haze near the Eta Carinae Nebula. A favourite of John Herschel, it contains 150 stars and covers one degree of sky, twice that of the full Moon. With a telescope, a number of small lines and orange stars can be seen. NGC 2516, another open cluster, can be seen by eye on a moonless night. Its scattered groups of stars can be seen through binoculars or a telescope, and three bright orange stars stand out within it.

Highlights

• Sunspots are reappearing on the Sun as it emerges from solar minimum. A number of coronal mass ejections occurred during August, sending out charged particles into the Solar System. These can cause the Aurora Australis to be seen, although they must be bright to reach the northern latitudes of most land in the southern hemisphere. The chances of seeing these Southern Lights will increase with solar activity.