September 2013

Sun and Moon
NEW MOON is on the 5th at 13:36 PM. FIRST QUARTER (half moon in the evening sky) falls on the 12th. FULL MOON occurs on the 19th  at 13:13 PM. LAST QUARTER (half moon in the morning sky) falls on the 27th.

On the 6th the young lunar crescent will be visible from most of North, Central and South America as well as most of Africa (including South Africa) and Australia. On the 7th the crescent Moon will be visible worldwide apart from the extreme north.

The Moon will be at perigee (closest approach to Earth) at a distance of 367 387 km on the 15th at  18:35 PM. On the 27th at 20:18 PM the Moon will be at apogee (furthest from Earth) at a distance of 404 308 km.

 Planetary and Other Events – Morning and Evening
Mercury and Venus are visible in the evening sky during September. Both are up from sunset, with Mercury setting around 20:00 PM mid-month and Venus setting around 22:00 PM mid-month. (Both set earlier at the beginning of the month and later at the end of the month). Mars is visible in the morning, rising around 05:30 AM at the beginning of the month and just before 04:30 AM by the end of the month.

Jupiter is visible in the morning rising around 04:00 AM at the beginning of the month and around 02:30 AM at the end of the month. Saturn rises in the early evening and is visible until around 23:00 PM at the beginning of the month and around  21:15 PM at the end of the month.

Uranus is up all night from around 21:00 PM at the beginning of the month and from sunset at the end of the month. Neptune is up from sunset and sets around sunrise at the beginning of the month and around 05:00 AM at the end of the month.

 The Evening Sky Stars
The winter Milky Way still sweeps majestically across the sky from NNE to SSW in early September evenings, and the centre of our Milky Way galaxy is almost overhead. Just to the west of the zenith (directly overhead) is the Scorpion, with the reddish star Antares at its heart. Antares (or ‘rival of Mars’) is a huge star 600 light years away, shining in visible light with 12 000 times the power of the Sun. Most of its energy output is in the infrared (hence the red colour), and its total power output is 40 000 times that of the Sun. If Antares were suddenly placed at the centre of our solar system, Mercury, Venus, the Earth, Mars and the asteroid belt would be inside this monster star, whose vaguely defined surface would lie 4/5 of the way from the star’s centre to the orbit of Jupiter. Gravity at the surface of Antares is so weak that it is losing mass fast enough to create a visible nebula or gas cloud around it, lit by Antares hot companion star. In the next few million years or so, Antares may explode as a supernova — so keep your eyes on the Scorpion if you’re the patient sort. Just NE of Scorpio in the Milky Way are the stars of Sagittarius the Archer, making a pattern a bit like a teapot. It’s in this constellation that the centre of our galaxy is located, but you can’t actually see the centre directly because of the thick dust clouds in between. Only one in a billion photons of visible light from the Galactic Centre can get through, and infrared cameras are needed to show what’s there. Infrared observations of stars orbiting the centre suggest that right at the centre is a black hole about 3 million times as massive as our Sun.

High in the NE, toward the edge of the Milky Way, is the bright star Altair in Aquila the Eagle. Altair is easy to recognise because of the dimmer stars more or less equally spaced on each side. Another bird constellation, low in the NE and thoroughly tangled in the Milky Way, is Cygnus the Swan, also known as the Northern Cross. At the NE end (top) of the Cross is Deneb, the tail of the Swan. Deneb is much more distant than most of the stars we see with the unaided eye, and its true brightness has been estimated at 160 000 Suns. To the right (W) of Deneb is another bright northern star, Vega, only about 1/100 as far away at only 25 light years. Vega is really much dimmer than Deneb, but appears brighter because it is so much closer. If Deneb were as close as Vega, it would be by far the brightest star in the sky, as bright as a thickish crescent moon!

Low in the NW in early evening is the bright star Arcturus, with Spica glowing low in the west. Higher in the west (and just north of the Scorpion) is the curious constellation of Ophiucus the Serpent Holder. One half of the Serpent stretches from the hands of Ophiucus toward Arcturus and the Northern Crown in the NW, while the other extends along the Milky Way toward the Eagle. South of the Scorpion are the Altar, the Level and the Wolf, while further south we find the Centaur (including the Pointers) and the Southern Cross. Fomalhaut is now high in the east, with bright Achernar low in the southeast, ninth brightest star in the sky.

The Morning Sky Stars
By the time the Earth’s rotation allows us to see the predawn September sky, Achernar (the ‘mouth’ of the celestial river Eridanus) is much higher in the south, with Fomalhaut (in the Southern Fish) low in the SW. Achernar is spinning so fast that its equatorial diameter is about 11.8 times that of the Sun, while its polar diameter is only 7.6 times solar. Partly due to its very fast spin, Achernar is losing mass thousands of times as fast as our own Sun, and is thousands of times as bright. According to ancient South African folklore, if the Senakane (the little horn) (Achernar) rises in the East very bright and gives off little lightnings, and the bullrushes are still in flower, men fear an early frost. The shield of the little horn is the Small Magellanic Cloud, known as mo’hora le tlala, ‘plenty and famine’. If dry dusty air made it appear dim, famine was to be expected.

High in the southeastern sky is Canopus, second brightest star in our skies and the brightest star in the ancient constellation of Argo, the great ship. High in the east is Sirius, brightest star in the sky as seen from Earth. If Canopus were at the same distance as Sirius, however, it would shine about 400 times brighter. Sirius is the brightest star in Orion’s Large Dog, and the stars of Orion, including bright Rigel and Betelgeuse, are high in the NE before dawn this month. Charging Orion is Taurus the Bull, with Aldebaran serving as an inflamed orange eye. No wonder Orion has his hide shield raised in front of him. Behind Orion, his Small Dog (with the bright star Procyon) is prudently staying on the safer side of the contest, while totally indifferent to all this drama, Auriga the Charioteer (with the bright star Capella) drives by low in the north.

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