What's Up - September

Sun and Moon
September begins with a thin crescent moon in the western sky at sunset, with first quarter (half moon in the evening sky) on the 7th, and full moon on the 15th. Last quarter (half moon in the morning sky) is on the 22nd, with new moon at 10:12 a.m. SAST on the 29th. The young crescent will not be visible from southern Africa until the following evening, and from the northernmost parts of Africa and much of the Middle East, sighting the crescent moon with the naked eye is unlikely until the evening of October 1st. 

The moon makes its closest approach to Earth in September (perigee – 368888 km) on the 20th. lts greatest distance from Earth this month will be on the 7th  (apogee – 404209 km). Apparent size and brightness of the full moon will therefore be about average.

Southern hemisphere spring begins officially in September. That's what everyone seems to agree on. But when it begins is another matter. According to some, the official beginning is on September 1. Others like to use the date of the spring equinox, when the moon crosses the celestial equator into the southern hemisphere. This year the equinox will be on the 22nd. 

The name 'equinox' suggests that this must also be the day when day and night are equally long. But it's not so. Instead the 22nd is the day when (if the Earth had no atmosphere) the centre of the sun would come closest to rising 12 hours before the centre of the sun went below the horizon. But the Earth does have an atmosphere, which bends sunlight so that when the sun appears perched on the horizon, its true position is entirely below the horizon! To put it another way, when it's near the horizon the sun appears higher in the sky than it is, and we don't count sunrise or sunset by the moment when the centre of the sun is on the horizon. Hence the last day when sunrise and sunset are less than 12 hours apart (from the time when the 1st sliver of the sun disappears until the last sliver disappears) is September 18. Starting with the 19th (time from sunrise to sunset: 12 hours 1 minute 38 seconds) the days will be longer than the nights. If you want to count the beginning of spring from that date, or not – feel free. None of these dates says much about when the weather feels like spring.

Evening and Morning Sky: Planets
This is a great month for planets in the evening sky, with Jupiter nearly overhead in early evening, and Mercury, Venus and Mars low in the west after sunset. On the 1st, the crescent moon and Venus form the base of a triangle, with Mars at the upper point and Mercury between Mars and the moon. As September progresses, the three planets will get closer to Spica (the brightest star in Virgo), but at different rates. By the 16th, Spica will be the upper point of an equilateral triangle with Venus and Mercury. (Mercury will be to the left of Venus and much dimmer.) By the 20th, the three planets and Spica will form a fairly tight right triangle with Venus at the top, Mercury and Mars at the base, and Spica between Venus and Mercury. On the 24th, Mercury, Spica and Mars (from left to right) lie almost in a straight line with Venus above all three. As a kind of grand finale to this month's planetary dance, Mercury, Spica and Mars (lower left to upper right) form an even straighter line low in the evening sky, with the young crescent moon just to the left of Spica. Don't miss the show!

The only bright planet missing from the above is Saturn, which is visible rising before the sun by month's end.

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 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 12000 times the power output of our own sun. But Antares is also so much cooler than the sun (hence the red colour) that most of its energy output is in the infrared, and its total power output is 40000 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. One of these first 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 amidst the 'dance of the three planets' described above. 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
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 diametre is about 11.8 times that of the sun, while its polar diametre 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 giving 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 Aldeberan 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.

A map of the Cape sky (latitude -34 deg)

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