August 2015

Sun and Moon

LAST QUARTER (half moon in the morning sky) falls on the 7th. NEW MOON occurs on the 14th at 16:53 PM. FIRST QUARTER (half moon in the evening sky) falls on the 22nd. FULL MOON is on the 29th at 20:35 PM.

The young lunar crescent may be visible in Central and South America, Namibia and the west of South Africa on the 15th of August. Under perfect conditions it may be visible over southern and central Africa and Mexico. It may be visible with optical aid from the southern US, parts of north Africa and western Australia. On the 16th the crescent will be visible from most of the world. The first possible sighting in South Africa occurs on the 15th of August.

On the 2nd of August at 12:12 PM the moon will be at perigee (closest to Earth) at a distance of 362,134 km. On the 18th at 04:34 AM the moon will be at apogee (furthest from Earth) at a distance of about 405,851 km. The moon will again be at perigee on the 30th at 17:25 PM at a distance of 358288 km.

Planetary and Other Events – Morning and Evening

Mercury is visible early evening all month. Venus shines brightly as an evening star for the first half of the month before conjunction. During the second half the month Venus emerges from the Sun’s glare and is visible in morning twilight. Mars will be visible during morning twilight all month. Jupiter is visible in the early evening in the first half of the month, before being lost in the Sun’s glare about a week before solar conjunction which occurs on the 27th. Saturn is visible from evening twilight until around 02:45 AM at the beginning of the month, setting around 00:45 PM by month’s end.

Three meteor showers are active in August: the Piscis Austalids, the Southern delta Aquariids and the alpha Capricornids. Observing prospects for all three are poor this year due to bright moon conditions around their peaks. The Piscis Australids are active from the 19th July to the 17th August, peaking on the 28th July. To view the Piscis Australids find a dark spot and look east near the constellation of Piscis Austrinus for the Piscis Australids radiant. The best time to view the shower is from around 21:30 PM until 05:00 AM when they’ll be in the west. The Southern delta Aquariids meteor shower is active from the 21st July to 29th August, peaking on the 29th July. To view the Southern delta Aquariids find a dark spot and look east near the constellation of Aquarius for the Southern delta Aquariids radiant. The best time to view the shower is from around 21:30 PM until 05:00 AM when they’ll be in the west. The alpha Capricornids meteor shower is active from the 15th July to the 25th August peaking on the 30th July. To view the shower look east near the constellation of Capricornus for the alpha Capricornids radiant. The best time to view the alpha Capricornids is from around 20:00 PM until 04:00 AM when they’ll be in the west.

The Evening Sky Stars

In the north, orange Arcturus shines brightly in the evening sky, with the half circle of the Northern Crown to the right, and bright white Vega rising low in the north east. Higher in the ENE, tangled in the northern Milky Way, is Aquila the Eagle with its bright star Altair. Vega and Altair are relatively nearby stars, but like most of the stars visible to the naked eye would appear much brighter than the Sun if they and the Sun were at the same distance.

To the south of Arcturus, high in the NW, shines blue-white Spica, the brightest star in Virgo. Spica actually consists of two stars 260 light years away from us, orbiting each other once every 4 days. Both are much hotter and brighter than the Sun. The brighter of the pair is 11 times as massive as our Sun and 13 000 times as bright, the other 7 times as massive as our sun and ‘only’ 1 700 times as bright.

Red Antares and the stars of the Scorpion are almost overhead in the early evening, and you should easily be able to see the shape of the Scorpion – this is one of the few constellations which really resembles the creature it’s named after. Antares is about 600 light years away, and radiates about 10 000 times as much energy as the Sun at wavelengths perceived by the eye. But Antares has a surface temperature of only 3 300 degrees (compared to 5 500 for the Sun), and radiates most of its energy as infrared radiation. The total energy output of Antares is about 65 000 times as much as the Sun, and this ‘red supergiant’ is so large that if it were placed at the centre of our own solar system, the orbit of Mars would be less than halfway from the centre of Antares to its swollen surface.

High in the south are the Giraffes of Venda starlore (the Pointers – Alpha and Beta Centauri – and the stars of the Southern Cross). For observers away from city lights, the winter Milky Way is spectacular on August evenings when the moon doesn’t interfere. The centre of our Galaxy is nearly overhead, and it is easy to see the ‘pancake’ shape of our wheel of hundreds of billions of suns, complete with the bulge near the middle. For the Indian tribes of South America, the dark patches in the Milky Way were also constellations. We know today that such dark areas as the Coalsack (near the Southern Cross) and the Great Rift are dense dust clouds where new stars are forming.

The Morning Sky Stars

Orion is once again prominent in the eastern sky, raising his club to swat the pesky Bull that forever charges at him. Orion’s two dogs are with him as well, with Procyon (brightest star in the Small Dog) lower in the east and Sirius (brightest star in the Large Dog) to the southeast of Orion. It’s unclear, however, whether the dogs are paying attention to Orion’s problems or to the hare which is hopping by between Orion and the Large Dog.

High in the southeast shines Canopus, second brightest star in the sky and brightest star in the huge ancient constellation of Argo, the great exploring ship that sought the Golden Fleece. Today it’s split up into several smaller constellations such as the Sails and the Keel. If the Argo is headed to or from Cape Town, it’s only logical that Table Mountain is in view, and it is just possible (on a dark night far from city lights) to see the dim stars of the Table Mountain (Mons Mensae) constellation in the far south. Thanks to Nicolas de Lacaille, who added more than a dozen constellations to the southern sky as he observed from Cape Town, Table Mountain is the only geographical feature on Earth to have its own place in the stars. You’ll probably find it easier to see the Tablecloth, represented by the Large Magellanic Cloud. The LMC, as astronomers know it, is a satellite galaxy of our own Milky Way, and a mere 180 000 light years away. It’s the closest galaxy to our own which is not actually being ‘digested’. Our Milky Way is cannibalizing two small galaxies at the moment, and neither is fully separate anymore. Part of the LMC (which looks to the eye like a stray patch of the Milky Way) slops over into the nearby constellation of the Swordfish, and the remainder of the southern sky is taken up with an odd collection of birds, water creatures and scientific instruments such as the Peacock, the Crane, the Water Snake, and the Rhomboidal Net. The instruments, like Table Mountain, owe their place in the sky to Lacaille’s mapping of the southern sky in the mid-eighteenth century.

Flowing from the southwest corner of Orion is the long winding constellation of Eridanus the celestial river, with bright Achernar near its ‘mouth’ high in the southwest. Canopus was ‘Naka’ (the ‘Horn’ star), in the Sotho calendar, while Achernar was ‘senakane’, the ‘little horn’. To the west of Eridanus is a rather watery part of the sky including the Whale, the Fishes, the Water Bearer and the Sea Goat. Fomalhaut (meaning ‘mouth of the southern fish’) is the brightest star in the western sky, but almost all the bright stars in this month’s morning sky are in the east. It’s relatively nearby (only 25 light years away) and a mere 200 million years old, making it just a baby compared to the Sun. A huge disk of icy dust, four times the diameter of our solar system, surrounds Fomalhaut. There is a clear area around the star itself which may have been caused by the formation of a system of planets, but the actual planets elude detection so far. It’s quite likely that there are planets.

Dr Nicola Loaring.

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