Want to know what’s up in the sky tonight? Our night sky chart will provide you with the information you need to locate the brightest planets, stars and deep-sky objects …
This month’s chart shows the night sky looking South in mid August at 2300. (click on image for larger view)
The Perseids
On any dark night, you should be able to see about ten naked-eye meteors per hour.
However, when the Earth passes through a comet’s tail or the debris left by a periodic comet, the number of meteors seen is greatly increased. We enter cometary debris during the last week in July and continue to pass through for a couple more weeks.
When the debris enters the Earth’s atmosphere they become meteors. They charge at the Earth at very high speed (72 kilometers per second) producing a great deal of heat by friction. This heat is so intense that it ionizes the atoms and molecules in the air surrounding the meteor, causing it to glow by incandescent light. (This light is the same as produced in an incandescent light bulb.) An average meteoroid becomes a meteor at about 115 kilometers above the Earth’s surface and it usually burns out at an altitude of about 70 kilometers as it breaks up into tiny particles called micro-meteors.
The name of the meteor shower that will occur during this month is called the Perseids – otherwise known as ‘Old faithful’ because it never fails to impress.The best time to view the show is after local midnight as the radiant, from which the meteors appear from, is starting to climb high into the sky.
The meteors appear to radiate from a single point of the sky. This happens because the Earth intercepts the meteoroids in a series of parallel paths. To the observer, this point is like looking at a railway line as it appears to meet in the distance. The best time to view the Perseids is at their peak, on the night of August 11-12, when you can expect to see nearly one Perseid meteor a minute. However, if the weather looks better a few days either side of the peak date, it is worth observing then too.
The shower will appear from a point in the constellation of Perseus. This constellation lies just below and to the left of Cassiopeia.
Sagittarius, the Archer – this month’s sky highlight
Sagittarius the Archer is a zodiacal constellation that contains the ‘Teapot’ asterism.
To view Sagittarius you’ll need a very open Southern horizon. This is not the easiest of observations to make, but if you can succeed, it’s well worth the effort. Let’s start at 2130. It’s still not dark, but at this time the constellation is directly due South. The teapot we’re looking for is only 10 degrees above the horizon. Firstly find Altair, the lowest component of the Summer Triangle, as described last month. With this star in your vision, move straight down approximately 10 degrees, which is the equivalent to a fist held vertically at arms length in front of you. Here you should find a faint-ish star called Eta Aquilae. Now travel in a five o’clock direction for 30 degrees and you should see a wide grouping of stars in line with the direction you’re moving. The brightest star here is called Nunki and forms part of the handle of the teapot.
Using binoculars, go from Nunki to the left, in a three o’clock direction for 15 degrees. As you move along you should sweep up a large-ish, elongated misty patch. This is M8, the Lagoon nebula. It is a magnificent emission nebula with an embedded open cluster. This cluster of young stars is heating the nebula’s gas and causing it to emit light. In binoculars the dark lane (the lagoon) that divides the nebula’s brighter regions and gives this object its name might be apparent, but it is very much dependant on sky brightness. A small telescope begins to reveal this nebula’s intricate folds and dark regions amidst brighter areas. Dark ‘Bok globules’ in the nebula mark dense clouds of gas and dust which are the site of star formation. The Trifid nebula, M20, lies close (above and to the right of) M8 and both nebulae can be seen in the same binocular field of view.
The Trifid Nebula gets its name from the rifts of dark matter that trisect its southern region. The Trifid lies in a rich area of the Milky Way, with M21 (to the left in a 10 o’clock position) in the same binocular field of view. This makes a stunning sight and is best seen at low power. M20 and M8 may be physically connected, although this is uncertain. The Trifid Nebula is a hot red emission nebula of hydrogen gas surrounded by a blue reflection nebula made up of dust grains. Don’t expect to see this colour with your eyes – it’s only apparent in long exposure images.
Let’s end our tour at M21. This bright cluster contains about 60 stars that are readily visible in a small telescope. M21 has a strong concentration of stars towards its centre and makes an impressive sight in binoculars along with M20 and M8. In larger telescopes, a ‘diamond ring’ of 10 or so stars can be seen. This is an extremely young cluster in astronomical terms at only five million years old.
If you have succeeded in this observation, give yourself a pat on the back! Finally, when you are ready to leave Sagittarius take note that you have just spent time looking into the heart of our own galaxy, the Milky Way.
In history …
Saturn’s moon Enceladus was discovered on August 28 1789 by William Herschel.
On the night of August 5 1864, Giovanni Donati made the first spectroscopic observations of a comet. He found mysterious bands in this comet’s spectrum that we now know are produced by molecular carbon.
On August 18 1868 Norman Lockyer discovered helium in the Sun’s spectrum. Actually, on that day he discovered the bands of an unknown element in the Sun’s spectrum and named that new element “helium” after the sun-god “Helios”. Years later, earthbound helium was isolated and its spectrum confirmed Lockyer’s discovery.
Asaph Hall of the US Naval Observatory discovered Mars’ small satellite, Deimos, on the August 11 1877. Six days later, August 17, 1877, he found Mars’ other moon Phobos!
The first photograph of the Earth was taken from orbit by Explorer 6 on August 7 1959.
On August 7 1996, NASA scientists announced that they had found structures and chemicals in the Martian meteorite ALH84001 suggesting these could be Martian micro-fossils. This ‘evidence’ has been disputed and most experts in the field believe the data is inconclusive.