Night Watch

Updated 1st August 2008

The Sagittarius star cloud. Image by Werner Kutsche
(more images by Werner Kutsche)

If you're brave enough to venture outside these cold winter nights, you'll be greeted by the heart of our Milky Way galaxy directly overhead. Find yourself a dark space in your backyard on a clear moonless night, and look straight up. You'll see the unmistakable form of Scorpius, the Scorpion, and the group of stars that make up the constellation of Sagittarius, the Archer. Look carefully in this area of the sky and you'll notice the misty glow of the countless millions of stars that are too far away for us to see clearly. Their light has journeyed for more than thirty thousand years to reach us.

As you scan the Milky Way, you'll notice vast rifts that don't seem to contain any stars. These rifts are actually enormous clouds of gas and dust that hide the Milky Way's stars behind them. It's not that there are no stars, they are just hidden from our view. The giant clouds of gas and dust will one day condense and form new stars.

Look low to the north. A beautiful blue-white star awaits. Vega is one of the closest and brightest stars in the night sky. It was one of the first stars whose distance was measured. The measurement has been refined since the 1800's, and today astronomers know the distance quite precisely: a bit more than 25 light-years. That means the light we see from Vega actually left the star a bit more than 25 years ago.

Astronauts, looking out of the space shuttle windows at night, see great clusters of lights; the glow of hundreds of thousands of individual lights that mark cities on earth. From the ground, we can see great clusters of lights in the heavens, too; cities of stars we call globular clusters. They're somewhat lost in the background of stars, but binoculars or telescopes reveal their true nature: spherical clumps of thousands or even millions of stars. Our Milky Way galaxy contains a couple of hundred globulars.

Omega Centauri. Image by Tony Virgo
(more images by Tony Virgo)

The greatest of these is Omega Centauri. Imagine a spherical region of space with our Sun at the centre and the outer edge four light-years away - the same distance as Alpha Centauri, our closest star system. Now imagine that this sphere contained a hundred thousand stars. The night sky would look alive with stars - bright stars in every direction, an overpowering cosmic light show. That's how the sky would appear if the solar system were transported to the centre of Omega Centauri.

You can see Omega Centauri with the naked eye, as a small fuzzy patch in the southwestern sky. It's visible to the unaided eye, but only binoculars or a telescope reveal its full glory; a dazzling city of stars.

Planets abound in the early evening sky during August. On the evening of August 2nd, only 30 minutes after sunset, look very low to the north-west and you'll find a very thin crescent Moon, with brilliant Venus not far above it. The following evening the Moon has climbed a little higher in the sky, and is joined by the ringed planet Saturn just above it. Look out again on August 4th, and the crescent Moon is joined by the red planet Mars, just to the right of it. Continue watching this dance of the planets over the ensuing nights. On the evening of August 15th, the planets Venus, Saturn and elusive Mercury all occupy an area less than 2 degrees wide.

If you keep observing higher into the sky on August 15th, you can actually bag yourself some more planets. Red Mars is higher, and almost overhead you can't miss the giant planet Jupiter. Come back outside a little later, about 10pm, and you can find the planet Uranus, low in the eastern sky, just below the constellation of Aquarius. In a very dark sky, you can see Uranus with the naked eye, but the light polluted city skies make it impossible to see it without binoculars. It looks like a tiny blue-green star. Use a star chart to help you find it.

The other planets all stand pretty much upright as they orbit the Sun. In other words, their poles point up and down. That's not the case with Uranus, though. Its poles point sideways. So instead of looking like a spinning top, Uranus looks more like a rolling billiard ball.

The most likely explanation is a big "whack." Collisions were common in the early solar system. Earth probably was a victim itself. A Mars-sized body hit our planet, vaporizing its outer layers and spewing gas and molten rock into space. This material coalesced to form the Moon. The Moon is New on August 1st, at First Quarter on the 9th, Full on the 17th, at Last Quarter on the 24th, and New again on August 31st.

A partial lunar eclipse occurs on the morning of August 17th. The Full Moon begins to enter the Earth's shadow at approx 4.00am, greatest eclipse begins at 6:40am, when the Moon is very low in the western sky.