The Night Sky in September 2006

Looking at the night sky with the naked eye or using a small telescope can be fascinating, so each month these notes keep you up-to-date with unfolding events.

Maps of the current night sky can be seen by clicking HERE

Phases of the Moon in September


New Moon Friday 22	First quarter Saturday 30	Full Moon Thursday 7	Last quarter Thursday 14

Click HERE to see the phase of the Moon on each day of the month.

Monthly Highlights

Sadly, the summer holidays are nearly over and no doubt you'll be heading back to school shortly. As we move on into Autumn the nights will start drawing in, and once past the Autumnal Equinox on September 23rd, there will be more nightime than daylight - more about that later. The good news is that we don't have to stay up too late to get a good look at the night sky, which means we have a chance of spotting one of the following monthly highlights:

September 23rd - Autumnal Equinox

The autumnal equinox marks the mid-point between the summer solstice (longest day) and winter solstice (shortest day), which occur in June and December. In physical terms, an equinox is the point during Earth's 365 day orbit, where the tilt of its rotation axis is at right-angles to the Sun, i.e. not tilted toward it, as in summer, or away from it, as in winter.

Animation of Earth's tilt in Summer and Autumn

This all means that at equinox we have around 12 hours of daylight and 12 hours of nightime. There is another equinox in the spring (around March 21st), known as the Vernal equinox. This occurs when the Earth has moved round in its orbit and is once again starting to lean towards the Sun. The word equinox comes from a Latin word meaning "equal night".

If you find equinoxes a bit confusing or you wish to learn more about our seasons, then try following this link.

The Planets

Click HERE and start the Electric Orrery to see where all the planets are today in relation to each other.

Mercury

Mercury starts the month on the other side of the Sun, having reached superior conjunction (other side of the Sun to Earth) on September 1st. It will start to emerge from the Sun's glare within a few days as an evening object, and by the end of the month will be found low above the western horizon for around 30 minutes after sunset. Because Mercury is always low down when the Sun is about, it is one of those objects that the Liverpool Telescope cannot safely observe.

Venus

Venus continues to move slowly around the far side of the Sun, but will remain a morning object and the brightest planet during September. It will finally reach superior conjunction (other side of the Sun to Earth) on the 27th October, before becoming an evening object toward the end of the year. You can still spot Venus during the morning twilight by looking low above the east-northeastern horizon around 60 minutes before sunrise. Take care not to confuse it with a much dimmer Saturn which rises around 30 minutes beforehand. Why not use the electric orrery to see how Mercury, Venus and Saturn are all on the opposite side of the Sun to Earth just now.

Mars

Mars is another one that will pass behind the Sun during October. Because we orbit closer to the Sun, we travel more quickly than Mars and will therefore be catching it up. This means that Mars will go from being an evening star to a morning star, unlike Venus which travels faster than both Earth and Mars and does the reverse. You can still spot a dim Mars low towards the west-northwest for around 30 minutes after sunset, although this will decrease during the month. Now only slightly brighter than most of the stars around it, Mars will be noticeable only by its slightly red hue and the fact that it moves from night to night. Sadly it will be next year before we get detailed images again.

Jupiter

Jupiter remains the best target to observe, but even it is decreasing in both size and brightness as we continue to move away from it on our inside track. By the middle of the month, Jupiter will be found low above the south-western horizon around sunset and remains there for nearly 90 minutes before setting in the west around 9pm. September is probably your last chance to get a resonable image of Jupiter using Go Observing.

Saturn

Saturn starts the month above the eastern horizon, appearing a good 2 hours before sunrise. It is currently another of the planets on the far side of the Solar System, making this side a little lonely for poor old Earth. See what we mean using the electric orrery. By the month's end we will have caught it up a little, with it rising 3 hours before the Sun. By next month it will start to become a decent target to observe once again using the Liverpool Telescope.

Constellation of the month

Each month, a new constellation is highlighted for you to identify in the night sky.

This month we will be looking for the constellation of Cygnus, the Swan, which can be found soaring high above your heads in the night sky. It can easily be found because it lies within the famous summer triangle of the stars Vega, Altair and Deneb; although mind you don't fall over backwards from staring up for too long. In fact Deneb also marks out the head of the swan and is the most northerly of the triangle stars.

Look for the shape of a cross, which represents a swan with wings outstretched. Not surprisingly, the constellation is also known as the Northern Cross.

September Night Sky

One of the more notable objects within the constellation is Cygnus X-1, the first black hole to be discovered by astronomers, way back in 1972. Cygnus X-1 stood out because of the very high levels of x-rays (another form of light) coming from it. We now know that Cygnus X-1 is a binary star containing a supergiant star over 20 times heavier than the Sun and a compact object around 10 times the mass (i.e. 10 solar masses) which orbit each other every 5.6 days.

Artist's impression of material from a nearby star falling onto a black hole

As we can only see the supergiant in normal light, we know that the unseen object must be the remains of a supernovae. However, astronomers have worked out that the largest possible mass of a neutron star can not be more than 3 solar masses, so Cygnus X-1 must be a black hole. The X-rays are produced in a swirling disk of material flowing from the supergiant into the black hole - a bit like water going down the plughole. As the material approaches the black hole it collides with other material and heats up due to friction. It eventually gets so hot that it emits X-rays, unlike most stars which are much cooler.

Mythology: Cygnus

There are many different myths relating to the naming of the constellation of Cygnus, some dating back to pre-Greek times. However, perhaps the most well known of these myths is that the constellation represents Zeus in the story of Leda and the swan, Leda being the very beautiful wife of King Tyndareus, of Sparta.
Zeus, noted for his affairs with mortal women, fell in love with Leda and changed himself into a swan in order to seduce her. She produced an egg from which emerged Castor, the mortal son of her husband, and Polydeuces (Pollux), immortal son of Zeus.