The Night Sky in October 2007

Last quarter Wednesday 3rd	New Moon Thursday 11th	First quarter Friday 19th	Full Moon Friday 26th

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

What's happening in the October night sky?

With the approach of winter, the trees are starting to lose their leaves as the colder nights really start to draw in. The clocks going back an hour on the 28th October signals the end of British Summer Time, and it won't be long after we get home from school before the Sun is scurrying over the horizon. The good news is that we will have more opportunity to get out in the back garden and enjoy the wonders of the night sky before it gets too late. Perhaps we'll even get to see one of the following monthly highlights.

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

October 11th - A Celestial Gathering

If you can brave an early morning on the 11th, you will be rewarded with a little gathering of bright stars and planets in the same part of the sky. The planets Venus and Saturn will form a small triangle with the bright star Regulus, and will all rise in the east at around 3:30am. The Sun will be up around 7:30am, so you have a good few hours in which to spot this gathering.

Meeting of Venus, Saturn and Regulus

Whilst Venus and Regulus will be around the same brightness, they will both be outshone by Venus, which will appear around 6 times brighter. In fact it will actually be around 200 times more powerful but for the fact that our eyes do not react to light in a linear way. For example, if we were to look at a 20 watt and 100 watt lightbulb some distance away, even though the 100 watt bulb is pumping out 5 times the amount of energy, our brain would interpret what our eyes could see as it only being around twice the brightness of the dimmer bulb.

Regulus is the brightest star in the constellation of Leo and one of the brightest stars in the night sky. It is approximately 77.5 light years from Earth and about 3.5 times more massive than our Sun. By comparison to other stars it is a young star, at only a few hundred million years old. It is also known to be hot, blue-ish in appearance and spinning extremely rapidly, with a rotation period of only 15.9 hours compared to the Sun's 27 days.

October 21st - Peak of Orionid Metor Shower

At certain times of the year, the Earth passes through a trail of dust and debris that was left by a passing comet many years ago. When it does so, we see an increase in the number of meteors or shooting stars as debris enters and burns up in the Earth's atmosphere due to friction. When these events occur they are known as meteor showers. The cause of the Orionids is actually the most famous of comets, Halley's Comet, which orbits the Sun every 76 years and has been seen and recorded by astronomers since 240 BC, making its last appearence in 1986.

Stellarium image showing Orionid radiant, where the meteors appear to start from.

The left-over comet trail is quite extensive; so although there's an increase in the number of meteors from the 15th October onwards the peak of the Orionid meteor shower occurs overnight on the 20th/21st October before reducing again toward the 29th. During the peak, you can expect to see around 20 fast meteors per hour. These will appear to originate close to the star Betelgeuse in the constellation of Orion. The best time to observe will be after 1am, once the Moon has set over the western horizon and Orion has risen in the east. The resulting dark sky should provide excellent conditions (weather permitting) for seeing orionid meteors.

What are the planets up to?

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

Mercury - starts the month very low above the west-southwest horizon shortly after sunset. Although it sets 30 minutes later, being so close to the horizon means it will be very difficult to pick out from the twilight. After passing greatest elongation (separation from the Sun) on the 29th September, Mercury will now come round to pass inferior conjunction (between Earth and the Sun) on the 23rd October before then becoming a morning object. By the end of the month it can be found in the east-southeastern sky for around 90 minutes before sunrise. Venus - shines clear and bright in the eastern sky for several hours before sunrise throughout the month. It is the brightest object in that part of the night sky, so you will have no trouble finding it. On the 1st, Venus rises in the east-northeast around 3am, nearly 4 hours before the Sun. With the clocks going back at the end of the month, the rising time will be around 3:30am, with the Sun following 4 hours 20 minutes later. Mars - rises in the east-northeast around 10:30pm at the start of the month and by sunrise can be found very high above the southern horizon - almost over our heads. During the month we continue to catch Mars up because our closer orbit to the Sun means we are travelling faster. This in turn means that the rising time of Mars will get earlier and earlier, such that by the end of October it will rise around 9pm, but still in the east-northeast. If you are not sure why these times get earlier then try looking at how things change during October using the Electric Orrery. Remember that the Earth spins anti-clockwise every 24 hours in the orrery's view of our Solar System. Jupiter - can be found low above the south-southwest horizon shortly after sunset at the start of the month, and sets in the southwest around 9:15pm. Although it is low to us, the Liverpool Telescope being located off the coast of Africa means that Jupiter will appear much higher than we see it and can therefore still be observed using Go Observing. By the end of the month, Jupiter will set in the southwest around 7:30pm, which is about 2 hours after sunset. Saturn - starts the month rising fairly close to Venus in the east-northeast at around 3am. It is then visible over the eastern horizon for a little over 3 hours. Although close to Venus, Saturn will be much dimmer and therefore harder to pick out. It will be the only other object around that appears to shine more steadily (i.e. not twinkle as much) than the stars around it. By the 15th of the month, Saturn will have reached its closest point to Venus in the night sky, before moving away again. By the end of October, Saturn will be rising in the east-northeast around 2am; a good 6 hours before the Sun comes up. We are slowly catching Saturn up, so it will become a much better target to observe in the coming months.

Constellation of the month

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

At this time of the year we are going to re-visit an old favourite called the constellation of Pegasus, the winged horse of the gods, which can be seen high in the southern sky close to midnight. The easiest way to find Pegasus is by looking for four bright stars that make up the shape of a box, known as the Great Square of Pegasus.

October Night Sky

By area, Pegasus is the 7th largest of the 88 modern constellations and contains 5 bright stars. The constellation has become more studied in recent years because it is home to the first extra-solar planet ever discovered, which orbits the star 51 Pegasus every 4.2 days (Mercury orbits every 88 days). The word extra-solar means that the planet is outside our own Solar System. Such a short period means that the planet orbits very close to its parent star, which was totally unexpected by astronomers. The planet is now known to be a gas-giant planet, like Jupiter or Saturn, with an atmospheric temperature well over 1000°C.

Planet Wobble

The planet, known as 51 Pegasus b, has never actually been seen, but was discovered due to the gravitational influence of the planet on its parent star. In other words the planet makes its star 'wobble' about as the star-planet system circles around a common centre-of-mass - just like in the animation above. Astronomers are able to spot this wobbling motion toward and away from us, called the star's radial velocity, by observing its spectrum. Modern technology allows us to detect radial velocities of just 1 metre per second - a fast walking pace. Jupiter causes the Sun to wobble by up to 12.5 metres per second, so it is no surprise that astronomers have since discovered hundreds of other extra-solar planets using this method.

If you want to learn more about extra-solar planets (or exoplanets) then follow this link.

Mythology: Pegasus

Pegasus was brought to Mount Helicon by Bellerophon and with one kick of his hoof, he caused the spring of Hippocrene to flow. Hippocrene is said to be the source of inspiration to poets. Bellerophon, who slayed the hideous beast Chimaera, became so headstrong that he ordered Pegasus to fly him up to Mount Olympus, the home of the gods. This impudence angered Zeus, who sent an insect to sting the winged horse, who bucked Bellerophon off its back. Needless to say, Bellerophon did not survive the fall to Earth. Pegasus went on to greatness, however, as the "Thundering Horse of Jove" who carried lightning bolts for Zeus.