Astronomical data for September 2009

NEW! Lunar Occultations information page here

The sun begins the month in the constellation of Leo, but on the 16th at around 17h40 crosses the astronomical border which separates Leo from Virgo, in which constellation it remains till the month ends. The Autumnal Equinox on September 22nd at 21h19 marks the official start of autumn. Day and night are almost equal in length again, and in the northern hemisphere, autumn lasts for 89.85 days. The earth-sun distance at this time is 150 125 903 km.

September is the best month to observe the ethereal zodiacal light during the early mornings when the moon is not present in the sky and you are well away from light pollution. Look towards the east before the onset of morning twilight, around 03h30, and you should see a faint cone of light pointing southwards at a steep angle of 60°. This phenomenon is caused by the sun illuminating a disc of fine dust, the remnant of solar system formation 4.5 thousand million years ago. The best days to observe this morning cone are from the 19th to the end of the month.

The Moon The moon is at perigee at around 08h00 of the morning of the 16th, and at apogee, its farthest from the earth, around 03h00 of the 28th, The Full Moon on the 4th in the early evening is situated near the Aquarius Pisces border. In farming circles, this was considered to be the Harvest Moon. Before the days of mechanisation, harvest was a long process and the labourers would have to work well into the night. For several nights before and after Full, the moon rises at about the same time each evening as the sun sets and so its light enabled the harvesters to continue their labours into the night. Look for the Harvest Moon rising almost due east just after 18h.
One of the highest Last Quarter moons of the year occurs on the 12th just after 02h00 and is in the constellation of Taurus. It will be at its highest at 06h on that day. New Moon is on the 18th just before 19h00 on the Virgo Leo border, the moon passing 5° south of the sun. First Quarter moon is on the 26th at around 05h00, low down in Sagittarius.
Earthshine may be seen on the dark hemisphere of the waning crescent moon from the 14th to the 17th.

The Planets
Mercury will not be seen until the second half of September, inferior conjunction occurring on the 20th. Thereafter it moves rapidly towards its best apparition in the morning sky early next month. If you look in the eastern sky on the 30th, you should be able to see Venus and Mercury at 05h15, just before the onset of civil twilight; Venus is 14° above the horizon, and Mercury is 6° above the horizon at that time. If you imagine a line from the east point on the horizon to Venus, Mercury is a third of the way along this line. Given a clear sky at that moment, scan with your binoculars just to the north of the east point, and you may possibly glimpse Saturn, 2° above the horizon as it begins to reappear in the morning skies.

Venus is now beginning to move in towards the sun, rising three hours before it at the beginning of September, but only two hours before sunrise at the end. The planet remains the brightest object in the morning sky. The waning crescent moon is in the vicinity of Venus during the mornings of the 16th and 17th. Earthshine on the night hemisphere of the moon will enhance the spectacle, as the two objects pass by each other. On the 16th, the moon is some 6° to the right of Venus, but on the 17th, it is just over 9° below Venus.

Mars spends the month moving eastwards through Gemini and by the end of September forms an interesting grouping with Castor and Pollux. The planet continues to brighten in the morning sky, and is as bright as Betelgeuse in Orion at the month's end. The planet rises at around 23h at the start of September, and by 22h30 at the end. On the 13th the broad waning crescent moon may be seen approaching the planet and by the onset of twilight the two are separated by 6° with Mars to the lower left of the moon.

Jupiter can be seen for most of the night as a bright steadily shining object in eastern Capricornus, achieving an altitude of approximately 20° when it crosses the meridian during the late evening. It is still moving towards the west as the earth overtakes it on the 'inside lane'. By the end of the month Jupiter sets at around 01h. There are two conjunctions of Jupiter with the moon in September. The first of these is on the 2nd, with the waxing gibbous moon 2° above the planet; the second conjunction on the 29th, looks similar.

Except as mentioned above, there is no chance of observing Saturn during September, as the planet is in conjunction with the sun on the 17th at 18h, and around this time the earth passes through the ring plane.

Uranus is at opposition and its nearest to the earth at 09h on the 17th just south of the Pisces 'Circlet', in the constellation of the "Fishes", just half a degree north of the Pisces Aquarius border. Its magnitude is 5.7 and therefore appears as a very faint star-like point. A small telescope will show a tiny grey-green disc. Neptune at magnitude 7.8 is in eastern Capricornus, below naked eye visibility, and is currently 2° north of Deneb Algiedi (Delta Capricorni), the constellation's brightest star.

Two minor meteor showers take place at the end of the month. They are the Capricornids, which are active from the 10th of July to the 5th of August, peaking around the 30th. On the 28th, we have the maximum of the Delta Aquarids, with up to 20 an hour, although the shower is active from the 15th July to the 19th of August.

A minor meteor shower peaks on the 1st, when just before morning twilight begins, you may see up to 8 Alpha Aurigid shooting stars an hour. The radiant, or apparent origin, is near to the star Capella (Alpha Aurigae). There will be interference from the moon until the early hours of the morning of the next day, when it sets.

Constellations visible in the south around midnight, mid-month, are as follows: Aquarius, the western fish of Pisces, sometimes known as 'The Circlet' because of its shape, and the large autumn square of Pegasus. Almost overhead are the great galaxy in Andromeda and the 'W' shaped constellation of Cassiopeia.

All times are GMT 1° is one finger width at arm's length.

Sky Notes are prepared by : John Harper