Welcome to the September night sky. This is the month of the autumn equinox, when the sun passes the midway point in its journey down from the highest level in summer, to its lowest in midwinter. This means that the whole world sees 12 hours of daylight and 12 hours of darkness. It also means that the sun rises exactly in the east and sets exactly in the west on the equinox day (September 22).
Something special happens to the full moon at the equinox time. Remember that a full moon is directly 180 degrees opposite the sun, so this requires the moon to be on, or near, the equator of the sky, where the inclination of the ecliptic is at its steepest. The most noticeable effect of this is that the full moon rises at practically the same time (sunset) for several nights consecutively. This is called the ‘harvest moon’, and was a great help to farmers in the past, as it provides light well into the night useful for night-time crop picking. With luck, last month you will have seen quite a few meteors. This was due to the Perseid Meteors shower, centred around August 12.
This month, we have less ‘shooting’ stars, but we will get a wonderful view of the Milky Way right overhead soon, when, after the sunset, the moon is not in the sky. This is a rare sight nowadays in built-up areas with light pollution. This is also a shame, as the Milky Way is our galaxy seen from the inside – a galaxy of around 200 billion stars.
September has no bright planets visible in the evening sky, but, if you are an early riser, the brilliant planet Venus is unmistakable in the eastern pre-dawn sky. The thin crescent moon was close to Venus on the morning of September 10 and, around that date, you may have caught a glimpse of the elusive planet Mercury. This planet was close to the eastern horizon one hour before sunrise.
The moon was at last quarter on September 6, the new moon was on September 14, the first quarter is on September 21 and the full moon is on September 29.
• The Astronomical Observatory of Tavira specialises in education and public outreach.Email: cdepa@mail.telepac.pt; homepage: http://www.cdepa.pt
