Autumn is coming: slowly but surely the sun is receding and already from nine o'clock in the evening it is dark night. Star lovers are delighted, because at last there is enough time again to discover the beauty of the night sky, including faint objects such as the Milky Way. We have put together the highlights of September here. For details and further observation tips, please refer to our Sky Guide for September 2020.
Double full moon on 1 and 2 September
Right at the beginning of the month the first highlight awaits us: We can enjoy two full moon nights. Strictly speaking, the moon is not yet completely full on the evening of September 1st, but only in the morning of September 2nd. However, on the first night of September the moon's mini-dent is not visible to the naked eye, so we can calmly speak of two full moon nights.
The beginning of autumn and the equinox fall three days apart
September is the beginning of autumn and the month of the equinox, but both events do not fall on the same day. On 22 September this year, the sun passes the autumnal point at exactly 15:31. In other words, its orbit intersects the celestial equator on this day, and the sun is exactly vertical above the equator. However, the day is still nine minutes longer than the night. The equinox will not be reached until 25 September 2020. How come? The upper edge of the sun's disc determines the exact time of sunrise and sunset. It does not appear above the horizon until 7:05 a.m. on the 25th of this month and disappears below the horizon at 7:05 p.m. on the evening of the same day. Day and night last exactly 12 hours. After that the days become shorter and shorter, the nights longer. Already at the end of the month it stays dark a quarter of an hour longer than on 22 September. Overall, the length of the day shrinks by one hour and forty minutes from September 1 to September 30.
Four bright and two almost invisible planets accompany us through the night
Four bright planets accompany us through the night in September: in the evening the duo of Jupiter and Saturn, followed by the ever brighter Mars, which dominates the firmament all night, and the morning star Venus.
Although the duo of the two gas giants Jupiter and Saturn no longer shines as brightly as in July, it is still clearly visible at dusk. The brighter of the two is Jupiter, by the way, which with its apparent brightness of -2.4 mag outshines Saturn (0.5 mag). Later, when it is really dark and the horizon is clear, the constellation Capricorn appears to the left of the two constellations, and the delicate band of the Milky Way appears to the right. But the star of the night is Mars. It has moulted more and more in August and is already very conspicuous from about 22:00 at the beginning of September with an apparent brightness of -1.8 mag. In the course of the month our neighbouring planet becomes brighter and brighter and bigger - it approaches its opposition - and at the end of September it reaches an apparent brightness of -2.5 mag, which makes it brighter than Jupiter.By the way: Mars has two companions. However, Neptune and Uranus are only for experienced observers, because you need a telescope to observe the two outer planets of our solar system. You will find Neptune about 3.5 hand width above Mars, Uranus about 1.5 hand width below Mars.
One planet, however, still trumps Mars in terms of brightness: morning star Venus, which appears above the horizon from about 3 a.m. onwards, is the brightest light in the night sky with an apparent brightness of about -4.2 mag, except for the Moon.
Autumn constellations take a stand
But others will also make their big appearance in September. Pegasus, Andromeda and Perseus announce the coming autumn: High in the south, the typical autumn constellations have already taken position at midnight and the stars of the autumn quadrangle - Algenib, Scheat, Markab and Sirrah - are easy to spot. In the south-east, the winter hexagon, especially Orion, already heralds the approaching cold season, while the summer triangle is slowly receding in the south-west.
In mid-September you will also have another chance to marvel at the Milky Way: the galactic centre is just above the horizon. The best time to see the Milky Way is around 17 September, as the new moon nights offer an uninterrupted view of our home galaxy.
Our Sky Guide tells you what else is going on in the night sky in September 2020 and when you can observe which celestial objects.