The beginning of autumn in the starry sky:
Equinox, two galaxies, three bright planets and a striking square of stars
Autumn is here: slowly but surely, the sun is receding and by nine o'clock in the evening it is already dark night. Stargazers will be pleased, because there is finally enough time to discover the beauties of the night sky, including faint objects like the Milky Way. We have compiled the highlights for the month of September here. For details and further observing tips, please see our Sky Guide for September 2021
Watch out for shooting star hunters in early September
The first highlight was last night, when the Aurigids stream of shooting stars reached its maximum shortly before midnight with a violent outburst of around 100 shooting stars. Those who missed them, be comforted: Next week, on 9 September, the September Perseids will reach their peak. Although this is by no means comparable to the August Peresids, it is the last stream of shooting stars until December.
Around five meteors per hour are sent by the September Perseids - and this year you might be lucky to spot some of them: By the time the constellation Perseus, where the radiant of the shooting stars lies, is at a favourable altitude from 23:00, the slender crescent moon has long since set (around 21:00) and observations will not be disturbed by moonlight.
Strikingly bright square of stars
And once you've spotted Perseus, Pegasus and Andromeda are not far behind. The three constellations herald the coming of autumn. High in the south, the typical autumn constellations have already taken up position shortly before midnight and the bright stars of the autumn quadrangle - Algenib, Scheat, Markab and Sirrah - are easy to spot. Three of them belong to the constellation Pegasus, only Sirrah is from the neighbouring constellation Andromeda. This conspicuous constellation moves from northeast to southeast until the end of October.
View of our neighbouring galaxy
Speaking of Andromeda, the constellation to the left of the autumn quadrangle gives a glimpse of our neighbouring galaxy. The Andromeda Galaxy is visible as a nebula spot (hence the galaxy is also called the Andromeda Nebula) in cloudless skies and from a dark location, as it appears almost twice the size of the full moon. This corresponds roughly to the width of 4 fingers on an outstretched arm. Despite this size and its relative proximity to us, the light from the galaxy takes about 2.5 million years to reach us. So we see the galaxy as it actually looked in the earliest Stone Age. But the spiral cluster of stars or dust bands can only be seen with the help of a telescope.
Home galaxy in full glory
Our home galaxy, the Milky Way, can now once again be marvelled at in all its glory in September - ideally around 7 September, because the new moon nights allow an uninterrupted view of the galactic centre of the Milky Way.
Three bright planets and two secret visitors
To the left of the Milky Way, you can easily make out two bright points of light: These are the two planets Jupiter and Saturn. The conspicuous pair of planets is close together and looks like a double star. The left, conspicuously brighter of the two planets is Jupiter, the smaller one on the right is Saturn.
Jupiter, however, with its apparent brightness of -2.8 mag, has to admit defeat to Venus. Our inner neighbour planet, with its apparent brightness of -4 mag, is so low on the south-west horizon shortly after sunset that it is almost overlooked.
For experienced observers who own at least a good pair of fast astro-binoculars or even better a telescope, Neptune and Uranus will also show themselves. At the beginning of the month you can spot Neptune above the eastern horizon from about 10.15 pm, Uranus follows at about 11 pm. At the end of September, it is already dark enough to make out both planets from half past eight.
The beginning of autumn and the equinox fall three days apart
September is also the beginning of autumn and thus 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 9.21 pm. Or to put it another way: its orbit intersects the celestial equator at this time and the sun is exactly perpendicular above the equator. However, the day is still nine minutes longer than the night. The equinox is not until 25 September.
Why is that? The upper edge of the solar disc is decisive for the exact time of sunrise and sunset. It only appears above the horizon at 7:05 a.m. on 25 September and disappears below the horizon at 7:05 p.m. in the evening of the same day. Day and night therefore last exactly 12 hours only on 25 September.
After that, the days become shorter and shorter, the nights longer and longer. By the end of the month it will be dark a quarter of an hour longer than on 22 September. In total, the length of the day shrinks by one hour and forty minutes from 1 September to 30 September.