Photo: NASAJPL-CaltechSpace Science Institute

The September 2021 Night Sky

Welcome to the September night sky. Now that the long hot nights of summer are coming to an end, we are approaching the autumn in the northern hemisphere, with the Equinox being on the 22nd of the month.

On that day, the Sun crosses the celestial equator and, in this case, its heading south. This time of the year, the Movement of the Sun in declination (altitude) is at its greatest, so it becomes noticeable that the nights are drawing in.

When it is fully dark in September, the gas giant planets Jupiter and Saturn still dominate the southern sky.

Jupiter is by far the brightest and its four large Moons discovered by Galileo-Galilei in the early 1600s are easily visible using any optical aid.

The ringed planet Saturn is a must-see for anybody with a telescope – magnification of 30x or more. Its wonderful ring system is an unforgettable sight when first seen either by young or old.

The summer triangle has now passed the meridian and is moving towards the west. High overhead in September evening skies is the constellation of Pegasus “the Flying Horse” with its four bright stars making up the square of Pegasus. The brightest star of the four is called Alpharatz and it is a young white star brighter than our Sun and 97 light years away.

Many stars name start with AL, and this is of Arabic origin as is the AL in Algarve. The faintest star of the four is Algenib. This star is about 1000x the mass of the Sun and around 6000x times brighter. Algenib is also the most distant star of the square of Pegasus located at about 400 light years from us.

One of the most famous stars in Pegasus is called 51 Pegasi which has an Exoplanet orbiting around it and this is the first Exoplanet found in orbit around a relatively Sunlike star in 1995. This star appears relatively faint to us at the 5th magnitude, so it is just visible without a telescope, and it is 50 light years away.

There are at least six observable meteor showers in September starting on the 1st with the Aurigid meteors. There are also showers on the nights of the 6th, 7th, 14th, 21st and 23rd. As with almost all meteor showers, they are best seen from a dark sky location when the Moon is not too bright and after midnight.

The Moon is new on the seventh, first quarter on the 13th, full on the 23rd, last quarter on September 29.

By Clive Jackson
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Clive Jackson is the director of the Camera Obscura attraction (next to the Castle in Tavira), specialising in education and public outreach.
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To see the September Sky Map click on the pdf link below

2021-09 September nightsky