The Night Sky This Month: September 2020

Summer Equinox
Image credit: NASA (APOD)

This year, the Summer Equinox occurs at 13:30 (UTC) on the 22nd of September. At this time, the Sun will be directly overhead at the equator, and the hours of light and darkness will be of equal duration throughout the world.

An equinox occurs at exactly the midway point between two solstices, as shown in the composite image above that was taken in Bursa, Turkey several years ago. The bottom band of Sun images was taken an hour apart during the winter solstice in 2007, while the top band of images was taken during the summer solstice in 2008.

Although the middle band was taken during the spring equinox (vernal equinox) in March of 2008, we will see the Sun in the same position(s) on September 22nd this year.

The Moon Phases in September 2020

Full Moon
Third Quarter
New Moon
First Quarter
September 2nd September 10th September 17th September 24th

The Planets in September 2020

Mercury will be very difficult to spot throughout the month since it will remain low on the horizon. Nonetheless, as September starts, the little planet may just be visible with binoculars or a small telescope east of the Sun just after Sunset.

Venus still dominates the predawn sky, having been at its greatest eastern elongation on the 12th of August. The planet rises about 3.5 hours before dawn at the beginning of the month, but its brightness will reduce from magnitude -4.3 at the start of the month, to about magnitude -4.1 by month’s end, while its angular diameter reduces from 19.5 seconds of arc to 15.6 seconds of arc. As a result, its brightness will remain relatively constant, even though its illuminated phase will increase from 60% to 72%.

Mars is now approaching its point of closest approach to Earth in many years, which it will reach during the first week of October. The Red planet starts September rising in the southeast at about 9:45 PM (BST), but it will rise progressively earlier throughout the month to transit the meridian at 2:00 AM (BST) at the end of the month. Now located in the constellation Pisces, the planet will reach an elevation of about 45% when it is due south, shining at magnitude -1.8, which will increase to about magnitude -2.5 as its angular diameter increases from 18.9 seconds of arc to 22.4 seconds of arc. Even small telescopes should therefore be able to resolve major surface features such as Syrtis Major if seeing conditions allow.

Jupiter is still visible in the south after sunset as it transits the meridian at about 9:30 PM (BST), but sadly, it will not rise above about 16 degrees above the horizon throughout the month. The most notable aspect of observing Jupiter during September is that it will end its retrograde (western) motion on the 12th of the month to resume its eastward motion across the sky. Nonetheless, observers who have access to an atmospheric dispersion corrector might be able to obtain reasonably clear views of the Great Red Spot on several occasions during September on the following dates-

  • September 1st   – 23:01
  • September 8th     – 23:48
  • September 11th   – 21:18
  • September 13th   – 22:57
  • September 20th – 23:44
  • September 25th   – 22:53
  • September 30th   – 22:03

Saturn lags behind Jupiter by about 8 degrees. It will therefore be due south at around 10:18 PM (BST) during first few days of the month, and due south by about 20:19 (BST) during the last few days of September. Like Jupiter though, the planet will remain at, or slightly below about 16 degrees above the horizon throughout the month, which will make it difficult to obtain clear views of the ring system. Also, like Jupiter, Saturn will both resume its eastward motion on the 29th and begin to approach Jupiter until the two planets will be less than a tenth of a degree apart on the 21st of December.

Meteor Showers in September 2020

No significant meteor activity is expected to occur during September.

Deep Sky Objects to Look For in September 2020

Prominent constellations in the south at this time of year include Cygnus, Lyra, and Aquila, with Pegasus just to the eastward of Cygnus. Also still visible and prominent is the Summer Triangle, a trio of bright stars that consists of the bright stars Deneb in Cygnus, Vega in Lyra, and Altair in Aquila. meanwhile, two other notable constellations, Cassiopeia and Perseus, are located towards the north. All of these constellations contain spectacular deep sky objects, albeit relatively few that can be viewed with modest amateur equipment, some of which are briefly described below-

The Double Cluster (Caldwell 14, NGC 869 & NGC 884)

The Double Cluster
Image credit: ESO, S. Brunier

Located about 7,600 and 6,800 light-years away respectively in the constellation Perseus, this pretty pair of clusters (indicated by red arrows), are among the youngest star clusters known. Their respective ages are only about 3.2 and 5.6 million years.

The most notable aspect of these two open clusters is that they share a common proper motion, approaching Earth at a velocity of about 80,000 km/h. It is not certain if the two clusters have a common origin. Nonetheless, both open clusters are members of the Perseus OB1 association, which is characterized by the extreme masses of member stars.

In Greek mythology, the two clusters represent the jeweled handle of Perseus’ sword, which is perhaps fitting given that each of the two clusters contains more than 300 supergiant stars.

Cumulo de Pegaso – Messier 15 (M15, NGC 7078)

Cumulo de Pegaso
Image credit: ESA, Hubble, and NASA

Located about 33,600 light-years away in the constellation Pegasus, M15 is a magnitude 6.2 globular cluster. It contains at least 100,000 stars, most of which are concentrated in the cluster’s core. With an absolute magnitude of -9.2, Cumulo de Pegaso is at least 360,000 times more luminous than the Sun.

Apart from its extreme luminosity, the cluster is also known for its great age. At an estimated age of 12 billion years, it is almost as old as the Universe, which likely explains the large number of variable stars and pulsars it contains. It is also famous for the fact that it contains a planetary nebula known as Pease-1, the first planetary nebula to be discovered in a globular star cluster.

NGC 7742

NGC 7742
Image credit: Hubble Heritage Team

Located about 73 million light-years away in the constellation Pegasus, NGC 7742 is a large spiral galaxy seen directly face-on to our line of sight. This galaxy is known for the fact that its core does not contain a bar like most other spiral galaxies, but a ringed structure that likely resulted from a merger with a smaller and less massive galaxy in the distant past.

Although it is not certain that the galaxy did indeed swallow a smaller galaxy, the fact that the bright central region rotates in an opposite direction to the outer regions is best explained by a merger with a smaller galaxy.

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