The Night Sky This Month: September 2017

Night Sky 5
Image Credit: Felix Mittermeier

Astronomers use the dates of solstices and equinoxes to delineate the start and end of seasons in a year, and in the Northern Hemisphere, autumn begins with the start of the September equinox, whose dates range from the 21st to 24th. Constellations that will be dominating the night sky in September includes Perseus, which rises in the northeast sky, below which can be seen the bright yellow star Capella in Auriga. The most conspicuous of all the northern autumn constellations, however, is Pegasus, whose huge body comprises the “Great Square”, as well as the constellation Andromeda, which is attached to Pegasus, and found between the constellation and the W asterism of Cassiopeia.

The Moon

The month of September offers lunar observers two excellent opportunities, first on the 12th, and then again on the 28th, to view the Alpine Valley, a 7-mile wide cleft that bisects the Apennine Mountains, a mountain range that bounds the Mare Imbrium. While the Alpine Valley itself is easy to spot, see if you can spot the narrow rille that runs along the floor of the valley!

Full Moon: 6th
Last Quarter: 13th
New Moon: 20th
First Quarter: 28th


– Mercury is now an early morning object, and on the 5th of the month, it will form a tight grouping with the star Regulus in Leo, and the planet Mars about 15 degrees to the southward of the planet Venus. However, the grouping will only be visible with binoculars, so exercise extreme care not to view the spectacle after the Sun has risen. Mercury will also gain some altitude as the month wears on, and will reach a point furthest from the Sun on the 12th, when it will shine at magnitude 0 about 0.5 degree to the lower right of the star Regulus. On the 14th of the month, Mercury will be located about 11 degrees or so to the lower left of Venus, while it will be separated from Mars by less than half a degree on the morning of the 16th. However, Mercury will become lost in the Sun’s glare during the last days of September as it falls back toward the Sun.

– Venus becomes visible in the east about 2 hours before sunrise at the beginning of September. Throughout the month, the brightness of Venus will subsequently remain fairly constant at magnitude -3.9, since while its angular diameter reduces from 12.4 to 11.2 seconds of arc, its illumination increases from 84% to 91%.

-Mars starts its new apparition as a morning object in the constellation Leo. However, it remains difficult to spot before sunrise, so look for Mars as a member of the grouping formed by Mercury and the star Regulus on the 5th of the month about 15 degrees or so below Venus. Mars’ angular diameter remains fairly constant at 3.6 seconds of arc, as does its brightness at magnitude 1.8. Mars will however gain some altitude as the month wears on, and will appear to be chasing Venus as Venus falls towards the Sun.

– Jupiter remains visible low on the south-western horizon about 40 minutes after sunset, but at an elevation of only about ten degrees. Moreover, at the month’s end Jupiter will be only about 4 degrees above the horizon, which means that the planet will be all but impossible to spot, even though it will shine at magnitude -1.7 and have an angular diameter of about 31 seconds of arc. Since Jupiter is now falling toward the ecliptic, it is at its smallest and dimmest during this apparition for northern hemisphere observers. Note that during 2018, Jupiter will reach a maximum elevation of only 25 degrees, while for the following two years it will never rise above 18 degrees.

– Saturn will be visible in the southwest after sunset, and starts the month shining at magnitude +0.4, but dims slightly to magnitude +0.5 at the end of September, when it will have an angular diameter of around 16.5 seconds of arc. However, Saturn will not rise above about 17 degrees when it is due south, which means that telescopic views will be severely hindered by atmospheric murk. Nonetheless, northern observers close to the equator may yet have good views of the planet and its ring system that is now tilted toward our line of sight by 26.8 degrees. A full tilt of 27 degrees will be reached in October.

– Neptune is now moving toward its point of closest approach to Earth, and will be in opposition on the 2nd of September. Shining at magnitude +7.9 and having an angular diameter of 3.7 seconds of arc, the planet should be a relatively easy binocular target under dark skies in good seeing conditions. Neptune will reach an elevation of about 27 degrees when it is due south, so look for it in the constellation Aquarius. At maximum elevation, telescopes with 8-inch aperture and larger will reveal Neptune’s moon Triton, provided the sky is clear and seeing conditions are steady.

Meteor Showers

The Southern Taurids is a minor shower that runs from the first week in September to around the middle of November, with an expected peak on the night of 9/10 October. However, the Southern Taurids hardly ever produces more than about 5 meteors per hour, even at its peak. Nonetheless, the Southern Taurids often produces very bright fireballs, but this year the Moon will still be almost full, which means that if anything, only the brightest fireballs may be visible.

Deep Sky Objects

While the constellations Cygnus and Lyra are still prominent, the change in season brings the constellations Andromeda, Triangulum, and Pegasus into view. Although Pegasus does not have much to recommend it except the globular star cluster M15, both Andromeda and Triangulum offer galaxies that can be viewed with binoculars:

– M31 (Andromeda Galaxy)

Located close to 3 million light years away, this great spiral galaxy that contains about twice as many stars as the Milky Way can be seen without optical aid under dark skies. However, a pair of binoculars reveals a distinct patch of light, and although binoculars won’t reveal any structure in the galaxy, a binocular view does remind the observer that this great spiral is our closest large neighbor, and is like the Milky Way, a member of the Local Group, a cluster of galaxies with about 3 dozen or so members. Look for the galaxy about one binocular field of view to the right of the star Mu Andromedae.

– M33 (in Triangulum)

M33 is a satellite galaxy of M31, and at just over 3 million light years away, it is the most distant object that unaided human vision can discern. With 8 × 40 binoculars under clear, dark skies M33 looks like a very faint cloud about twice the diameter of the full Moon. Most observers agree that the galaxy has a uniform brightness, but it is not possible to resolve structure with binoculars or small telescopes. Look for M33 below M31 at about the same distance that M31 is from Mu Andromedae.

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