The Night Sky This Month: September 2018

Night Sky 2
Image Credit: Andy Holmes

The Northern Hemisphere’s Autumn Equinox occurs on September 23rd this year, presenting a perfect time for stargazers to make the most of the improved seeing conditions as the summer’s heat haze begins to die down. Of particular interest is the “Summer Triangle” that can be seen in the southern sky during the late evening, while to its east can be found the The Great Square of Pegasus, whose numerous star contained within the asterism can be used to test a person’s visual acuity. Meanwhile, to its north-east lies the circumpolar constellations, including Cassiopeia and Perseus.


Lunar observers have a great opportunity on the night of the 18th to view both Mons Piton and the crater Cassini, both located in the eastern reaches of the Mare Imbrium, just below the crater Plato. Cassini is nearly 60 km in diameter, and its lava-filled floor holds two smaller craters, designated Cassini A and Cassini B, respectively.

Mons Piton, just to the west of the crater Cassini is an isolated mountain or outcrop that rises about 2,250 meters above the Mare floor.


Last Quarter: 2nd
New Moon: 9th
First Quarter: 16th
Full Moon: 25th


September is not a good time to view the planets, all except Neptune being low on the horizon for much, if not all of the month. Nonetheless, below are some details of what can be expected to be seen of the visible planets during the month:

Mercury is visible low above the east-north-eastern horizon for about 45 minutes before sunrise during the first few days of the month, shining at magnitude -1.0. By the 6th, Mercury will be separated from the bright star Regulus in the constellation Leo by just more than one degree. However, the little planet will become lost in the Suns’ glare by the 11th as it moves towards a position of superior conjunction with the Sun, which it will reach on the 20th.

Venus was at its point of greatest separation from the Sun on the 17th of August, and is therefore visible low on the west-south-western horizon until it sets about 80 minutes after the Sun. However, Venus will brighten from magnitude -4.6 to -4.8, making it easier to spot in the Sun’s glare, although binoculars may be required to spot the planet. While Venus’ illumination decreases from 40% to only 17%, its angular diameter increases from 29 seconds of arc to 46 seconds to arc, meaning that it will remain relatively bright as the month progresses.

Mars, which was engulfed in a major dust storm during August, has now resumed its prograde motion, and is moving into the constellation Sagittarius at the beginning of September. Look for Mars at an elevation of about 14 degrees just east of south where it will shine at magnitude -2.1, reducing to only -1.3 or so by month’s end. Although the dust storm on Mars has not fully subsided, a small telescope may reveal major surface details such as Syrtis Major- provided that seeing conditions allow clear views through the murk of the atmosphere.

Jupiter becomes visible in the west soon after sunset, but sadly, it will not rise above 10 or so degrees, since it is now approaching the southern part of the ecliptic. Although Jupiter will have a magnitude of about -1.9 at the beginning of the month, an atmospheric dispersion corrector may be required to obtain reasonable views of the King of the planets.

Saturn will also be difficult to observe during September, since it will only reach an elevation of 14 or so degrees above the southern horizon. However, if seeing conditions allow, look for Saturn a few degrees from M8 (Lagoon Nebula) and M20 (Trifid Nebula) on the 6th, although an atmospheric dispersion corrector may be required to obtain clear views of the ring system that is now tilted towards us by about 25 degrees.

Neptune reaches its point of closest approach to Earth on the 7th, at which time it will rise to an elevation of about 27 degrees or so when it crosses the meridian. Telescopes of 8-inch and larger apertures will easily resolve the planet, since its angular diameter will be 3.7 seconds of arc and it will therefore shine brightly just to the leftward of the star Lambda Aquarii in the constellation Aquarius. If the sky is sufficiently dark, it may even be possible to spot Neptune’s moon, Triton, as well.

Deep-Sky Objects

The planets may not be much to look at during September, but there are always spectacular deep-sky objects that can be viewed with nothing more than a pair of binoculars. Below are some details of three easy binocular targets:

M31 (Andromeda Galaxy)

The Andromeda Galaxy (M31), our biggest neighbor in the Local Group of galaxies, is now visible above, and to the left of the star Sirra (Alpha Andromedae) that marks out the top left corner of the Square of Pegasus. Located about 2.5 million years away, the great Andromeda Galaxy is somewhat bigger than the Milky Way but it contains about twice as many stars, which makes it bright enough to be visible without optical aid if the sky is sufficiently dark.

M33 in Triangulum

Meanwhile, M33 is the smaller galaxy that can be seen to the lower right of M31, but it is slightly further away, and at a distance of about 3 million light years, is said to the furthest object in the Universe that can be seen without optical aid. Irregular in shape, this galaxy has a uniform brightness, and no structure is visible- even with large telescopes. However, while M33 can be seen with binoculars, dark skies and excellent seeing conditions are required to see more than just smudge of light that is about twice as big as the full Moon.

North American Nebula (Cygnus)

Located just above and to the left of the star Deneb, this vast region of nebulosity is an easy target for binoculars. The area includes two distinctly separate nebulae; the large structure to the left of its center resembles the North American continent, while a smaller region to its right resembles a pelican, hence the name, Pelican Nebula. Note that the best views of this spectacular structure can be had when the sky is very dark and seeing conditions are stable.

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