The Night Sky This Month: September 2015

Night Sky 5
Image Credit: Felix Mittermeier

Early fall is the perfect time for all-night observing as most of the heat haze has died down, and the nights are cool, rather than cold. The atmosphere is also stabilizing, making for improved seeing conditions, so make the most of this time to log some observing before winter really begins, and in particular, look out for the lunar eclipse at the end of September. Therefore, lets start our exploration of this month’s night sky with Earth’s only natural satellite, the moon.

The Moon

If you observe nothing else this month, make sure you watch the total lunar eclipse on 27-28 September, with the date depending on which side of the Atlantic Ocean you are located. The eclipse will take place when the Moon has its largest apparent diameter, 33.5 arc minutes, for all of 2015. The entire eclipse will last three hours and twenty minutes, and since the Moon will pass through the southern part of the penumbra, the southern lunar limb will appear more luminous that the northern limb. During totality, which is expected to last for slightly more than an hour, the Moon will be 27 degrees above the horizon in the south-west. Use this link to see where in the world totality will occur.

Moon Phases

Last Quarter: September 5th
New Moon: September 13th
First Quarter: September 21st
Full Moon: September 28th

The Planets


During the first few days of September, Mercury will be visible low on the western horizon, before it reaches maximum elongation from the Sun on September 4th. By the middle of the month, Mercury will disappear from view as it moves into Inferior Conjunction (in front of the Sun) with the Sun on September 30th.


Together with Jupiter and Venus, Mars will form a spectacular object visible just before dawn close to Regulus (alpha Leonis) in the constellation Leo. Regulus, which is a blue star, will be at a magnitude of +1.4, which will make for a striking color contrast with the salmon-pink of Mars. On September 25th, Mars will be only 47 arc seconds removed from Regulus, but no details of the red planet will be visible as its disk only 3.8 arc seconds in diameter at a magnitude of +1.8. Wait for the last few days of the month to obtain vastly improved images when Mars rises about three hours before dawn.


At the beginning of the month, Venus rises in the east-northeast about 90 minutes before dawn, but as it moves away from the Sun, it will rise a full 4 hours before dawn by the end of September. During the third week of September, Venus will be at magnitude -4.8, and show a thin crescent with 9% of its disk illuminated by the Sun. By the end of September, however, the phase will have increased to 35%.


Having moved out from behind the Sun (Superior Conjunction) on August 26th, Jupiter now rises just before dawn in the first few days of September, but will rise progressively earlier as the month wears on. By the end of the month, Jupiter will be visible about 18 degrees above the horizon in the northeast at dawn. Early risers should look out for the equatorial bands on Jupiter, as well as the four Galilean moons as the earth approaches Jupiter, with the giant planet’s disk increasing in size from 30.8 to 31.4 arc seconds as the month wears on.


Located in the eastern reaches of the Libra constellation, Saturn, at a magnitude of +0.6 is now visible low on the south-western horizon just after sunset. However, its close proximity to the horizon somewhat limits the view of its 16.4 arc second-diameter disc, which is further impeded by the facts that that by month’s end, Saturn will only be a few degrees above the horizon, and that as the month wears on it will move in behind the Sun. However, with some perseverance the ring system which is now inclined at 24.3 degrees to our line of sight, should still be observable in the first half of the month. Also look out for Titan, Saturn’s largest moon, at the same time.

September Constellations


Looking toward the south, four constellations, Lyra, Cygnus, Pegasus, and Andromeda are visible during September almost overhead in a spectacularly beautiful part of the Milky Way. Look out for the “Summer Triangle”, which is made up of the stars Vega in Lyra, Deneb in Cygnus, and Altair in Aquila. Also visible will be the great square of Pegasus, bordering on the constellation of Andromeda which houses the spectacular Andromeda Galaxy (M31). To the northward lay look for the “W”-shaped Cassiopeia and the constellations of Perseus.

Vega: Three times as massive as the Sun, and around 50 times as luminous, the star Vega is the brightest star in Lyra, and also the fifth-brightest in the entire night sky with a magnitude of 0.03. Only a few hundred million years old, Vega is at the center of a disc of dark material out of which a solar system is being formed.

Epsilon Lyrae: Just to the upper-left of Vega is an easy binocular target called the “Double-double”, so named for the fact that the two components of this “binary” system are both binary systems in themselves that are orbiting each other.

M57: Look out for the famous Ring Nebula located about halfway between beta and gamma Lyrae. In a small telescope, M57, which is the remnant of an exploded star, appears as a wispy ring of smoke, but looking at it long enough with a large aperture telescope may reveal a greenish tinge to the ring component. Apart from Albireo, this is one of very few deep sky objects that shows any trace of their true color. Most objects emit light in frequencies that human vision cannot perceive, which makes M57 an unforgettable sight once its true color has become apparent.

M56: Also look for this globular cluster located roughly between Albireo, and gamma Lyrae. At a magnitude of 8, this 60 light year-diameter star cluster is an easy binocular target.

Deep Sky Objects in Cygnus

Deneb: Look for Deneb, a 1.3 magnitude star in the tail of the “Northern Cross”, as Cygnus is also known. Deneb is only 2,000 light years away, and if it were placed at the same distance from earth as Sirius, its luminosity which is about 80,000 times that of the Sun, would appear as bright as a half-moon, which means that as long as Deneb was above the horizon, it would never get fully dark on earth.

Albireo: In a small aperture telescope, this binary star system of which the one components is blue-green, and the other amber, is an unforgettable sight and there is ample justification for its reputation as the most beautiful double star system in the entire sky, on account of the stark color contrast it presents.

Brocchi’s Cluster: Look for this small cluster of stars, that resemble a coat hanger, by following the neck of the Swan downwards toward Albireo, before panning down to the lower left of the double star system. With binoculars, the cluster should be easy to spot against the dark backdrop of the dust lane behind it.

Deep Sky Objects in Pegasus

Pegasus is not known for its wealth of bright deep sky objects, apart from the great square that forms the body of the Flying Horse. Almost fifteen degrees on a side, or the width of a clenched fist held out at arm’s length, the great square has no notable stars, or other noteworthy objects. However, look for the star cluster M15, a feint, but viable binocular target toward the upper right of the constellation.

Deep Sky Objects in Andromeda

M31, the Andromeda Galaxy, or Andromeda Nebula, as it is also known in older texts, is without doubt the star attraction of the constellation, but it might be difficult to find if you are not an experienced star hopper. About twice as big as the Milky Way, the Andromeda Galaxy is also a spiral galaxy, but it lacks the dense core of typical spiral galaxies, presumably because of an interaction with a smaller galaxy that punched right through the center, or nucleus of the larger galaxy. Although M31 is visible without optical aid in dark skies, it is best viewed with binoculars or a telescope to appreciate the splendor of our closest galactic neighbor, a mere 2.5 million light years away. Look for the galaxy at RA 0° 42′ 44″/ Dec 41° 16.152′.

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