SKY REPORTER: September 2011 main content.

SKY REPORTER: September 2011

by Steve Beyer on


Sky Data

Kepler Field on the Sky
Field of view where NASA’s Kepler mission orbiting observatory is searching for extrasolar planets.
Credit: NASA, Carter Roberts

Thursday September first, civil twilight begins at 5:54 a.m. and sunrise is at 6:22. That evening the sun sets at 7:28 p.m. and civil twilight concludes at 7:56.

On the last day of this month, Friday September 30th, civil twilight begins at 6:24 with sunrise at 6:51 a.m. That day sunset occurs at 6:40 p.m. and civil twilight ends at 7:07.

Autumn begins in the northern hemisphere Friday September 23rd. At this time of the year the apparent roller coaster track of the Sun moving through the zodiac reaches its greatest downward angle and daily decreases in length of daylight are most pronounced.

During September average overnight temperatures recorded in Central Park drop from 65 degrees Fahrenheit on the first of the month, to 55 degrees on the 30th.

Principal Phases of the Moon during this month are:

First Quarter Sunday, Sept. 4th
Full Moon Monday, Sept. 12th “Harvest Moon”
Last Quarter Tuesday Sept. 20th
New Moon Tuesday Sept. 27th

On September first, Mercury appears as a morning star in the constellation Leo, rising at 4:54 a.m., an hour and 28 minutes before the Sun. During the predawn twilight of Saturday September 3rd, Mercury has a separation of 18 degrees of arc from the Sun. Although this distance slowly decreases during subsequent days, for about a week the span provides reasonable opportunities for spotting that planet during early September.

At the start of civil twilight on Saturday the third, Mercury has an altitude of 11 degrees of arc and an azimuth of 81 degrees, about one fist length above the horizon and slightly to the left of the east point on the horizon. As with the usual case with planets, Mercury shines with a steady light, unlike the twinkling seen in stars such as fainter first magnitude Regulus, located beneath and about one-third the altitude of Mercury when civil twilight begins that Saturday morning.

Early in September Venus is also in Leo, but on the opposite side of the Sun from Mercury. Venus is very low in the western sky at sunset. On Thursday the first, Venus sets at 7:45 p.m. just 17 minutes after the Sun. On that date the angular distance between the Sun and Venus is less than five degrees of arc and even that planet’s usual brilliance is overwhelmed by peripheral solar glare. Times between sunset and the setting of Venus gradually increase throughout the remainder of 2011, and by the holiday season, this planet will again dominate the western evening sky. Venus will set more than two hours after the Sun by the end of this year.

Mars begins the month as a first magnitude object in Gemini. The so-called Red Planet rises at 2:09 a.m. on Thursday the first of September. It continues its prograde, direct motion eastward, and crosses into the constellation of the Crab at midmonth.

Jupiter remains in the constellation Aries, rising at 9:54 p.m. on the evening of September first. This largest planet in the solar system currently shines at magnitude minus 2.7 and it is by far the brightest object in the overnight sky except when the moon is visible. At month’s end Jupiter will rise at 7:56 p.m.

Saturn, which has been a visual treat in our sky for many months, is now low in the west as twilight ends. The great ringed planet sets at 9:01 p.m. on September first. On that date the four day old waxing crescent moon is 19 degrees to the left of Saturn, both at an altitude of 12 degrees of arc, in the west southwest at the end of civil twilight. By September 30th the beautiful ringed planet will be visually lost in the glow of evening twilight.

The bright waning gibbous moon is near Jupiter the night of September 15-16.

During early morning of September 23, Mars is in the eastern sky about five degrees to the upper left of the waning lunar crescent. The Beehive star cluster, M44, is about the same distance to the moon’s left, and these three objects form a nearly equilateral triangular pattern.

At 5:05 a.m., also on September 23rd, autumn begins in the northern hemisphere. At that moment the Sun is at the zenith above a point in the Pacific, on the equator halfway between the Solomon and Marshall Islands.

M44 Praesepe Cluster in Cancer
Open star cluster M44, also known as the Beehive or Praesepe cluster.

During the last week of September Mars approaches Messier 44, the lovely “Beehive” open star cluster. On the 30th, Mars rises at 1:44 a.m., and appears mingled among stars at the edge of the cluster, which is located about 500 light years from the solar system.

On Thursday September 15th the brilliant star Vega is at the southern meridian at 8 p.m. In its vicinity are Deneb and Altair, marking other corners of the prominent asterism called the Summer Triangle. Between Vega and Deneb is the sky region where instruments aboard the Kepler space observatory search a series of survey grids as astronomers seek and discover planets orbiting stars far beyond the Solar System. Kepler’s viewing fields include stars to distances of several thousand light years from the Sun.

By the end of September Mercury, the fastest orbiting planet in the Solar System, will have moved into the western evening sky within boundaries of the constellation Virgo. Mercury sets at 6:48 p.m. on September 30th, just eight minutes after the Sun drops beneath the horizon.

Friday September 30th, Venus sets at 7:12 p.m., shortly after conclusion of evening civil twilight and 32 minutes after sunset.

Sky Lore

The night of Monday September 12th, the bright gibbous Moon is just south of the asterism known as the Great Square of Pegasus. This figure, consisting of three stars in the constellation Pegasus, the flying horse, and one star in Andromeda, is a fine marker to other locales in autumn evening skies. Stars forming the Great Square are only of second magnitude, but the figure’s symmetry makes the pattern an eye-catching sight. The size of the Great Square, spanning a diagonal of about 20 degrees of arc, is helpful for making scale comparisons to star separations in other parts of the sky.

The Great Square of Pegasus also offers a way of estimating the darkness and clarity of the sky. Although its main second magnitude stars may be rather difficult to see from some urban areas, binoculars will help concentrate our view, and the Square presents a noteworthy pattern. When star watching from areas away from urban glare, and moonless nights reveal traces of the Milky Way, it is possible to see a half dozen stars within borders of the Great Square. From truly dark sites this asterism presents dozens of stars to unaided eyes.

With Pegasus high in the sky, other features of autumn evenings may be noted in our eastern sky field of view. Constellations sometimes considered along with Pegasus as members of the “Royal Family of the Sky”, are reminiscent of tales associated with Andromeda and Perseus in Hellenic mythology. Other constellations visible on autumn evenings, including Aquarius, Delphinus, and Capricornus, have aqueous associations and comprise a sky region sometimes called the “Celestial Sea.”

While the Great Square is usually considered the prime asterism of autumn evenings, stars of the Summer Triangle as still very much in view during September. At nine p.m. around the middle of this month, the Triangle’s stars are directly overhead.