The star-studded summer constellations are moving westward, making way for the fainter autumn ones rising in the east. The full Harvest Moon occurs on the 20th, while the first day of autumn occurs on the 22nd. The Milky Way remains well-positioned for evening viewing, spanning the entire sky on clear, dark September evenings.

The Milky Way, from Scorpius (bottom right) to Cygnus (top left), begins to set between the San Pedro Mountains (bottom right) and Cerro Pelon (a.k.a., the ‘Galisteo Wave’ – bottom center).

September is a feast for planet watchers, with seven planets visible before midnight. Mars, Uranus, and Neptune may need optical aid to see, but Mercury, Venus, Jupiter, and Saturn are easily found with the naked eye after sunset.

September Planets:

As September opens, Mercury marks the midpoint of a line consisting of the three closest planets to Earth, with Mars to the west and Venus to the east. They’re visible in twilight, low in the WSW, in the constellation Virgo. By mid-month, Mercury is at greatest elongation from the Sun. At the end of September, Mercury will be back in the glare of early twilight, not far from first magnitude Spica.

Venus continues to dominate the western sky, setting shortly after dark, as it moves from Virgo into Libra. It’s in the waning gibbous phase. By the end of September, only 63% of its face will be illuminated. However, the planet actually brightens during September, from magnitude -4.1 to -4.2, as it gets closer to Earth.

At the beginning of the month, Mars sets around 8:00pm MDT, in the constellation Virgo. Binoculars may be needed to find the Red Planet in the glare of twilight. Mars will soon be too close to the Sun to be seen.

All month, Jupiter will be the brightest object in the night sky (with the exception of the Moon), once Venus sets. At sunset, Jupiter is already above the ESE horizon and it will remain visible almost all night in eastern Capricornus, close to the Aquarius border. Just past opposition, Jupiter remains extremely bright, but will fade slightly as the Earth pulls away from the giant planet.

Jupiter and its four Galilean Moons, viewed one week after Jupiter’s opposition.

Saturn, also just past opposition, is visible most of the night in the constellation Capricornus. Saturn’s rings remain wide open, but the planet’s rings and disk are shrinking as the Earth moves away.

Saturn and its rings can be seen in this image, taken with a 600mm lens (no telescope), three weeks after opposition.

Uranus, in Aries, is just visible to the naked eye in dark skies at magnitude 5.7.

At magnitude 7.7, Neptune reaches opposition at mid-month in Aquarius. Mounted binoculars or a telescope to will be needed to view it.

September Spotlight: Sagittarius

There are few areas of the sky richer in easily viewed deep sky objects than the constellation Sagittarius. Sagittarius, the Centaur of Greek mythology, is one of the thirteen constellations of the zodiac that the Sun appears to pass through as the Earth orbits it. The constellation is visible in the SSW as darkness descends on September evenings.

Many people, upon viewing Sagittarius in the night sky, see a tea kettle, with a handle on the left and a spout on the right. While the stars marking the constellation are beautiful, the real fascination comes from the many clusters and nebulae that lie within its boundaries, even visible to the naked eye on clear, dark nights.

Some of the deep sky objects in Sagittarius and neighboring Scorpius, many of which can be seen with the naked eye (the ‘spout’ of Sagittarius consists of the triangle of bright stars at the bottom center; two of the four stars of the ‘handle’ are visible at the lower left).

The center of our Milky Way galaxy lies within Sagittarius, not far from the tip of the spout. The galactic center is invisible to us because of all the intervening stars and clouds of dust that lie between Earth and the middle of our galaxy, 26,000 light-years distant.

But, while the galactic center eludes us, there are a plethora of easily visible objects that more than compensate. Let’s start with the Milky Way itself. The Milky Way is brightest in Sagittarius since we’re looking toward the center of the galaxy. The pale, ghostly light of myriad stars is visible near the end of the spout, turning dark on the right side of the glow. This bright patch of starlight is the Large Sagittarius Star Cloud; the dark lane to its right is part of an enormous cloud of gas and dust known as the ‘Great Rift’ (in the photo of the Milky Way at the top of this post, the Great Rift can be seen bisecting the glow of the Milky Way, from bottom right to top left).

The Large Sagittarius Star Cloud is just that – a cloud of stars. Lacking the gas and dust needed to create young, blue stars, the Large Sagittarius Star Cloud is full of older red giant stars. As a result, the cloud has a yellowish tinge, slightly different than neighboring parts of the Milky Way.

Not far from the Large Sagittarius Star Cloud is the Lagoon Nebula, also referred to as Messier 8 or M8 (Note: Charles Messier was a French astronomer who observed deep sky objects visible from Paris and published a list of them in the second half of the 18th century). The Lagoon Nebula is a stellar nursery – a giant cloud of gas and dust that coalesces into new stars.

Light from the nebula takes about 5,000 years to reach Earth. But, the nebula is so large (about 50 light-years x 110 light-years) that it’s visible to the naked eye as a tiny, 6th magnitude oval of light (whenever I see it, I think of the Beatles’ ‘Yellow Submarine’).

Just north of the Lagoon Nebula is the Trifid Nebula (a.k.a., M20), another star nursery. The Trifid Nebula is unusual in that it’s composed of three different types of nebulae – part of it is an emission nebula (a cloud that shines by ionization from a nearby star); part is a reflection nebula (a cloud that shines by reflecting light from a nearby star); and part is a dark nebula (a concentrated cloud of gas and dust that obscures the stars behind it). There’s also an open star cluster embedded in it. A telescope is needed to see some of the details of this beautiful and mysterious object.

Speaking of mysterious objects, Sagittarius is home to several globular clusters. A globular cluster is a group of hundreds of thousands of tightly spaced, old stars, bound by gravity. They exist in almost all galaxies and represent some of the oldest objects in their respective galaxies.

The brightest globular cluster in Sagittarius is Messier 22. With a magnitude of 5.1, it’s visible to the naked eye, appearing as a single faint star. A little magnification will reveal M22 to appear ‘fuzzy;’ greater magnification (and long exposure times with a camera) will begin to resolve the fuzzy star into individual stars around the edges.

The globular cluster in Sagittarius known as Messier 22 (M22). Photo courtesy of Universe Today.

While there are numerous other gorgeous deep sky objects in Sagittarius, I’d be remiss if I didn’t give a shout out to Messier 6 (M6) and Messier 7 (M7), two naked eye open star clusters visible in front of the spout, just across the border in neighboring Scorpius. M6 is the smaller cluster of the two – it’s known as the Butterfly Cluster and is aptly named. M7, called Ptolemy’s Cluster, is just to the lower left of M6. Open star clusters consist of stars that were born in the same cloud of gas and dust and are traveling together through space.

September Night Sky Events:

September 1-30: The Zodiacal Light (sunlight reflecting off of interplanetary dust particles) is visible in the east, about an hour before dawn.

The Zodiacal Light is the glowing cone in Gemini, above the garden gate. Orion is on the right; a meteor slashes through the front paws of Ursa Major on the left.

September 5: Venus passes above the first magnitude star, Spica.

September 7: New moon.

September 14: Mercury is at greatest elongation from the Sun, visible in Virgo and setting about 8:00pm MDT.

September 14: Neptune is at opposition in the constellation Aquarius, just inside the border with Pisces. Mounted binoculars or telescopes are needed to see this blue gaseous planet since it shines at magnitude 7.7, beyond naked eye visibility.

September 20: The full Harvest Moon occurs at 5:54pm MDT in the constellation Pisces. Take five minutes to listen to ‘Shine On Harvest Moon’ on this night (my favorite version is by Leon Redbone).

September 22: The autumnal equinox occurs at 1:11pm MDT. At this time, the Sun is directly above the equator, marking the first day of fall in the northern hemisphere.