There’s a lot to see in the sky during these fall nights and early mornings in September, including a full Corn Moon and stunning planetary pairings. Mark your calendars!
All times are listed as Eastern Daylight Time for the Northern Hemisphere.
Take note that we sometimes will use angular degrees to define the separation between two objects, such as (for example) the Moon and a bright planet. Keep in mind that the width of your clenched fist, held at arm’s length, measures roughly 10°.
When we speak of magnitude, we are referring to the brightness of an object; the lower the figure of magnitude, the brighter the object. The brightest stars are zero and first magnitude. Under a dark, clear sky, the faintest objects that you can see with just your eyes are fifth or sixth magnitude. Objects with negative magnitudes are the brightest. Sirius, the brightest star, is -1.4. Venus can get as bright as -4.8. A Full Moon is -12.7 and the sun is a blindingly bright -26.7.
September 1 – Mercury is going to be a rotten evening apparition for skywatchers at north temperate latitudes this month. This in spite of the fact that it increases its angular distance east from the sun from 14° to 26° by October 1st, the time of greatest elongation. However, the zero-magnitude planet also moves well south of the sun, and from latitude 40° north is unfavorably situated, setting only ¾ hour after sunset. Nevertheless, you can try to spot it on a clear evening by scanning with binoculars 20 to 30 minutes after sundown. Look near the horizon due west early in September, and west-southwest late in the month.
September 2 – Full Corn Moon at 1:22 a.m. EDT (If you live in the western US and Canada, you’ll greet this Moon on the 1st). Usually the September full Moon gets the title of “Harvest Moon.” That’s the full Moon on the calendar closest to the autumnal equinox. This year, however, it will be the October 1st full Moon that gets the “Harvest Moon” honors. When that happens, September’s Moon becomes the full Corn Moon.
September 5—That brilliant yellow-orange “star” sitting just above the waning gibbous Moon late this evening is Mars, approaching opposition, and making a grand entry every evening in the eastern sky. It comes up a little over two hours after sunset on the 1st, and 50 minutes after sundown (during mid-twilight) by month’s end. Give it at least two hours to climb above the horizon where visibility is poor. By then it will be at an altitude of just over 20° (as seen from 40° north latitude). Mars appears much sharper and steadier when it transits the meridian (at around 4 a.m. local daylight time on September 1st, at 3 a.m. mid-month, and at 2 a.m. on September 30th). At that time, its altitude is nearly 60°. Its distance from the Earth decreases from 45.6 to 38.8 million miles this month, while the red planet brightens dramatically from magnitude -1.8 to -2.5. In fact, on September 28th, it will replace Jupiter as the second brightest planet.
September 6—Venus dominates the night sky from the time it rises, about 3½ hours before the Sun all month. It shines at magnitude -4.2 and is positioned about 1/3 of the way up in the eastern sky by the middle of morning twilight. Despite the fact that Venus was at greatest elongation (maximum angular separation) from the Sun on August 17th, the planet is even a bit higher in September—about 40° high at each September sunrise around latitude 40° north. On the morning of the 6th, Venus is in line with the Gemini Twins, Castor and Pollux (though 10½° from Pollux).
September 10—Last Quarter Moon at 5:26 a.m. In this phase, the Moon appears as a half Moon due to the direct sunlight, the illuminated part is decreasing toward the new Moon phase.
September 13—This morning, Venus glides about 2½° south of M44, the Beehive Star Cluster in Cancer.
September 14—An hour before sunrise, a lovely celestial pairing adorns the eastern sky as a waning crescent Moon, three days before new, sits 4½° to the left of brilliant Venus.
September 17—New Moon at 7:00 a.m. In this phase, the Moon is not illuminated by direct sunlight and is completely invisible to the naked eye.
September 22—First Day of Fall. At 3:50 p.m. EDT, the Sun crosses the celestial equator, heading south. This moment is known as the autumnal equinox and marks the beginning of the fall season in the Northern Hemisphere and spring in the Southern.
September 22—Later this evening, Mercury passes 0.7° to the upper left of 1st-magnitude Spica, brightest star in Virgo. Mercury is more than twice as bright as Spica, but as previously mentioned is quite difficult to see this month because of its very low altitude and being deeply immersed in bright twilight. Viewers in the Southern Hemisphere, on the other hand, have a grand view of Mercury in September; it gets fairly high in the west in twilight, affording a fine view of its conjunction with Spica.
September 23—First Quarter Moon at 9:55 p.m. In this phase, the Moon looks like a half-Moon in the sky. One-half of the Moon is illuminated by direct sunlight while the illuminated part is increasing, on its way to becoming a full Moon.
September 24—Jupiter, glaring at magnitude -2.4, is highest in the south at evening twilight. It reversed its retrograde (westward movement relative to the stars) on September 12th after halting above the handle of the Teapot of Sagittarius. This evening at dusk/nightfall you’ll find it about 4½° to the upper left of the waxing gibbous Moon.
September 25—Saturn glows in the south after dusk, about 7½° east (left) of Jupiter. A steady yellowish light in the otherwise rather empty constellation of Capricornus. This evening it is conveniently placed about 3½° above the waxing gibbous Moon for early evening viewing now that it is well past its July 20th opposition. In addition, since the Earth has sidled off the line from the sun to Saturn, we no longer look squarely into Saturn’s sunlit face. This means we see a little around the planet’s eastern edge to view some of its shadow falling on the rings. A small telescope shows the rim of a black shadow quite clearly.
September 30—Venus ends the month just 3° to the upper right of 1st magnitude Regulus in Leo – which it will meet in a very close conjunction on October 3rd.
Compiled by contributing astronomer, Joe Rao. Our schedule is adapted from “Skylog,” a regular feature appearing in Natural History magazine, written by Mr. Rao since 1995.