September, the ninth month, is named after the Latin word septem, meaning seven. In case you might think the Romans couldn’t count, their calendar used to start in March and contained only 10 months. In 45 B.C., Julius Caesar reorganized the calendar so that the year started on Jan. 1. He also added 67 days to bring the calendar in line with a 365-day solar year and renamed the sixth month, sextus, after himself – Augustus Caesar.

Focus on the planets

Mercury is located very low on the southwestern horizon, where it may be spotted about a half-hour after sunset early in the month. Scan the horizon to the extreme lower right of Venus.

Venus is at its greatest brilliancy in late September but also is at its lowest point on the horizon in years. Look for Venus just above the southwestern horizon at sunset. It remains in view for an hour and a half as the month opens but sets an hour after the sun at September’s close.

Mars will enter the morning sky in September and will reach aphelion, the most distant point in its orbit at midmonth. Consequently the distant planet will not be visible in the dawn light.

Jupiter rises about three hours after midnight and is situated well up on the eastern horizon about an hour before dawn. Look for a narrow crescent moon just to the upper left of Jupiter on Sept. 4 if you are out and about during the early morning hours.

Saturn rises on either side of midnight throughout September and remains up for the rest of the night where it may be found in the constellation of Orion. Saturn is best seen during the early morning hours and the favorable inclination of its rings make losing an hour or so of sleep very worthwhile.

Uranus and Neptune are in the constellation of Capricornus and, while a bit brighter than in previous months, still require a small telescope and the aid of a chart, to spot them.

Pluto is in Ophichus but requires both a powerful telescope and chart to have any hope of locating it.

Our celestial neighborhood

Still in the neighborhood, but just barely, are the two Voyager spacecraft launched in 1977. Originally the two were scheduled to visit Jupiter and Saturn and be shut down in 1981. Instead they visited Neptune and Uranus as well and today, 25 years later, Voyager 1 is 7.8 billion miles from Earth and about to reach the boundary where the sun’s gravitational influence ends.

It takes more than 23 hours for signals to complete a round trip between Earth and Voyager 1, making it somewhat difficult for the dozen scientists, down from more than 300 in the 1980s, to maintain contact, but they hope that data will continue to filter back until 2020.

Even then the spacecraft may one day tell other worlds about Earth as they carry gold-plated disks that contain images and sounds of our planet. “The Voyagers probably represent the greatest mission of discovery in the history of mankind,” says former Voyager mission manager Charles Kohlhase.

September events

1 Sunrise, 5:57 a.m.; sunset, 7:13 p.m.

3 The two stars to the immediate left of the moon tonight are Castor and Pollux of Gemini.

4 Tonight the thinning crescent moon lies to the upper left of Jupiter.

7 New moon, 11:10 p.m. Today marks Rosh Hashanah, the first day of the Jewish year 5763.

8 Looking to the left from the crescent moon, you first encounter the bright star Spica and then Venus. Directly below the moon, and barely peeking over the horizon, is Mercury. Check these out a half-hour after sunset.

13 Moon in first quarter, 2:08 p.m.

16 The sun enters Virgo on the ecliptic.

21 Full moon, 9:59 a.m. The full moon of September is called the Fruit Moon and, being the full moon nearest the autumnal equinox, is also the Harvest Moon.

23 Fall or autumnal equinox, 4:56 a.m. This marks the sun’s crossing of the equator into the Southern Hemisphere, bringing spring to its inhabitants and fall to us. The sun also enters the astrological sign of Libra although, astronomically, it has just entered Virgo.

30 Sunrise 6:31 a.m.; sunset, 6:18 p.m.