Zodiacal light can mimic approaching dawn

A piece in this month’s issue of Astronomy magazine reports that this is a prime time to observe the “zodiacal light” or “false dawn.” This is a hazy band of light that stretches across the sky along the ecliptic and is best seen just before sunrise or after sunset. It is caused by sunlight scattering off countless tiny interplanetary dust particles.

The article says that in areas where artificial light does not overwhelm it, zodiacal light can be mistaken for the approaching dawn or even the glow from a distant town. The zodiacal light can be seen on the eastern horizon before sunrise or the western one after sunset.

A second phenomenon that may be observed is a small, bright spot of light on the ecliptic opposite from the direction of the sun. This is called “gegenschein” or “counterglow,” which is caused by light reflected straight back to the viewer from dust concentrated in the plane of the ecliptic. The best time to look for the zodiacal light is after Sept. 9 when the moon will not interfere.

Focus on the planets

Mercury lies very low in the southwest buried in the glow of sunset and is hard to spot. On Sept. 21 Mercury and the star Spica are separated by less than a degree and should be readily seen with binoculars.

Venus opens the month low in the east at dawn but climbs rapidly with each passing day. On Sept. 8, a slender crescent moon lies to the upper left of Venus and may be seen the next night to the lower left. By month’s end Venus is shining so brightly that it can be spotted even after sunrise.

Mars rises in the east shortly before midnight in the company of Aldebaran, the “red eye” of Taurus the bull. Together they allow Taurus to glare with two flaming red eyes. As September draws to a close, Mars’ disk grows to the point where some surface features are visible by telescope.

Jupiter gleams low in the southwest as the evening twilight fades. Immediately below Jupiter is the orange-red star Antares in Scorpius. Although low in the sky, Jupiter is large when viewed through a telescope, and many of its surface features, such as the equatorial belts, plus its four major moons, are readily visible.

Saturn lies low in the east at dawn, well below the much brighter Venus, and in the company of Regulus from the constellation of Leo. The next few months will be a disappointment for those wanting to view Saturn’s fabled ring system for they will have closed to their smallest tilt, and poorest opportunity to view them, in nearly a decade.

Uranus in Aquarius and Neptune in Capricornus are seen as greenish and bluish disks, respectively, by means of binoculars and with the use of a finder’s chart such as the one in the July 2007 issue of Sky & Telescope.

September events

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

3 During the early morning hours, the east-northeast horizon features the nearly last-quarter moon, with Mars directly beneath and Aldebaran to the moon’s lower right.

4 Moon in last quarter, 10:34 p.m.

9 A thin waning crescent moon hangs above Saturn and Regulus on the eastern horizon about an hour before dawn with brilliant Venus to its upper right.

11 New moon, 8:44 a.m.

15 The moon is at apogee, or farthest distance from the Earth.

17 Jupiter, Antares and the waxing crescent moon form a slightly crooked diagonal, reading from top to bottom, in the southwest about an hour after sunset. The sun enters Virgo on the ecliptic.

19 Moon in first quarter, 12:48 p.m.

21 Mercury is less than half a degree from the bright star Spica on the southwestern horizon shortly after sunset. This is a great opportunity to test your skill with binoculars or telescope while catching the elusive pair together.

23 Fall-autumn equinox, 5:54 a.m. This is the point where the sun crosses the equator to take up residence in the southern hemisphere. The length of daylight and darkness are nearly equal. The sun enters the astrological sign of Libra at the equinox but astronomically has only just entered Virgo.

26 Full moon, 3:46 p.m. As the full moon nearest the fall equinox, the full moon of September is defined as the harvest moon. This was a time when farmers could work at harvesting their crops well into the evening by the light of the moon.

28 The moon is at perigee, or closest approach to the Earth.

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

Send astronomical queries to Clair Wood at cgmewood@aol.com or care of the Bangor Daily News, Style Desk, P.O. Box 1329, Bangor, Maine 04402.