Ducks and geese flying south in the fall are a familiar sight to Mainers. Many species of birds migrate vast distances each year. The record holder appears to be the Arctic tern, which spends one summer in the Arctic and then migrates halfway around the world to spend another summer in the Antarctic. They have an annual round trip of more than 40,000 miles. How do they and other long-distance commuters know where they are going?

Some, such as the Arctic tern, apparently use the sun and the Earth’s magnetic lines of force guided by small magnetic mineral particles in their beaks. Others actually “fly by the stars.” Stephen Emlen, a Cornell University ornithologist, showed that indigo buntings orient to the northern sky using the pole star Polaris as a focal point.

“It is instinctive [for the bunting] to learn configurations of stars in the pole of the sky, i.e. the axis of rotation,” Emlen said. Many other species, among them warblers, robins and hummingbirds, are avid stargazers as they migrate from one place to another.

Focus on the planets

Mercury is very low in the west-southwest during the first half of the month, where it may be spotted with difficulty with binoculars. The best bet is on Sept. 14, when it is to the left of the waxing crescent Moon.

Venus gleams in the east about an hour before sunrise and is at its greatest brilliancy of the year on Sept. 21. Venus’ crescent can be spotted by telescope as early as the first of the month.

Mars rises in the east at dawn but is too distant to show any appreciable detail. Look for Mars on Sept. 10, when it is to the lower left of Venus with a crescent Moon between them.

Jupiter also rises in the east at dawn and is situated well to the lower left of Venus. There is a very narrow window for viewing as Jupiter rises only a half-hour before the sun on Sept. 1, but this broadens to two hours at the end of the month. Jupiter’s cloud formations at its poles plus the four major moons should be seen by telescope.

Saturn comes up in the southwest at nightfall and sets well before midnight. Saturn’s ring system is still favorably tilted for viewing the two rings systems, their division and the major moon, Titan.

Uranus appears low in the east in Pisces and Neptune in the southeast among the stars of Aquarius after midnight. Blue-green Uranus and blue-gray Neptune can be found utilizing Sky & Telescope magazine’s finder chart at skypub.com/urnep.

September events

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

4: Mercury is at its highest elevation for the month but still will be difficult to spot in the evening twilight.

5: Moon in last quarter, 5:54 a.m.

9: This is the peak night for the September Epsilon Perseid meteor shower. This is a very weak shower compared to the August Perseids and will yield only five to 10 meteors per hour radiating from the vicinity of Algol in Perseus.

10: Check the eastern horizon an hour before dawn to spot Mars, the thin crescent moon, and Venus in an ascending line from left to right.

13: New moon, 2:41 a.m.

14: The moon is at apogee, or greatest distance from the Earth, today.

17: The sun enters Virgo on the ecliptic.

18: Saturn stands directly to the left of the moon tonight with orange-hued Antares even further to the left.

21: Moon in first quarter, 4:59 a.m.

23: Fall or autumnal equinox, 4:21 a.m. The sun enters the astrological sign of Libra as it crosses the celestial equator into the southern hemisphere.

25: Look for Mars nestled close to Regulus well up in the east an hour before sunrise with Venus to the upper right and Jupiter to the lower left of the pair.

27: Full moon, 10:50 p.m. The full moon nearest the fall equinox is the Harvest Moon. The moon also is at perigee, or closest approach to Earth, and the combination of these two factors gives rise to what is called a Supermoon and abnormally high tides. If all this were not enough, there will be a total eclipse of the moon starting at 9:07 p.m.; totality lies between 10:11 p.m. and 11:23 p.m.; and ends at 12:27 a.m. on Sept. 28.

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, Features Desk, P.O. Box 1329, Bangor, ME 04402.