Last month, as a beautiful Harvest Moon graced the skies did you ever wonder how the Moon came to be? There have been several theories proposed over the years.

The most widely accepted is the `Big Whack’ theory proposed by William Hartmann in 1975. He said the primitive Earth was struck a glancing blow by an object about the size of Mars knocking off a massive fragment that became the Moon.

A paper in the latest issue of Nature Geoscience gives support to this theory with one new twist. Examination of rocks brought back by Apollo astronauts found them identical to Earth rocks except for a slight excess of one lighter element in the lunar rock. The authors speculate that the Earth sustained a direct hit rather than a glancing blow and this caused the object to disintegrate entirely while stripping the Earth of its crust. The resulting debris ring eventually coalesced into the primitive Moon while raining materials back to the surface of the Earth. Heavier elements preferentially returned to Earth causing lighter elements to be higher in lunar rocks.

Focus on the planets

Venus, Saturn, and Mars grace the evening sky while Mercury and Jupiter light up the predawn hours.

Mercury starts October about a third of the way up on the eastern horizon 45 minutes before sunrise. It sinks lower with each passing day and becomes lost in the Sun’s glare by midmonth.

Venus starts the month low in the southwest at sunset and creeps a little higher each night. By month’s end Venus is a third of the way up on the horizon about an hour after sunset.

Mars rises in the south as Venus and Saturn sink towards the horizon. Look for Mars about an hour after sunset on Oct. 7 when it is just to the lower left of the Moon. Mars is growing steadily fainter and smaller so few features can be made out even with a telescope.

Jupiter emerges in the east a half hour before sunrise nestled alongside of Mercury. While Mercury soon vanishes from the morning sky Jupiter continues to climb rising over two hours before the Sun at month’s end.

Saturn starts the month less than a third of the way up on the southwestern horizon and steadily sinks all month. While far less vivid than in months past, Saturn’s rings remain open for viewing and the major moon, Titan, plus several others are easily spotted with a good telescope.

Uranus technically is in view all night but it will be most easily spotted high in the southwest around midnight. Look for its blue-green disk among the stars of Pisces.

Neptune is in the southwest rising two hours before Uranus where its blue-gray disk is nestled among the stars of Aquarius. A current finder chart for Neptune and Uranus may be found in the October issue of Sky & Telescope.

October events

1 Sunrise, 6:33 a.m.; sunset, 6:17 p.m.

3 Venus is low in the southwest as darkness falls with the thin crescent Moon to its upper right.

4 The Moon is at apogee or farthest distance from Earth today.

5 Saturn is just to the left of the Moon with orange-red Antares immediately below the pair.

7 Look for Mars to the lower left of the Moon about an hour after sunset.

9 Moon in first quarter, 12:33 a.m.

11 Mercury and Jupiter are extremely close together on the eastern horizon a half hour before sunrise.

16 Full Moon, 12:23 a.m. The full Moon of October is known as the Hunter’s Moon. The Moon is at perigee or closest approach to Earth. This fact, coupled with the full Moon, could make for an abnormally high tide called a spring tide.

19 The Moon will occult or cover Aldebaran, the `Red Eye’ of the Taurus the Bull during the overnight hours.

21 The Orionid meteor shower peaks during the predawn hours. The waning gibbous Moon may obscure some sightings but look for about 15 meteors per hour in the southeast out of Orion.

22 Moon in last quarter, 3:14 a.m. The Sun enters the astrological sign of Scorpio.

27 Antares, Venus, and Saturn are in an ascending diagonal line on the southwestern horizon a half hour after sunset.

30 New Moon, 1:38 p.m. The Moon enters Libra on the ecliptic.

31 All Hallow Eve or Halloween, a cross-quarter day marking the midpoint between the fall equinox and winter solstice. The Moon is at apogee for the second time this month. Sunrise, 7:12 a.m.; sunset, 5:26 p.m.

Send astronomical queries to Clair Wood at or care of the Bangor Daily News, Features Desk, P.O. Box 1329, Bangor, Maine 04402.