Celestial Lights—October 2009
By Phyllis Burton Pitluga

How far can you see?

Ground-based telescope image of Andromeda Galaxy, by Bill Schoening and Vanessa Harvey of NOAO/AURA with funding from NSF.

The Moon is 240,000 miles away and the Sun is 93 million miles. Both are easy to see; no telescope needed. On a clear night, the most distant celestial target visible without a telescope is the Andromeda Galaxy. This autumn Andromeda is faintly visible overhead in our night sky. Because we are orbiting around the Sun, the time of night when Andromeda Galaxy is overhead occurs earlier and earlier (October 1 at 1am, October 15 at midnight, November 1 at 10pm, November 15 at 9pm and November 30 at 8pm).

Andromeda Galaxy is an enormous island of stars far beyond the stars of our own Milky Way Galaxy (see “Celestial Lights,” September 4, 2009). From a dark sky you can see a white fuzzy oval smudge of light with your eyes alone above the constellation of Andromeda. With binoculars you see it more easily because it is brighter. With a telescope, Andromeda Galaxy is so large you only see a portion of it as you scan across this galaxy.

Andromeda Galaxy is almost twice as big as our Milky Way Galaxy, shining with the light from trillions of stars. Andromeda and the Milky Way are gravitationally drawing together and will merge their stars in two to three billion years. Future observers might call it the Milkomeda Galaxy.

So how far are you seeing? Andromeda Galaxy is 13 quintillion miles away (13 followed by 18 zeroes), so far that, as with stars, we more easily express the distance in light years. One light year is how far light travels at incredible speed in one year. The light shining into your eyes from Andromeda Galaxy has been racing toward planet Earth for two million two hundred thousand years. That’s how far you can see on a clear autumn night: 2,200,000 light years.



Sky Calendar, October 2009

By following the Moon as the biggest and brightest “pointer” in the sky, during the month you can identify different planets and bright stars. On following nights you can relocate them but without the Moon—the Moon moves about 25 times its own diameter from one night to the next. The Moon is much closer than the planets of our solar system and the stars are even farther. So, when the Moon appears close to a celestial light they are truly separated by millions, billions or trillions of miles.

October 3, Saturday: Full Moon this month is the Harvest Moon because it is the full moon closest to the autumnal equinox, which occurred on September 22. Traditionally the evening light of the Full Moon illuminated fields as farmers harvested their crops. The Full Moon rises at sunset, then, for the next two weeks, the waning Moon rises about 50 minutes later each night.

October 5, Monday: Mercury is highest in the dawn sky and offers the best viewing opportunities during the first half of October before moving back toward the glare of the rising Sun.

October 8, Thursday: Saturn and Mercury are close together in the dawn sky. Mercury is the brighter of the two planets.

October 11, Sunday: the Last Quarter Moon rises after midnight and passes below Mars.

October 16, Friday: Saturn and Venus are joined by the waning crescent Moon in a beautiful dawn display. Venus is brighter than Saturn. Mercury is beneath the grouping.

October 18, Sunday: New Moon (= no Moon because dark side faces Earth). The Zodiacal Light will be visible for the next two weeks at dawn away from other bright lights; this cone of light is from myriad grains of meteoric material in the plane of our solar system; each grain is like a teeny moonlet reflecting the Sun’s light—together they create the Zodiacal Light. This time of year the cone of light is angled up nearly perpendicular so it stands up from the glow of dawn.

October 21, Wednesday: The waxing crescent Moon is passing above the red star Antares in the heart of the scorpion, Scorpius.

October 25, Sunday: First Quarter Moon (halfway across the sky at sunset) and Daylight Savings Time ends in Mexico.