Sky Roundup: September, 2007

The Night Sky Before Dawn

Get up before the Sun rises and one can see a world perhaps not as familiar as our usual sky at night.  Most folks will view the evening sky after sunset, find their favorite objects with a binocular or telescope, and then retire well before midnight.  The more intrepid will stay out past midnight to catch a glimpse of celestial wonders that rise well into the wee hours of the morning. 

And those that stay out all night or rise before dawn will get a sneak preview of evening skies that one would usually see one season later.  That is, the sky we see just before sunrise is the same sky we’ll see one season later just after sunset.  So, since Autumn begins this month, we can glimpse the early evening sky we’d see in Winter after the Sun goes down by viewing the sky now before the Sun comes up.  That’s the way it works every season. 

Before the dawn’s early light in Autumn, one can glimpse the zodiacal light, a faint glowing wedge stretching up from the horizon.  The zodiacal light is caused by sunlight reflected from distant dust particles scattered along the orbit of Earth.  From our vantage we see only a portion of this band of dusty debris either before dawn in Autumn or after sunset in Spring.  (Image by Jerry Lodriguss.)

And by rising before the Sun, we may even have a chance to glimpse one of the sky’s lesser seen phenomena.  Known as the zodiacal light, this phenomenon is seen best well away from city lights.  Also known as the “false dawn,” the zodiacal light appears as a soft glow in the eastern sky before sunrise.   This is not an atmospheric phenomenon; rather, it resides well beyond the Earth in the plane of our planet’s orbit.  And it’s visible without aid of binocular or telescope.

The zodiacal light is the faint reflection of sunlight from dust particles that lie along the orbit of Earth.  In Autumn (from September to November), the angle at which we can see the plane of Earth’s orbit allows the us to glimpse these cosmic dust particles as a pale glow in our eastern sky well before sunrise.  The glow itself can appear as a triangular patch of light peaking away from the horizon before morning twilight.  City dwellers never see this phenomenon due to street lighting and local pollution.  Rural, mountainous, or shoreline locales are often better venues to see this phenomenon.  The same phenomenon is visible from these same remote locales in Spring (from February to March) but in the western sky and well after evening twilight. 

The source of this cosmic dust is the Solar System itself.  Largely the material left over from collisions of asteroids with one another and the debris left behind by passing comets, the dust concentrates along the ecliptic, which is the apparent path the Sun takes across our sky in the course of a year as the Earth orbits the Sun, and, the expression of the plane of the Earth’s orbit as seen from our vantage. 

The high angle that the ecliptic makes to our horizon in Autumn and in Spring allows us to see the zodiacal light.  And the phenomenon gets its name from the path along which the triangular patch of light appears to stretch, that is, along the Zodiac, the band of constellations where the Sun, Moon, and planets are always seen. 

Up in the sky this month before dawn are also several planets.  Venus, Mars, and Saturn all put in appearances before the Sun comes up. Venus was last seen in our evening sky in early Summer.  But by early this month, Venus has already reappeared in our morning sky, bright and low in the east.  Venus achieves its greatest brilliancy on September 23, the same day as the vernal equinox, the astronomical start of Autumn.

Though it rises very late the previous evening, Mars, too, is visible high in the east-northeast before dawn.  At mid-month, Mars shines in the same part of the sky as the ruddy star Aldebaran, the eye of Taurus (the Bull); both planet and star glow with a similar orange hue. 

From early in the month till mid-month, Saturn rises and shines pale yellow next to the bright white star Regulus.  These two objects are in conjunction; that is, they are seen close together in the pre-dawn twilight.   No doubt that a good binocular or telescope will show the contrast of both pale planet and bright star in the same field of view. 

Even the Moon gets in the act as it appears to graze both Saturn and Regulus.  (For Clevelanders, the waning crescent Moon in the morning sky will hide neither the planet nor the star; so, there is no occultation of either of these objects; see Almanac below.)  By late in the month, both Saturn and Regulus are seen farther apart and higher in the pre-dawn sky, and will appear all in a row with brilliant Venus. 

Finally, the pre-dawn sky offers a pleasing sight with the Moon riding just above the pretty star cluster known as the Pleiades.  Coming just fours days after the Harvest Moon (the Full Moon closest to the start of Autumn), the waning gibbous phase will hang close to that cluster of stars forming the shoulder of Taurus.

There’s always plenty to see in the night sky after the Sun goes down, but just as much worth seeing in the night sky before dawn.  Rise early and check it out.

Sky Roundup Almanac for Cleveland, September, 2007

September   3:  Waning gibbous Moon just over 1° north of the Pleiades
September   4:  Moon at Last Quarter
September   8:  Waning crescent Moon just north of Beehive Cluster
September   9:  Bright star Regulus in Leo (the Lion) less than ½° south of waning crescent Moon; Saturn less than 1° north of Moon
September 11:  New Moon
September 13:  Mercury 2° north of waxing crescent Moon
September 18:  Bright star Antares less than 1° north of Moon; no occultation seen in Cleveland
September 19:  Moon at First Quarter
September 22:  Mercury appears next to star Spica in Virgo, seen low in western sky after sunset
September 23:  Vernal Equinox, i.e., first day of Autumn
September 26:  Full Moon
September 29:  Mercury at greatest elongation, i.e., farthest east of Sun as seen in the western sky after sunset
September 30:  Waning gibbous Moon just over 1° north of Pleiades

Questions or comments?
Please contact Roy Kaelin at (216) 231-4600, x3405, or rkaelin@cmnh.org