Stars over Sisters

During the summer and winter months, when the sky is clear and dark, the majestic Milky Way can be seen arching overhead in a generally north-south orientation. That band of light, composed of billions of stars, is the disk of our home galaxy.

But in the spring and autumn seasons a lesser-known and dimmer astronomical sky glow can also be observed. Called the zodiacal light, this faint triangular-shaped luminosity is seen after sunset and before sunrise and is produced by sunlight reflecting off tiny bits of dust. The zodiacal light is so named because it is always observed within the band of zodiac constellations, where the sun, moon and planets all reside.

According to a leading theory, our solar system formed from a slowly rotating cloud of interstellar gas and dust. Over a period of millions of years this material gradually coalesced under the influence of gravity to form the sun, planets, moons, asteroids, comets and other minor members of the solar family. But not all of this matter was used up. There is a disk of leftover debris, comprised of particles ranging in size from about three feet to as small as 1/25,000 of an inch, which still exists in the orbital plane of the planets.

The zodiacal light is best seen in the spring on the western horizon because the zodiacal plane is highly inclined with respect to the horizon at this time. For the same reason, the best views occur in the fall on the eastern horizon.

A related but even dimmer light is the gegenschein. Gegenshein is also a brightening in the sky, but its shape is more oval and fainter than the zodiacal light. It occurs at a point directly opposite the sun, therefore is highest in the southern sky (for northern-hemisphere observers) at around midnight. It takes very dark skies and a keen eye to see this one.

April is a good month to try to spot the zodiacal light. Choose a dark location with very little inference from moonlight. Look toward the western horizon about an hour after sunset.

In the early morning of April 4 a lunar eclipse is visible from our area. First contact of the earth's umbral shadow on the moon happens at 3:16 a.m. PDT. For the next hour and 41 minutes the moon slips ever deeper into the dark shadow of the earth. Maximum eclipse occurs at 5 a.m. Because the moon won't track near the center of the shadow, the eclipse won't quite be total for those of us who live here in Central Oregon. To complete the lunar phase cycle for the month, last quarter will occur on April 11, new moon on April 18, and first quarter rolls around on April 25.

The Lyrids meteor shower occurs from April 16-25, peaking on April 22. These meteors often produce long trails of light, visible for several seconds. They radiate from near the bright star Vega in the constellation of Lyra and are associated with comet Thatcher, which takes about 415 years to orbit around the sun. More meteors will be seen from midnight to dawn than from dusk to midnight.

Venus shines brightly in the western sky and cannot be missed. A much-fainter Mars is there too, but look early in the month because after that it will be lost in the sun's glare. At sunset Jupiter is already riding high in the sky, and conveniently placed for observing the planet's cloud bands and four Galilean moons with a telescope. Saturn is currently in the constellation of Scorpius, near the reddish star Antares, and is visible in the southern sky from late evening until dawn.

To learn more about the night sky consider attending the first Stars over Sisters star watch of the 2015 season. It will be held Saturday, April 18, beginning at 8:30 p.m. with a presentation in the Sisters Park & Recreation District building, located at 1750 W. McKinney Butte Rd. in Sisters. After the presentation, telescopes provided by members of the Sisters Astronomy Club will be available in the northwest corner of the high school parking lot for viewing the night sky. The event is free.