Did you notice the Moon this past Wednesday night? Not only was it a full Moon, but because it was at perigee (closest to the Earth), it may have appeared slightly bigger and brighter - about 9% wider than normal and about 20% brighter. The extra moonlight was caused partially by the Moon's nearness to Earth. However, in addition, the Sun's also closer to the Earth at this time of year. During winter in the Northern Hemisphere, the Earth is about 1.6% closer to the Sun than it is for most of the rest of the year. Since the Moon reflects sunlight, and since both the Earth and Moon are closer to the Sun in winter, the Moon's a tad bit brighter now.

Now that the moon is just past full, nine bright stars (first magnitude or brighter) and 3 bright planets accent the late winter night sky. Also, once Mr. Moon steps aside, some of the fainter phenomena in the night sky will become a bit more easy to see.

Currently, Jupiter is the brightest object in the night sky, The bright star Sirius is the second brightest object (the brightest star we can see in the mid latitudes of the Northern Hemisphere), and Saturn is the third brightest object. Jupiter gleams overhead in the constellation Gemini, and Saturn's in Taurus, just one constellation away in the zodiac. The zodiac refers to those 12 constellations that are on or near the ecliptic - the imaginary path the Sun takes as it moves across the sky during the day and also the path the planets take as they revolve around the Sun. Late February and early to mid March is the zodiac sign of Pisces, and you can see it hugging the western horizon not too long after sunset. Pisces is where the ruddy Mars can be found now.

When the Sun is said to be in Pisces, for example, it's supposed to means that the Sun is rising with the stars that comprise Pisces. However, because the Earth wobbles as it moves around the Sun (referred to as precession or precession of the equinoxes), the Sun has changed its position in the zodiac from the time when the twelve zodiac constellations were first identified a couple of thousand years ago. Precession results in shifting of the zodiac by one constellation approximately every 2,150 years. So even though late February is the zodiac sign of Pisces, the Sun doesn't rise in Pisces now - instead it rises with the constellation Aquarius.

At the time the pyramids were built, the vernal (spring) equinox occurred when the Sun rose with Taurus. Two thousand years ago, the vernal equinox occurred when the Sun rose with Aries, and today on the first day of spring, the constellation Pisces accompanies the rising Sun. In about two thousand more years, spring will be marked when Aquarius rises with the Sun.

For some reason when the zodiac constellations were being named, mostly animal figures were used. For example, Pisces (fish), Aries (ram), Taurus (bull). The Greeks consequently called this band of constellations the circle of little animals or zodiakos kyklos.

In addition to the band of constellations that mark the ecliptic, there's a faint pyramid or wedge-shaped band of light that's associated with the ecliptic and the grouping of stars forming the zodiac. It's called the zodiacal light (pronounced zo di' a cal). Like most low light features, in order to see the zodiacal light, you need to be away from city lights - dark skies are a must.

Whereas auroral light emanates from the Earth's upper atmosphere, and the glow from the Milky Way is light years away, the source of the zodiacal light lies within our solar system. The explanation for the zodiacal light is that meteoric dust particles (sub millimeter or micron size) found in the plane of the solar system scatter sunlight in such a way that it's faintly visible from Earth. This dust mainly comes from asteroids and comets that periodically drop off debris on their way around the Sun. Over the eons, thousands of comets have replenished the solar system with dust, most of which is concentrated along the plane of the solar system.

The diffuse light scattered by the dust particles can only be seen during evening or morning twilight. Zodiacal light is more readily seen at those times of the year when the Sun rises or sets more abruptly - the spring and fall. In the Northern Hemisphere, we can see it best an hour or two before the Sun rises in October and November and after the Sun sets in February and March. It's possible to see it at other times of the year, but when the Sun rises or sets at a more acute angle, this weak light is generally too close to the horizon to be noticeable. Near the Equator, where the Sun appears and disappears at a steeper angle than in does in the mid latitudes or near the poles, the zodiacal light may be observed throughout the year.

Zodiacal light is sometimes referred to as the afterglow of day or the false dawn. Remember the old ballad by the Platters "Twilight Time?"

"Deepening shadows gather splendor as day is done,
Fingers of night will soon surrender the setting sun...
Here, in the afterglow of day, we keep our rendezvous
beneath the blue"

Under favorable conditions, the zodiacal light is as bright as the Milky Way, and it can be seen 30 to 40 degrees up from the horizon - about 1/3 of the way to the zenith. Actually, the zodiacal light doesn't just stop half way up the sky but extends as a faint band from evening twilight to morning twilight. Under the darkest skies, if you know where to look, you might be able to convince yourself that you're seeing this band.

However, at the point in the night sky opposite of where the Sun is on the daylight portion of the Earth (antisolar point), the feeble band brightens slightly. It's not all that obvious, but nonetheless it's pretty impressive to think that in the middle of the night, a spot of light opposite the Sun can be detected with the unaided eye. This oval-shaped spot is known as the gegenshein or the counterglow and the time to look for it is around midnight (in February and March and again in October and November). The myriad dust grains along the ecliptic, when they're opposite the Sun, act like little moons that reflect light back toward the Earth. Even though it's very faint, the gegenschein is surprisingly big, about the size of your fist held at arm's length, and it's not a stationary feature - it migrates across the night sky during the course of several months.

Again, don't bother to look for the zodiacal light or counterglow in the confines of a city or the suburbs. But on a clear night sometime during the next several weeks, if you find yourself in the country, and the Moon isn't too bright, see if you can see these elusive lights.

For more about the gegenschein see the October 2000 issue of Sky and Telescope Magazine
For more about this month's big Moon see
http://science.nasa.gov/headlines/y2002/26feb_bigmoonshine.htm?list451840