The Zodiacal Light as seen from Paranal. (Credit: ESO/Y. Beletsky).
This week’s astronomy challenge is seasonal for mid-latitude observers. Around the time of the equinox, the ecliptic meets the horizon at a favorable angle and a unique phenomenon may become apparent: the zodiacal light. This diffuse band of light can be briefly seen after sunset or before sunrise from a moderately dark location. Around the fall equinox, the angle of the ecliptic favors a morning sighting for northern hemisphere viewers and around the springtime, it favors the evenings. This band looks like a diffuse pyramid of light extending upwards from the horizon, and a time exposure take on a moonless night might just reveal it.
But just what is this subtle band of light? What you’re seeing is the reflection of sunlight off of tiny dust grains scattered along the solar ecliptic. None other than Queen guitarist turned-astronomer Brian May has done a PhD thesis on the topic, and a related sister counter glow may be seen at the anti-sunward point known as the gegenschein. In equatorial latitudes, the zodiacal light may be seen year round; at higher latitudes it’s more of a seasonal affair. I’ve seen the zodiacal light rival the band of the Milky Way from a truly dark sky site. Good luck in spotting this elusive band, and if you can see it, consider yourself under fairly dark skies!
The astro-word for this week is Zodiac. This is the band of constellations centered on the ecliptic, or the apparent path the sun takes around the celestial sphere in one year. The classical zodiac of the Greeks was divided into twelve houses corresponding in constellations each spanning 30 degrees of arc, but of course the modern constellations now only roughly correspond. In fact, solar system objects such as the Moon, Sun, and planets do not neatly stay within these borders, but routinely cross into other lesser known constellations such as Ophiuchus and Sextans, as noted in a recent “non-troversy!”