The zodiacal light is a triangular white glow visible before dawn and after dusk that extends from the Sun along the elliptical plane toward the zodiac. The glow is also called false dawn. The zodiacal light is brightest after dusk in the west (northern hemisphere) or before dawn in the east (southern hemisphere) for about a month before and after the spring equinox or before dawn in the northern hemisphere and after dusk in the southern hemisphere around the autumnal equinox. However, telescopes and time-lapse photography can pick up the glow much of the year, spanning the length of the sky along the zodiac. The zodiacal light accounts for most of the light on a dark, moonless night.
How to See the Zodiacal Light
Viewing the zodiacal light requires a dark sky, so it isn’t visible within cities or when the moon is up. It’s brightest closer to the equator, but still visible the higher latitudes. In the northern hemisphere, look for the glow after sunset in the spring (pointing toward the constellation Taurus) or before sunrise in the fall. In the southern hemisphere, look for the glow before sunrise in the spring and after sunset in the fall. What you’re looking for is a sort of bent triangle or column of white or yellowish light angled toward the zodiac. While you’re looking, check the region of night sky opposite the Sun to see if you can spot the gegenschein. The gegenschein is a slightly brighter oval of light caused by backscattered sunlight.
Reason for the Zodiacal Light
The zodiacal light comes from sunlight scattering off dust particles in the zodiacal cloud. The zodiacal cloud is a region of particles floating in the space between planets. Most of this interplanetary dust lies within the plane of the solar system, so the zodiacal light appears along the ecliptic or plane of the Earth’s orbit around the Sun.
Astronomers estimate the total mass of the zodiacal cloud is about the mass of an asteroid with a radius of 15 kilometers and density of around 2.5 g/cm3. At one time, scientists thought asteroids and comets were the source material for the dust. But, recent data collected by the Juno spacecraft indicates most of the dust occurs along the orbit of Mars. The likely explanation is that the particles escape the Martian atmosphere from the planet’s great dust storms.
References
- Espy, Ashley J.; Dermott, S.; Kehoe, T. J. (September 2006). “Towards a Global Model of the Zodiacal Cloud”. Bulletin of the American Astronomical Society. 38: 557.
- Pavlov, Alexander A. (1999). “Irradiated interplanetary dust particles as a possible solution for the deuterium/hydrogen paradox of Earth’s oceans”. Journal of Geophysical Research: Planets. 104 (E12): 30725–28. doi:10.1029/1999JE001120
- Peucker-Ehrenbrink, Bernhard; Schmitz, Birger (2001). Accretion of Extraterrestrial Matter Throughout Earth’s History. Springer. ISBN 978-0-306-46689-2.
- Reach, W. T. (1997). “The structured zodiacal light: IRAS, COBE, and ISO observations”. Diffuse Infrared Radiation and the Irts. 124: 1.