The Sky Guide Challenge
See the Zodiacal Light.
The Solar System is a dusty place. The majority of what’s known as interplanetary dust is thought to have originated from Jupiter-family comets. These are short-period comets with orbits influenced by Jupiter’s gravity. They typically have periods less than 20 years. The cometary dust is small with typical particle sizes in the range 10-300 microns. Over time, collisions further reduce particle size. Below 10 microns, dust is removed from the inner Solar System by radiation pressure but is continually replenished by around 400 Jupiter-family comets.
Sunlight interacting with the dust particles creates the Zodiacal Light phenomenon and this month’s challenge is to to see and photograph it. Before you do, however, it’s important to understand that the Zodiacal Light is extremely subtle and easily lost in even low-level light pollution. A good, dark sky is essential, as is perseverance.
As its name suggests, the phenomenon is connected with the Zodiac, the circle of constellations through which the Sun appears to travel during the year. In reality, it’s the movement of the Earth along its own orbit that makes the Sun appear to move. The apparent path of the Sun defines the great circle known as the ecliptic.
Interplanetary dust occupies a lenticular-shaped volume centred on the Sun with most of the material concentrated along the ecliptic plane. Consequently the dust is best seen along the ecliptic and this gives us some clues as to the best times of the year to try and see it.
Certain months present better opportunities for mid-northern latitudes. Around the Northern Hemisphere’s spring equinox for example, the Zodiacal Light is optimally tilted to the western horizon after sunset. This is because the ecliptic creates a steep angle with the western sunset horizon at this time of year. Around the Northern Hemisphere’s autumn equinox, the same is true of the pre-sunrise ecliptic relative to the eastern horizon.
When visible, the Zodiacal Light presents itself with a distinctive conical shape. The cone’s vertical axis runs along the ecliptic and the edges of the cone bulge outward. The best time to try and see the Zodiacal Light is during the 90-minute window that starts 90 minutes after the Sun sets.
If you fail to see it visually, a wide-angle camera pointed in the directions mentioned above at the correct times may fare better. The camera should be set to produce a relatively deep but non-overexposed sky. A time-lapse sequence may also help because it can be played back to show whether the characteristic Zodiacal Light shape was really there. Do let us know if you manage to see or photograph it by posting your pictures on www.facebook.com/skyatnightmagazine or @skyatnightmag.