The zodiacal light is sunlight scattered by dust in the plane of the planets
A thin disk of dust, shed by comets and asteroids and much of it from Jupiter-family comets, fills the inner solar system along the plane the planets orbit in. Sunlight scattered by the dust makes a faint cone of light that rises from the horizon along the ecliptic, the Sun's yearly path across the stars through the zodiac constellations. Under a dark sky it can rival the Milky Way, and the Persian astronomers called it the false dawn.
Look in the west after dusk in January to April, and in the east before dawn in September to November
The cone lies along the ecliptic, so it shows best when the ecliptic stands steeply up from the horizon at the end of twilight. At latitude 40°N that happens on spring evenings and autumn mornings; on autumn evenings and spring mornings the ecliptic lies low and the light is lost in the haze. The angle at the end of evening twilight and the start of morning twilight, on the 15th of each month:
| Month | Evening angle | Morning angle | Best view |
|---|---|---|---|
| January | 65° | 46° | Evening, west |
| February | 72° | 36° | Evening, west |
| March | 73° | 30° | Evening, west |
| April | 65° | 27° | Evening, west |
| May | 51° | 27° | - |
| June | 36° | 31° | - |
| July | 28° | 44° | - |
| August | 27° | 60° | - |
| September | 29° | 70° | Morning, east |
| October | 33° | 73° | Morning, east |
| November | 41° | 69° | Morning, east |
| December | 53° | 58° | - |
The months swap in the Southern Hemisphere: evenings from August to October, mornings from March to May. Near the equator the ecliptic is steep all year.
A moonless sky away from city lights is the one requirement
- Find a site of Bortle class 3 or darker, with a clear western horizon for the evening or eastern for the morning. The dark sky places list rates sites by Bortle class.
- Go in the two weeks around new Moon, so the Moon is not up during the hour after evening twilight or before morning twilight.
- Start looking when astronomical twilight ends, about 90 minutes after sunset, or 90 to 120 minutes before sunrise. The light lasts about an hour.
- Look for a tilted cone, wider at the bottom, leaning along the ecliptic. It is easy to mistake for the glow of a distant town, but it lines up with the zodiac stars and the planets, and it moves with them.
The gegenschein is a faint glow exactly opposite the Sun, best in October-November and February-March
The gegenschein, German for counterglow, is a faint oval patch about 10° across at the antisolar point, the point in the sky opposite the Sun, where the dust reflects sunlight straight back like the full Moon. It is highest around midnight. It is harder than the zodiacal light: it needs a Bortle class 1 or 2 sky, no Moon, and a time when the antisolar point is away from the Milky Way. In December and January it is highest but lost in the Milky Way, and in June and July it is low and in the Milky Way of Sagittarius. From the darkest sites the zodiacal band, a faint strip along the ecliptic, links the gegenschein to the zodiacal light on either side.
A wide lens and a 20-second exposure show the cone easily
Use a camera on a tripod with a wide lens, ISO 1600 to 3200 and an exposure of 15 to 30 seconds, aimed along the ecliptic toward the horizon. The camera shows the cone even when the eye barely sees it, and a wide view takes in the Milky Way crossing it.