One of the least known components of our solar system is the interplanetary dust that fills the disk in which the inner planets revolve. These microscopic dust particles can be seen only under special circumstances. They are very small and very few and far between, but numerous enough to cause most of the meteors that we see streaking through the night skies.
They are also visible en masse as a faint haze along the ecliptic really dark nights, known as the zodiacal light.
The zodiacal light is brightest when we look towards the sun ? when it's not up. The best times of the night are just after evening twilight and just before morning twilight, but even then the light is hard to see. Chances improve at certain times of year when the ecliptic is nearly vertical in the sky, particularly September and October in the morning and February and March in the evening.
This week we enter the first of two viewing windows for the zodiacal light in the morning sky. For the next two weeks, it will be visible about half an hour before the beginning of morning twilight, forming a huge dim cone of light in the eastern sky.
The attached chart marks the ecliptic, where the light is concentrated, but it actually is a huge glow covering a much larger area of sky. In fact, the zodiacal light is the third brightest ?object? in the sky, after the sun and moon, with an integrated magnitude of ?8.5 but, because it?s spread over such a huge area, it is impossible to see with any optical aid, and only visible with the naked eye with difficulty. Strangely, it?s possible for something to be too big to be seen.
The zodiacal light is easily mistaken for the Milky Way, but this is a good time to observe it because the zodiacal light is aligned up and down in the sky while the milky way runs almost horizontally at this time of the year.
If the zodiacal light is hard to see, the Gegenschein is even harder. ?Gegenschein? is German for ?counter-glow,? and refers to the concentration within the zodiacal band directly opposite the sun in the sky. Like the lettering on highway signs, the area of the zodiacal band directly opposite the light source (the sun) glows brighter than the rest.
Very few astronomers have ever seen this, but it?s certainly worth trying for, if you have a truly dark observing site. Here you must wait until local midnight, so that the sun is directly beneath your feet. But that is a target for another time, as it is best observed on a moonless midwinter night.