What are the relative sources of light on a moonless night?
This week's question comes from John Lawrence in Dunrobin, Ontario. He asks:
Is there a measure for ambient light from a moon-less night sky, with no human light pollution? How much light do we get from Jupiter? How much do we get from the brightest star? And what portion of this night sky light comes from the Milky Way?
Craig Heinke, a professor of physics from The University of Alberta, says there is a measure for all light sources referenced in the question. If you look at the light from the night sky, without moonlight or light pollution, about one-fifth of the total light comes from the stars. Most of those comprise the Milky Way. Distant galaxies are very faint and contribute very little light.
That means the remaining four-fifths come from other sources. One of those sources is the Earth's atmosphere, which is called air-glow. Another is sunlight reflected off dust particles in our Solar System, called zodiacal light. The amounts of these sources will vary, depending on how active the Sun is, for example.
The amount of light that comes from Jupiter ranges from one-thirtieth to one-ninetieth, depending on how close we are to the planet at any given time.
The brightest star, Sirius, produces one one-hundredth, or about one percent of the total light in the night sky.