Seeing Stars

If you step outside at night from a bright room, you won’t see many stars at first. It takes up to 30 minutes for your eyes to become “dark-adapted.” But then how many stars can you see?

A reader writes, “Suppose you had a clear, moonless night. How many stars could you see with the eye alone?”

There’s really no definitive answer to this question. No one has counted all the stars in the night sky, and astronomers use different numbers as theoretical estimates. Some estimate as many as 10,000 visible stars. Others place the number at more like 5,000. Even under ideal conditions, there’s a fair amount of variation between how well people can see the stars — depending on things like the strength of your vision — and your age. As you get older, for example, your eyes become much less sensitive to faint light.

You also have to take into account the brightness of your night sky. Even on a moonless night, the glow of lights from Earth’s surface brightens the sky. Also, suppose we use the estimate of 5,000 visible stars. At any given time, half of Earth is in daylight — so only half that number — maybe 2,500 stars — would be visible from Earth’s night side. And another fraction of those stars would be lost in the haze all around your horizon. Still — far from city lights — under absolutely perfect conditions of darkness and sky clarity — a person with normal vision should indeed be able to see thousands of stars. To ask us your science question, go to our contact page.

What you have in your mind?