Obviously, the sky is dark at night because there is no Sun. But to Heinrich Olbers (1758-1840), a German physician and astronomer, the answer was a little more complicated. In his day not very much was known about the universe at large. In particular, the size and extent of the Milky Way Galaxy was unknown. Also, we did not know about the existence of other galaxies. When people looked up at the night sky, they saw only the Sun by day and the Moon and stars by night. For all they knew to the contrary, the stars could be distributed more or less evenly through all of infinite space-although the band of light across and around the sky that gives our galaxy its name was a tantalizing curiosity.

Olbers reasoned that if the universe were truly infinite and if the stars are distributed throughout all space, there should be a star - actually an infinite number of stars - along every possible line of sight. If this were actually the case, he thought, the whole sky should be bright as the Sun.

As the 19th century progressed, it became clear to astronomers that the Milky Way was not an infinite expanse of stars but, rather, a kind of disk-shaped cloud of stars, actually a vast pin-wheel shaped cloud with a bulge in the centre. If you cut out a cardboard disk roughly 30 cm (12 inches) in diameter, make a hole in the middle and stuff in a tennis ball, you'll have a pretty good model of the Milky Way Galaxy.

If the Milky Way was simply a cloud of stars, limited in extent, here was an obvious answer to why the sky is dark at night. We are mere light minutes from the Sun, light-years from all other stars. Their combined light was not sufficient to come even close to rivaling the Sun in brightness. The night sky had to be dark.

But around the same time, astronomers began to understand that many of the oddly shaped luminous clouds that they could see in space must be other galaxies external to the Milky Way. And they seemed to be scattered around the sky quite randomly. Unlike the stars of the Milky Way, there was no concentration of galaxies in any part of the sky. And galaxies, of course, are composed of stars. So if the galaxies extended forever through infinite space, there must be an infinite number of galaxies along every imaginable line of sight. Why, then, is the night sky not as bright as the Sun?

Along came Edwin Hubble. He it was who discovered that all the galaxies are rushing away from us at velocities proportional to their distances. At some point, those velocities become so great that their light is red-shifted to invisibility.

And that, dear reader, is why the sky is dark at night.

Bill Broderick is a director of Humanist Canada and chair of Humanist Quinte. Meetings, January through June, are in Room P-22, Loyalist College, fourth Sunday of each month, 1:30 p.m.. He can be reached at This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

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