How Distant Are the Stars?

Abstract

Copernicus (1473–1543) and Galileï already knew that the only object in the solar system that emits light is the sun, and that Earth, moon and planets are cool objects that we see only because they are illuminated by the sun. Some, like our moon and Mercury are quite dark and reflect only of order 10 % or less of the sunlight. Others, like Venus, Earth and the four large outer planets, reflect a large fraction of the sunlight, between 30 and 70 %. Still, the total amount of sunlight reflected by the largest planet, Jupiter, is a billion times less than what the sun itself emits. Copernicus, who in his book De Revolutionibus Orbium Celestium (About the orbital revolution of celestial bodies) proposed in 1543 that the sun and not Earth is the centre of the solar system, already suspected that the stars are in fact “suns”, objects that emit their own light. And that we see them only as faint specks of light in the night sky because their distances are enormously much larger than that of our sun. One can easily calculate that in order for us to see the light of the sun as faint as that of the brightest stars in the sky, the sun should be placed at a distance of about 3 light years (a light year is the distance that light travels in 1 year).

Space is big. You just won’t believe how vastly, hugely, mind-bogglingly big it is.

I mean, you may think it’s a long way down the road to the drug store, but that’s

just peanuts to space.

Douglas Adams, British author