What happens when you point a telescope at the sky?

That time, that place

Word arrived from the Netherlands of a tube that made distant things look near. Galileo, hearing only the principle, built one himself, then ground far better ones than anyone else's. And he turned it on the sky.

Why this question

Here the real problem starts. If two pieces of glass show you what the naked eye cannot, may you believe it?

Opponents said the telescope manufactured illusions — that passing through glass created what was not there. It was not an unreasonable objection.

What was found

Galileo verified on the ground first. He looked at a distant building through the tube, then walked there and checked — over and over. He established that the instrument does not lie before asking anyone to trust it about the sky.

Then, what he saw. The Moon has mountains and valleys, and their shadows change length: it is not a perfect sphere. Four small stars accompany Jupiter, shifting position nightly — they orbit Jupiter. So not everything circles the Earth.

Venus was decisive. Venus waxes and wanes like the Moon, all the way to fully round. In Ptolemy's system that phase is impossible.

Terms on this card

InertiaThe telescope