Galileo's original diagrams

Elsewhere on this site, you'll find photos of Newton's original statements about mechanics, which I like to show pupils to give a bit more historical context than they might get otherwise. I've also been able to look through an original copy of Galileo's Dialogo of 1632, which he got into a spot of bother for publishing because it rather unsubtly proposed a heliocentric model for the solar system.

In the Dialogo, Galileo gives some diagrams which might strike us today as fairly unexceptional, but in the history of science they were highly significant:

This is Galileo's proposed model of the solar system. The Sun is, fairly obviously, at the centre with the Earth and Moon shown orbiting third out after Mercury and Venus. After Mars, Galileo then shows Jupiter with the four moons that he had been the first human to see, just twenty or so years earlier. Nothing is shown orbiting beyond Saturn because none of the planets out there could be seen with 17th century telescopes.

In this diagram, Galileo shows how heliocentric orbits can easily explain the retrograde motion of superior planets. The Earth's orbit has capital letter labels, Jupiter has lower case letters, and the arc at the top of the image shows the distant stars against which Jupiter seems to occasionally move backwards as seen from the Earth. This diagram is often seen - hardly altered at all - in modern astronomy textbooks.

In this diagram, Galileo explains how the seasons occur. The Earth is shown at four significant points in its orbit around the Sun: as experienced in the Northern hemisphere, they are

On the left: at midsummer (June)

At the top: at the autumn equinox (September)

On the right: at midwinter (December)

At the bottom: at the vernal equinox (April)

As with the previous diagram, this also appears almost identically in textbooks and on the web to this day.


More than half a century after Galileo's Dialogo, Newton set out his Laws of Motion in Book I of the Principia and built up to showing in Book III that his law of gravitation explained everything then known about orbits of all kinds. Along the way, he gave the reader some great diagrams:


Here, Newton shows how observations of the great comet of 1680 show that it followed a parabolic path around the sun. I think it's fair to say that the tail lengths shown are not exactly to scale, but it's a good representation of the way comets' tails point away from the sun.