Steve’s Virtual Planet

Jan 7, 1610

As we start the New Year, this is as good a time of year as any to find inspiration from history. Today is one of those recollections of a day when our world changed over night (well, at least once the data was published in March 1610!).

On Jan 7, 1610, the discovery of three of Jupiter’s moons by Galileo Galilei and a fourth one on Jan 15, provided the evidence that the heliocentric theory proposed by Copernicus was indeed valid. As the details were actually put in print in Galileo’s book Siderius Nuncius (The Starry Messenger), the simple fact of this discovery was not only a wonderful revelation of nature but a complete rebuttal of the idea that the Earth was the center of … well, everything!

This so-called geocentric hypothesis had held sway since the days of Ptolemy (c. 100 – c. 170 AD). However, what Galileo clearly saw, magnified in the telescope, was another world in our solar system that had not been revealed like this before. And since the moons could be shown to orbit the planet, it meant they were NOT orbiting the Earth!! One might say that this discovery shook things up a little!

He might have seen something like this representation on January 7th through his homemade 20 power telescope. From left to right, the objects in the view are Ganymede, Jupiter, Europa, Io & Callisto.

In the simulation above, Callisto and Ganymede are the farthest out but Io and Europa are very close together. So, Galileo may not have been able to split them that first night. Nevertheless, his persistence and continued observations over the next few nights showed clearly how the moons were shifting position in relation to Jupiter itself and the entire collection was drifting against the background star field. To him, this movement of the objects were the clincher in concluding this was not just a planet, but one with orbiting moons.

This set of observations comprised one of the most consequential discoveries in the history of science. Due to Galileo’s keen sense of the importance of the observation and the empirical method he was indeed a giant of the age of modern science. Einstein himself in 1933 referred to Galileo as “the father of modern physics and and in fact of the whole of modern natural science.”

That is powerful inspiration indeed.

originally posted on Jan 7, 2025