Galileo's First Jupiter Observations

Sidereus Nuncius, published in 1610, is Galileo's account of his first astronomical observations using a telescope. He found that the surface of the Moon, like Earth, is rough and uneven, that the Milky Way and several nebulas are made up of numerous stars too faint to see individually with the naked eye, and most famously, that Jupiter has four large satellites.

Over the eight weeks from January 7 to March 2, 1610, Galileo sketched 64 observations of the positions of these four moons relative to Jupiter. The following pages reproduce all 64 sketches, along with a modern calculation of the moons' positions and some brief commentary. Look for the first night that Galileo realized the moons weren't stars, the night he first saw four moons, not just three, and the only night he drew a moon that wasn't there.

I've relied on the English translation of Sidereus Nuncius by Albert Van Helden as well as the edition in the original Latin held by the Linda Hall Library in Kansas City, MO and made available online by the Cultural Heritage Language Technologies consortium. I've redrawn each of Galileo's figures, being careful to preserve both position and relative scale.

I wrote a program to produce the modern diagrams using the methods described in chapter 44 of Jean Meeus's Astronomical Algorithms, second edition. The method was extended to provide shadow positions in the manner of Sky and Telescope's online Jupiter calculator. In order of increasing distance from Jupiter, the color-coded moons are Io, Europa, Ganymede and Callisto.

Each Galileo sketch was scaled to provide the best fit to the positions of the moons. Galileo had not yet devised a method of rigorously measuring what he saw, and the optics of his 1610 instrument wouldn't have allowed it in any case, so his sketches are qualitative, and the scale of the drawings here will vary significantly.

All of the times in the figures are UT. Galileo reported the times of his observations as a number of hours, and sometimes minutes, after sunset. The times in the figures are derived from my calculation of the time of sunset in Padua using the method described by Paul Schlyter.