Sidereus Nuncius: What Galileo Saw in 1610
On 13 March 1610 a short Latin book appeared in Venice, printed by Thomas Baglioni Sidereus Nuncius, Wikipedia[1]. Its title, Sidereus Nuncius, is usually translated as “The Starry Messenger” Sidereus Nuncius, Wikipedia[1]. In it Galileo Galilei reported what he had seen through a telescope he had built himself, and the sky never looked the same again. Our Galileo Galilei sticker sums up his career: telescope, tea, trouble with the Church.
What telescope did Galileo use?
Galileo had heard of the Dutch telescope and built his own improved version Sidereus Nuncius, Wikipedia[1]. One of his first instruments magnified 8 to 10 times, using lenses he had ground himself. The instrument he used for the observations in the book magnified 20 times Sidereus Nuncius, Wikipedia[1]. He was not the first person to point a telescope at the night sky, but his was the first published study of celestial bodies made with one Sidereus Nuncius, Wikipedia[1].
What did Galileo see on the Moon?
The book contains more than seventy drawings and diagrams of the Moon, of constellations such as Orion, the Pleiades and Taurus, and of the moons of Jupiter Sidereus Nuncius, Wikipedia[1]. The Moon was not the perfect sphere of Aristotelian cosmology. It had mountains and valleys, and Galileo could estimate their height.
How do you measure a mountain on the Moon?
Near the terminator, the line between lunar day and night, the peaks of mountains catch sunlight while the ground around them is still dark. A bright point at distance d beyond the terminator is a peak just tall enough to reach the Sun's grazing rays. Right-triangle geometry then gives its height:
h = √(R2 + d2) − R ≈ d2 / 2R
where h is the mountain height in m, R is the Moon's radius (about 1.737 × 10⁶ m) and d is the distance in m from the terminator to the lit peak. With d = 100 km the peak is about 2.9 km high (our arithmetic). Using the distance of sunlit mountaintops from the terminator, Galileo judged, quite accurately, that the lunar mountains were at least four miles high Sidereus Nuncius, Wikipedia[1].
What did he see among the stars?
The telescope showed hundreds of stars invisible to the naked eye, in the Milky Way and in familiar constellations Sidereus Nuncius, Wikipedia[1]. Some of the “nebulous” stars of the Ptolemaic catalogue resolved into many small stars. Galileo concluded that the nebulae and the Milky Way were clusters of countless stars too small and distant for the eye to separate Sidereus Nuncius, Wikipedia[1].
How did Galileo discover Jupiter's moons?
Galileo first observed the moons of Jupiter on 7 January 1610 Sidereus Nuncius, Wikipedia[1]. Simon Marius found them independently at nearly the same time, on 8 January 1610 Galilean moons, Wikipedia[2]. Galileo named them the Medicean Stars and dedicated the book to his former pupil Cosimo II de' Medici, Grand Duke of Tuscany, in the hope of patronage Sidereus Nuncius, Wikipedia[1]. The names Io, Europa, Ganymede and Callisto were Marius's choice Galilean moons, Wikipedia[2].
Why did four small moons matter so much?
One objection to a moving Earth was that the Moon would be left behind if Earth moved around the Sun. Jupiter visibly moves, and it keeps four moons. Here was a second centre of motion in the sky, something the Earth-centred picture did not allow for.
The moons also weigh Jupiter. In 1621 Kepler noted that his third law, published in 1619, applies to the four brightest moons of Jupiter Kepler's laws of planetary motion, Wikipedia[3]. With Newton's gravity it takes the form Kepler's laws of planetary motion, Wikipedia[3]
M = 4π2a3 / (G·T2)
where M is Jupiter's mass in kg, a is a moon's orbital semi-major axis in m, T is its orbital period in s and G = 6.674 × 10⁻¹¹ m³ kg⁻¹ s⁻². Io's orbit has a = 421 800 km and T = 1.769 days Galilean moons, Wikipedia[2], which gives M ≈ 1.90 × 10²⁷ kg. The other three moons give the same value to within about 0.1% (our arithmetic).
The inner three moons are locked in a 4:2:1 orbital resonance Galilean moons, Wikipedia[2]. In terms of their mean motions n, in revolutions per day, the resonance means
nIo − 3·nEuropa + 2·nGanymede ≈ 0
With the tabulated periods of 1.769, 3.551 and 7.155 days Galilean moons, Wikipedia[2], the left side is 0.565 29 − 0.844 83 + 0.279 52, which is zero to within 0.0001 rev/day, well inside the rounding of the periods (our arithmetic).
What did Galileo see after the book?
- Venus. At the end of 1610 Galileo observed Venus going through a full set of phases, though he did not publish this until 1613 Phases of Venus, Wikipedia[4]. He first sent Kepler a scrambled Latin anagram to establish priority Phases of Venus, Wikipedia[4]. The phases ruled out the Ptolemaic system, but they were compatible with both the Copernican and the Tychonic systems Phases of Venus, Wikipedia[4].
- Saturn. In 1610 he saw Saturn's rings but could not resolve their shape Rings of Saturn, Wikipedia[5]. He described them as Saturn's “ears”. They vanished in 1612, when Earth passed through the ring plane Rings of Saturn, Wikipedia[5]. In 1655 Christiaan Huygens was the first to describe them as a disk Rings of Saturn, Wikipedia[5].
- Neptune. Galileo recorded Neptune in his notebooks in 1612 as one of many dim stars, without realizing it was a planet Galileo Galilei, Wikipedia[6].
How was the book received?
Some readers doubted that the telescope could be trusted. Johannes Kepler was the first astronomer to publicly support Galileo's findings, in an open letter in April 1610. He could not publish an independent confirmation until August 1610, because sufficiently powerful telescopes were scarce Sidereus Nuncius, Wikipedia[1]. Galileo's later defence of Copernicus in his 1632 Dialogue led to his trial in Rome in 1633 Galileo Galilei, Wikipedia[6].
For what came next, see how Newton built on Galileo and Kepler. The direct proof that Earth rotates came two centuries later; see the Foucault pendulum. There is more sky in our Astrophysics collection.
Quick FAQ
Did Galileo invent the telescope?
No. He improved a Dutch invention and was the first to publish a study of the sky made with one Sidereus Nuncius, Wikipedia[1].
Did Galileo prove the Earth goes around the Sun?
Not on his own. His observations ruled out the Ptolemaic system, but the phases of Venus also fit Tycho Brahe's model Phases of Venus, Wikipedia[4].
Can I see Jupiter's moons myself?
Yes. Ordinary binoculars or a small telescope show them as points of light beside Jupiter that change position from night to night.
References
- Wikipedia, “Sidereus Nuncius”. https://en.wikipedia.org/wiki/Sidereus_Nuncius
- Wikipedia, “Galilean moons”. https://en.wikipedia.org/wiki/Galilean_moons
- Wikipedia, “Kepler's laws of planetary motion”. https://en.wikipedia.org/wiki/Kepler%27s_laws_of_planetary_motion
- Wikipedia, “Phases of Venus”. https://en.wikipedia.org/wiki/Phases_of_Venus
- Wikipedia, “Rings of Saturn”. https://en.wikipedia.org/wiki/Rings_of_Saturn
- Wikipedia, “Galileo Galilei”. https://en.wikipedia.org/wiki/Galileo_Galilei
Written by Pixelated Physics. Every factual claim is linked to the source we checked; points that are still debated are labelled open.