Earth 1

The first to look

Portrait of Galileo in old age, white beard, black robe with white collar, holding a telescope

Galileo Galilei

1564 – 1642 · Pisa → Padua → Florence

He pointed a telescope at the sky, timed balls rolling down ramps, and insisted that what you measure outranks what you are told.

Portrait: Justus Sustermans, 1636–40, oil. Public domain, via Wikimedia Commons

Moons round Jupiter

If everything circled the Earth, nothing could circle Jupiter. Galileo drew the little stars every clear night and watched them shuffle from side to side. Scrub through the nights below.

7 Jan 1610

night

3

east

1

west

Real periods (Io 1.8 d, Europa 3.6 d, Ganymede 7.2 d, Callisto 16.7 d); starting positions are illustrative. Galileo saw three 'stars' on 7 January and a fourth on 13 January. They went round Jupiter, so not everything goes round the Earth.

The phases of Venus

The decisive one. A full set of phases, with Venus small when full and large as a crescent, is only possible if Venus goes round the Sun.

100 %

lit

0

day of 584

Around the Sun, Venus shows a full set of phases and is smallest when full (far side of the Sun) and largest as a thin crescent. Galileo saw exactly this in 1610. In Ptolemy's scheme it could only ever be a crescent.

Falling: time squared

A falling ball is too fast to time with a water clock, so he slowed it down on a ramp. In equal beats it covers distances in the ratio 1 : 3 : 5 : 7, and weight makes no difference, against two thousand years of Aristotle.

8°

ramp

1 · 3 · 5 · 7 · 9

units per beat

A heavy ball and a light ball roll together. In each equal beat of time they cover 1, 3, 5, 7, 9 units, so the total runs 1, 4, 9, 16, 25: distance grows as time squared, whatever the weight.

  • d: distance fallen from rest
  • g: the pull of gravity, 9.8 m/s² at the Earth's surface (smaller along a ramp)
  • t: time since release: double it and the distance is four times as far
“It is written in the language of mathematics, and its characters are triangles, circles, and other geometric figures.”
Galileo, Il Saggiatore (The Assayer), 1623, tr. Stillman Drake

The trial

The 1632 Dialogue made the case for the moving Earth and put the Pope's favourite argument in the mouth of a character called Simplicio. In 1633 he was made to kneel and abjure, and lived his last nine years under house arrest, where he wrote his greatest book on motion. “Eppur si muove” (“and yet it moves”) is attributed: it first appears in print in 1757.

His evidence was strong but not yet proof: the Earth's motion was only shown directly by stellar aberration (Bradley, 1729), stellar parallax (Bessel, 1838) and Foucault's pendulum (1851).

Timeline

  1. 15 Feb 1564

    Born in Pisa, son of the musician Vincenzo Galilei.

  2. 1589

    Professor of mathematics at Pisa.

  3. 1592

    Moves to Padua, in the Venetian Republic, for eighteen years he later called his best.

  4. Aug 1609

    Hearing of a Dutch spyglass, builds his own and shows an 8× telescope to the Venetian Senate.

  5. 7 Jan 1610

    Sees three small 'stars' in a line beside Jupiter; on 13 January a fourth. Within days he sees them moving round it.

  6. Mar 1610

    Sidereus Nuncius (The Starry Messenger): mountains on the Moon, countless stars in the Milky Way, Jupiter's moons, named the Medicean Stars.

  7. 1610

    Chief mathematician and philosopher to Cosimo II de' Medici in Florence. By December he is watching Venus go through phases.

  8. 1613

    Letters on Sunspots: the Sun has spots and turns.

  9. Feb 1616

    Cardinal Bellarmine tells him not to hold or defend the Earth's motion.

  10. 1623

    Il Saggiatore (The Assayer).

  11. 1632

    Dialogue Concerning the Two Chief World Systems, written in Italian for everyone.

  12. 22 Jun 1633

    Tried by the Roman Inquisition, made to abjure, sentenced to house arrest for life at Arcetri.

  13. 1638

    Two New Sciences, smuggled out and printed in Leiden: the science of materials and of motion.

  14. 8 Jan 1642

    Dies at Arcetri, blind. Newton is born that year (25 Dec, old calendar).

  15. 31 Oct 1992

    Pope John Paul II accepts a commission's finding that the judges erred.

See how his instrument grew into Webb on Telescopes, and the theory he defended on Copernicus.

Sources