Atoms & Light

Part 9 - Astronomers

Until the 17th century, most Europeans accepted Ptolemy's theory of 150 CE. The Earth was stationary at the centre of the universe while the stars were embedded in a large outer sphere which rotated daily. The Sun and Moon were embedded in their own, smaller spheres. Ptolemy had devised elaborate ways to explain why the wandering stars (planets) did not follow simple, circular orbits around the Earth.

However, in the 10th century, Islamic scholars, including Ibn al-Haytham of Basra, criticized Ptolemy's theory and others questioned why the Earth should be immobile at the centre of the universe.  Abu Sa'id al-Sijzi, concluded that what appeared to be the motion of the heavens was actually due to the Earth rotating around its axis. 

In Poland, Nicolaus Copernicus (1473 – 1543) suggested that the Sun was at the centre of the universe (the heliocentric theory). Aristarchus of Samos had suggested this 1800 years earlier and both Aristotle and Plutarch had mentioned the idea. Between 1512 and 1515, before the telescope was invented, Copernicus carefully observed the solar system and discovered the variability of Earth's motion and of the movement of the solar apogee in relation to the fixed stars. (As the Earth's orbit is elliptical, the maximum distance between the Sun and the Earth (the Solar apogee or aphelion) is about 94.5 million miles. The perigee or perihelion is about 91.6 million miles. The Earth is nearest to the sun around January 3 and farthest about July 4).

Copernicus claimed that all the planets rotated around the Sun and the stars were infinitely more distant from the Sun than was the Earth. Also, the apparent movement of the stars across the sky was caused by the Earth's daily rotation and its annual journey around the Sun, 'while the firmament and highest heaven abide unchanged.'

Rumours of Copernicus's discoveries became widespread in Europe but Copernicus, perhaps concerned about religious objections, delayed publication of his work. The publication just before his death in 1543, was a major event in the history of the Scientific Revolution. His book, On the Revolutions of the Celestial Spheres, was dedicated to Pope Paul III.

In March 1616, the Catholic Church issued a decree against Copernicus's book until it could be "corrected," to ensure that it did not prejudice Catholic truth.

However, Copernicus's theory was not generally accepted until Kepler and Galileo produced substantial evidence that it was correct. 

Tycho Brahe (1546 -1601), was a Danish astronomer and one of the last to work without a telescope but he was known for his accurate astronomical observations. He developed his own model of the universe with the planets orbiting the Sun but with the Sun and moon orbiting the Earth.  In 1573, he refuted the Aristotelian beliefs with precise measurements proving that the "new star" of 1572 (a supernovae), was not a tailless comet in the atmosphere but, as it showed no parallax, it was beyond the Moon. In the same way, he demonstrated that comets were also not atmospheric phenomena and must pass through the supposedly unchangeable celestial spheres.

Johannes Kepler (1571 - 1630) was a German astronomer who lived at a time when there was no distinction between astrology and astronomy. But, he was a superb mathematician as well as a competent astrologer. He maintained that the Sun was the principal source of motive power in the universe and, in 1596, he published a defence of the Copernican system. In 1600, Kepler obtained work with Tycho Brahe and through most of 1601 he analyzed Tycho's planetary observations. After Tycho suddenly died on October 24, Kepler was appointed his successor as imperial mathematician and astrologer to Emperor Rudolph II.

In 1603, Kepler worked on the problems caused by the optics of atmospheric refraction and the unexplained phenomena associated with both lunar and solar eclipses, such as the unusual light surrounding a total solar eclipse and the red colour of a total lunar eclipse. He published his research, in The Optical Part of Astronomy, describing reflection by flat and curved mirrors, the optics of pinhole cameras and the inverse-square law governing the intensity of light. This included astronomical optics such as parallax and the apparent sizes of heavenly bodies.

A bright, new star appeared in October 1604 and, as an astrologer, Kepler expected this to foretell significant events but he became skeptical about astrology as the nova faded without incident. But, he did speculate about its origin, arguing that it must be in the sphere of fixed stars as it showed no parallax and this refuted the theory that the celestial spheres were perfect and unchanging. 

Kepler theorized that the power radiating from the Sun weakened with distance causing the planets to move more slowly the farther they were from the Sun. He calculated and recalculated various approximations of Mars' orbit using Ptolemaic circles (the mathematical tool that Copernicus had rejected) but he was dissatisfied with the inaccurate result.

Based on measurements of the aphelion and perihelion of the Earth and Mars, he devised a formula where a planet's rate of motion was inversely proportional to its distance from the Sun. But, this required extensive calculation and, in 1602, Kepler reformulated the problem in geometrical terms; a line between the Sun and a planet would sweep out an equal area in an equal time.

In 1605, he tried to calculated an ovoid (egg-shape) that fitted the observed orbit of Mars but, after about 40 failed attempts, he was astonished to find that an elliptical orbit fitted the Mars data perfectly. He concluded that all the planets moved in elliptical orbits with the sun at one focus of the ellipse. (Kepler had earlier studied conic sections, an ellipse being an angular section of a cylinder, the circle being a special case when the two foci of an ellipse coincide at the centre. A parabola being a section of a cone). 

Using one of the first telescopes early in 1610, Galileo Galilei was astonished to discovered four moons orbiting Jupiter and asked Kepler to confirm his findings. Kepler quickly confirmed Galileo's observations and later published his own telescopic observations of Jupiter's satellites.

Kepler also established the theoretical basis of double-concave diverging lenses and double-convex converging lenses and how they combined to produce a Galilean telescope. He also described a telescope, having a higher magnification than Galileo's, with two convex lenses later known as the astronomical or Keplerian telescope.

In 1615, Kepler completed the first of three volumes of his most influential work Epitome astronomiae Copernicanae (Epitome of Copernican Astronomy), published between 1617 and 1621, these culminated with his ellipse-based orbital system and contained his three laws of planetary motion but did not explain how elliptical orbits could be derived from observational data.

Kepler's laws of planetary motion.

1. The orbit of a planet is an ellipse with the Sun at one of the two foci.

2. A line joining a planet and the Sun sweeps out equal areas during equal intervals of time.

3. The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit.

The second law shows that when a planet is closer to the Sun, it travels faster. The third law shows that the farther a planet is from the Sun, the longer is its orbit.

Kepler's laws of planetary motion were not immediately accepted and their significance was not realized until Christiaan Huygens, Isaac Newton and Edmund Halley proved that the gravitational attraction between the Sun and the planets decreased by the square of the distance between them and that centrifugal force kept the planets in their orbits.

Later in the 17th century, a number of theories based on Kepler's work began to appear culminating in Isaac Newton's Principia Mathematica (1687).