First steps on the distance ladder
Aristotle (384–322 BC) was the first to estimate the size of the Earth, using the angle of the shadow of a pole at noon at a location 100 miles south of the equator. Eratosthenes and Posidonius later used a similar method. These latter estimates are within about 10% of the modern value.
In the second century BC, Hipparchos used a solar eclipse method to estimate the distance of the Moon and deduced a value of 59 Earth radii, a little under the modern value of 60.3. Aristarchos tried to estimate the distance of the Sun using a lunar eclipse, but was out by a factor of 20.
The Greeks also gave us Euclidean geometry (Euclid 300 BC), the idea of absolute, uniform time (Aristotle), and the idea of an infinite physical frame (the atomists, Epicurus). Interestingly, and contrary to the picture held by medieval thinkers, Aristotle believed that the stars were at a range of distances.
The Aristotelian Earth-centred (geocentric) universe reached its epitome in the detailed system of Ptolemy (first century AD), described in The Almagest. The Arabic-derived title reflects the crucial role of Arab scientists in preserving and extending the achievements of the Greeks. Certainly Nicolaus Copernicus (1473–1543) was aware of Arab work in his development of a Sun-centred (heliocentric) model of the solar system. A discovery of Copernicus that is less well-known is that he gave, for the first time, the correct relative distances of the Sun and planets. His values were within 5% of the modern values and the absolute scale of the solar system was not determined more accurately till the 19th century. The Copernican system also implied a huge increase in the minimum distance of the stars.
Galileo Galilei’s discovery in 1609 of the moons of Jupiter lent weight to Copernicus’s picture of the planets orbiting the Sun. His discovery of mountains on the Moon showed this was another world like the Earth. And his resolution of the Milky Way into stars was the first step into the universe of galaxies. Galileo’s work on kinematics demonstrated the limitations of Aristotle’s physics and paved the way for the system created by Sir Isaac Newton (Newtonian mechanics) and still used for most everyday situations today. In his interesting dialogue with the philosopher Richard Bentley, Newton also discussed the idea of an infinite universe of stars.
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