Excerpts from the book “La scoperta dell’ombra” by Roberto Casati
Not datable: Night is caused by the shadow of the earth.
Not datable: The moon follows a cyclical course.
Not datable: The moon is an orb as demonstrated by the geometry of the lunar phases.
Not datable: Lunar eclipses are caused by the shadow of the earth. A solar eclipse is caused by the moon slipping between sun and earth; the sun’s corona is still visible.
Not datable: The earth is not flat, since the length of the shadow of a certain object at noon varies depending on the latitude of the place.
Not datable: The fact that the shadows lengthen and then shorten again from one equinox to another proves that the apparent course of the sun has a certain inclination as compared to the course of the stars; this is what causes the seasons of the year.
Ca. 500 BC: The moon does not wax and wane but is variably illuminated by the sun.
Aristotle (384-322 BC): The earth’s shadow on the moon during a lunar eclipse proves that the earth is a globe and larger than the moon.
Eratosthenes (273-192 BC): By comparing the shadows at two different locations of the same latitude the earth’s perimeter may be determined: its length is 250 000 stadiums (one stadium measures 157,5 m), i.e. approx. 40 000 km.
Hipparch (second half of the 2nd century BC): The moon’s average distance from the earth equals 67 earth radii, the sun’s average distance from the earth corresponds to at least 490 earth radii.
Theon of Alexandria (380 AD): The geometry of the shadows proves that light propagates linearly.
Galilei, 1610: The shadows created on the moon by the rising sun reflect an uneven surface. Therefore, the surface texture of the moon does not differ much from that of the earth.
Galilei, 1610: The mountains on the moon are up to 8000 m high.
Galilei, 1610: Venus does not shine with its own light.
Galilei, 1610: Venus’s cyclic course is not consistent with the predictions of the Ptolemaic world view. Obviously Venus is revolving around the sun and therefore the sun has to be one of the rotational centres of the universe, along with the earth and Jupiter.
Galilei, 1612: The eclipses of the Jupiter moons are a gigantic cosmic watch which may be used for measuring longitude on earth.
Gassendi, 1616: The silhouette of Mercury, which may be watched during the planet’s course in front of the sun, demonstrates that the size of Mercury is only a sixth of what had been assumed before.
Campani, 1664: Shadows demonstrate that the bizarre shape of Saturn is due to the rings that surround it.
Grimaldi, 1665: Shadows prove that light does not only propagate directly and by reflection, but also by refraction.
Roemer, 1676: The time lag of the eclipses of the Jupiter’s moons that may be watched at various times during the year indicates that light propagates with a finite speed and makes it possible to figure out the speed of light.
1761: The planetary course of the Venus in front of the sun allows the first exact measurement of the sun’s parallax.
Eddington, 1919: Solar eclipses allow the observation that a star in the constellation of Taurus, through which the sun moves, seems slightly displaced. The deflection of the light by the mass of the sun coincides with the predictions of Einstein’s theory of relativity.
Dunham, 1980: The discrepancy between the real and the theoretical progress of a solar eclipse leads to the conclusion that the sun’s radius has shrunk by about one third of an arc second during the course of three centuries.
Stephenson & Morrison, 1984: The discrepancy between the real and the theoretical progress of approximately seven hundred historical eclipses leads to the conclusion that the earth’s rotation has slowed down by about a twentieth of a second during the course of two thousand and five hundred years.