I mean that the angle between these angular momenta is acute as opposed to obtuse (23.5 and not 180-23.5), so that the rotations are "roughly in the same direction".

Related but distinct question -- when was this actually discovered?

  • Wouldn't the Sun rise in the west otherwise? Meanwhile the Sun progresses in the same direction through the constellations along the ecliptic. – Rob Jeffries Oct 21 at 14:15
  • I am admittedly confused. I was under impression that "west" was where the sun sets "by definition" (just like "clockwise" is the direction of suns movement in the sky/gnomon's shadow in the sundials in the northern hemisphere). But I think you mean that the sun's path in the ecliptic relative to the fixed stars would change direction -- then you are right, and I thought "you can get a pretty accurate idea of the Sun's path by observing the rising & setting of stars and planets that are near the Sun's path" was the part of PM 2Ring's answer that addressed this. – Max M Oct 21 at 21:38
up vote 13 down vote accepted

Certainly! Ancient astronomers, eg in Babylon ~3000 years ago were sufficiently familiar with the geometry of the celestial equator and the ecliptic to be able to predict eclipses, and discover the eclipse series now known as the Saros series. But of course, in those days astronomers (mostly) didn't conceive of them in terms of the motion of the Earth.

OTOH, there were a few ancient astronomers in ancient Greece (notably Aristarchus of Samos), and even in Babylon (Seleucus of Seleucia), who did believe in a heliocentric system.

The celestial pole is easy to discover, (especially if you're in the northern hemisphere), and it doesn't take a lot of observation to notice that the ecliptic differs from the celestial equator.

Although it's not possible to observe directly that the Sun's path is the ecliptic, it was known that all of the other celestial wanderers stay fairly close to the ecliptic, and that eclipses only occur when the Moon is very close to the ecliptic. And you can get a pretty accurate idea of the Sun's path by observing the rising & setting of stars and planets that are near the Sun's path. So astronomers across the ancient world recognised that the ecliptic is the Sun's path. And by the 2nd century BC, knowledge of these movements was sufficiently detailed that Hipparchus of Nicea was able to estimate the precession of the equinoxes.

  • The zodiac is also based on the Sun's path being the ecliptic, isn't it? – Barmar Oct 21 at 17:49
  • Nice answer, just one question : why easier in the Northern hemisphere ? I've never been, but I'm told they have stars down South too. :-) – StephenG Oct 21 at 18:28
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    @StephenG they do, but they lack a conveniently close bright star to act as a visible reference like in the northern emisphere – Francesco Oct 21 at 20:29
  • @Barmar Yes. I suspect that ancient astronomers noticed that the planets and the Moon traveled through the zodiac before they figured out that the Sun does too. Once they started keeping detailed records of these movements it wouldn't take too long to determine that the ecliptic was the midline of these paths. FWIW, the Chaldeans / Babylonians kept records, especially of the Sun & Moon (for eclipse prediction) that spanned several centuries. That data was very important to the subsequent development of Alexandrian astronomy. – PM 2Ring Oct 21 at 22:37
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    @StephenG We currently do have a southern hemisphere pole star, Sigma Octantis, about 1 degree from the south celestial pole, but with a magnitude of 5.45 it's not very useful. So down here we learn to locate the celestial pole via Crux (the Southern Cross) and Alpha and Beta Centauri (aka the Pointers). Although it's a small constellation, Crux is fairly bright and rather easy to find. It was an important constellation to ancient cultures of the southern hemisphere, and south of the tropics, it never sets. – PM 2Ring Oct 21 at 23:05

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