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In the history of humanity, easily observable extra-(Solar System) objects greatly helped understanding certain phenomena inside the Solar System. Importantly, the “precession of the equinox”, and also true periods of heliocentric orbits lying near the ecliptic.

Imagine a system like our Solar System with indigenous astronomers who are unaware of far-away objects (or cannot determine their bearings). How can astronomers possessing some Early Modern technology learn of absolute (inertial) spacial directions then? Ī̲ see several possibilities: looking at highly inclined subsystems (think Uranus with its rings and satellites), noticeably eccentric orbits (such as of Mercury or, occasionally, of comets), or just inclined orbits (think Pallas). But do other reliable references exist?

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    $\begingroup$ Are you trying to imagine this as part of creating a fictional world? Perhaps this would be better on Worldbuilding. $\endgroup$ – James K Dec 23 '20 at 10:41
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    $\begingroup$ @JamesK: this site has several questions, such as astronomy.stackexchange.com/questions/34788/… , astronomy.stackexchange.com/questions/40100/… , having tenuous relevance to astronomy and discussing patent imagination without much scientific base. Who cares about exporting those threads? This question, in constrast, is strongly on-topic for Astronomy. $\endgroup$ – Incnis Mrsi Dec 23 '20 at 11:00
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    $\begingroup$ The fact that other questions exist is not really relevant. I'm just suggesting that you might get a better response elsewhere. This is probably on topic, I think it is marginal, but I didn't vtc. But if you are asking about creating a fictional world, then as a piece of friendly advice.... perhaps worldbuilding would be better? $\endgroup$ – James K Dec 23 '20 at 11:24
  • $\begingroup$ Indeed, I notice you had commented on those two questions previously, which make this question look like you are"making a point". $\endgroup$ – James K Dec 23 '20 at 11:26
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Say I have a single observatory, and the only things I measure are the times and azimuth angles for the high magnitude solar system bodies (the Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn) as they ascend into the sky and descend out of it (for a fixed elevation angle).

If I gather these for a few years, I will have enough data to have an overdetermined system to calculate the latitude of my observatory, the Earth's axial tilt, the Earth's rotation rate, and the orbital elements of all the bodies.

The trick would be to realize that all these bodies (including the Earth) can be approximated by rotating spheres that are revolving around the Sun (with the Moon also revolving around the Earth). Solar and Lunar eclipses certainly would have helped with this notion.

Though our own ancient astronomers used the relatively fixed locations of the stars as a reference to better track the motions of the planets, they certainly aren't necessary. In fact, perhaps we would have understood the motion in our Solar System sooner in our development if we hadn't been distracted by the rest of the Universe.

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  • $\begingroup$ Possibly Ī̲ am currently in a poor mood and cant interpolate points of this answer to get solutions, but again… the Earth’s axis changes by almost 1′ a year wrt fixed stars. How would the “overdetermined system” permit for learning about it? As for “orbital elements”, planets (except Mercury) orbit almost circularly, and there are indications that this is somewhat a rule rather than a Sol-specific curiosity. Without absolute directions, how to determine the orbital period for a circular orbit? To be fair, Ī̲ don’t even see my question properly addressed (let alone answered). $\endgroup$ – Incnis Mrsi Dec 24 '20 at 10:34
  • $\begingroup$ Sorry, it’s the First Point of Aries that changes by almost 1′ a year – this thing is relevant to periods of planets. The Earth’s axis changes by 20″ only. $\endgroup$ – Incnis Mrsi Dec 24 '20 at 11:00

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