When the Sun gets eclipsed, one can observe four objects near it: Mercury, Venus, the Earth, and Mars. Over an eclipse system, one can observe them go back and forth around the Sun, evidently orbiting it.
Moving about as fast as the Sun, and looking like a bright star, would be Jupiter. It would get as far as 33d from the Sun, making it easier to see than the inner planets.
Looking with a telescope will reveal the Earth’s moon and Jupiter’s four big moons. One will quickly find that those four moons follow Kepler’s third law. As to the inner planets, Jupiter, and Saturn, if their motions are referred to the stars, then they will also obey that law. So we notice these sets of objects all obeying it:
- Saturn’s moons
- Jupiter’s moons
- The Sun’s “moons”
So if one wasn’t sure of Kepler’s third law before, one would be sure of it now.
Interstellar astronomy would work much like from the Earth, but Saturn’s orbit will give parallaxes 10 times greater, though with 30 times the period. That will make it possible to measure distances 10 times greater, meaning about 1000 times more stars on average. Let’s see how the numbers work out:
- Photographic film:
- Earth: nearest stars
- Saturn/Titan: 2/3 of brightest stars, Hyades cluster
- The Hipparcos astrometric satellite, the Hubble Space Telescope:
- Earth: 2/3 of brightest stars, Hyades cluster
- Saturn/Titan: Galactic disk, Cepheids
- The GAIA astrometric satellite:
- Earth: Galactic disk, Cepheids
- Saturn/Titan: Galactic center, Magellanic Clouds (nearby dwarf galaxies)
The Hyades Cluster and Cepheid variables are some of the nearer rungs in the Cosmic Distance Ladder, a series of methods calibrated for their nearer objects and then extended to their more distant objects. Measuring their distances directly will reduce the uncertainties in the farther parts of the ladder, something that has already happened for us for the Hyades Cluster, and that should soon happen for the Cepheid variables.
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