Continuing our trek up the cosmic distance ladder, the rung of the planets and speed of light is quite a story. The ancient astrologers realised that all the planets lie on the ecliptic (a plane) due to the fact that they only moved via the Zodiac (the set of 12 constellations around the Earth. Ptolemy produced inaccurate results due to his geocentric model while Copernicus made highly accurate conclusions, initially poring over the annals of the ancient Babylonians who knew that the synodic period of mars repeated itself every 780 days. The heliocentric model allowed Copernicus to calculate the actual angular velocity as 1/170 and knowing that the earth took 1 year to go around the sun he would subtract implied angular velocities to find that the sideral period of mars was 687 days.Copernicus determined the distance of mars from the sun to 1.5 AU (astronomical units) by assuming circular orbits and using measurements of mars' location in the Zodiac across various dates. Brahe made similar predictions but they deviated from the Copernican regime, Kepler maintained that this was so because the orbits were elliptical and not perfect circles as Copernicus has assumed. Kepler would attempt to compute the orbits of the earth and mars simultaneously and since Brahe's data only gave the direction of mars from the earth and not the distance, he would need to figure out the orbit of the earth using mars. Working under the assumption that mars was fixed and the earth was moving in an orbit, Kepler used triangulation to use Brahe's 687 day interval to compute the earth's orbit relative to any position of mars. Such allowed the more precise calculation of the AU by parallax (measuring the same object from two different locations on earth), especially during the transit of Venus across the sun in multiple places (including Cook's voyage). But the anomaly of the precession of Mercury (where the points of aphelion and perihelion progressively wind around one another in a circular manner) could not be reconciled with Newtonian mechanics, so general relativity was invoked.The first attempts at accurately measuring the speed of light (

*c*) was by Rømer who measured

*c*by observing Io, one of Jupiter's moons that made a complete orbit every 42.5 hours. Rømer noticed that when Jupiter was aligned with the Earth, the orbit advanced slightly but when it was opposed, it slowed and lagged by around 20 minutes. Huygens inferred that this was because of the extra distance (2 AU) that light had to travel from Jupiter, so light travels 2 AU in 20 minutes; hence the speed may be computed to 300,000 km/s.

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