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How was the Earth to sun distance measured. How do we know how far from the Earth the Sun actually is.

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    $\begingroup$ Did you even try? google.com/… $\endgroup$ – Phil N DeBlanc May 25 '18 at 19:47
  • $\begingroup$ 150million kilometer. 1 mile is around 1.6 km. $\endgroup$ – peterh says reinstate Monica May 25 '18 at 20:13
  • $\begingroup$ Am I supposed to stump people with my question? I'm trying to understand things I don't know about. $\endgroup$ – user23178 May 25 '18 at 20:16
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    $\begingroup$ On SE sites people frown upon questions that show no research. Googling for your exact question title would have given you enough answers. $\endgroup$ – Jan Doggen May 25 '18 at 22:10
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    $\begingroup$ And I want to clarify, it's actually a very good question, it's just not a good question for the stack exchange format. $\endgroup$ – userLTK May 26 '18 at 11:37
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There are a bunch of ways. The simplest is triangulation.

Consider a transit of Mercury or Venus, when the planet gets between the Earth and the Sun. If you observe the planet's path across the face Sun from two different places, measuring the chords across the Sun's disk and the timings, using no more than trigonometry, you can get an accurate distance to the Sun.

Here's a good discussion of the method on the Physics SE

(It's worth noting that it is easy to measure the distance to, say, Mars in terms of the ratio of the distance to Mars compared with the Earth-Sun distance, than it is to measure the Earth-Sun distance in miles. Hence the Astronomical Unit. We accurately knew many astronomical distances in AU before we knew the length of the AU equally accurately.)

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  • $\begingroup$ In trigonometry you need two sides to calculate the third. The question is basically asking how the first two sides are generated specifically the second side. The first side is simple enough to measure, two points on earth (assuming a flat terrain) but the second is equally as difficult as generating the third. There is a lot that’s not properly accounted for in generating this distance $\endgroup$ – Autodidact Jun 17 at 12:12
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    $\begingroup$ We knew the two sides, in astronomical units (the distance of the Sun is 1AU, by definition). The point of the observation of transits was to measure a terrestrial distance in astronomical units. Unfortunately most explanations do not make that clear, confusing people like Autodidact who actually think clearly about the questions. $\endgroup$ – Martin Kochanski Jun 17 at 12:27

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