# How was the distance from the Earth to the Sun determined

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

• Did you even try? google.com/… May 25, 2018 at 19:47
• 150million kilometer. 1 mile is around 1.6 km. May 25, 2018 at 20:13
• Am I supposed to stump people with my question? I'm trying to understand things I don't know about. May 25, 2018 at 20:16
• On SE sites people frown upon questions that show no research. Googling for your exact question title would have given you enough answers.
– user1569
May 25, 2018 at 22:10
• And I want to clarify, it's actually a very good question, it's just not a good question for the stack exchange format. May 26, 2018 at 11:37

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.

(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.)

• 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 Jun 17, 2019 at 12:12
• 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. Jun 17, 2019 at 12:27

Here is the best answer I have seen to this:

It works through the calculations using simple observations.

Cassini (the guy, not the spacecraft) calculated the distance to the Sun. See http://curious.astro.cornell.edu/about-us/41-our-solar-system/the-earth/orbit/87-how-do-you-measure-the-distance-between-earth-and-the-sun-intermediate

Edmond Halley realised the transit of Venus across the Sun could be used to calculate the distance. NASA has a page explaining that: https://pwg.gsfc.nasa.gov/stargaze/Svenus1.htm

Nowadays things are easier. We can easily find the distance to the Moon to centimeter accuracy by bouncing laser light from the reflectors left on the Moon during the Apollo program. We can use that to calculate the distance to the Sun.