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distances - in the 18th century using the transits of Venus...

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How do we know the distance to the stars? In particular how do we know, today, that the stars really are very very far away, much farther even than the planets, as Copernicus claimed in response to critics of his theory. When we look at the sky and see stars we are unable, as individual observers, to measure the distance to the objects we see. We lack the two key things we use to estimate distance by eye to objects here on Earth and in our immediate surroundings. These two things are 1: A sufficiently long baseline to estimate parallax. 2. Context to establish perspective We can overcome 1 to establish a long baseline by the following steps: 1. use the whole Earth or a significant part of it. To do this we must know the size of the Earth. This was worked out roughly by ancient Greeks but was first measured really accurately only in the 18th century by Europeans able to travel to different parts of the globe. 2. Using this baseline we can employ parallax to establish the distance to other objects in our Solar System, for instance the Sun. This was done
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Unformatted text preview: in the 18th century using the transits of Venus, rare events where Venus crosses the Sun. Viewed from different parts of the Earth Venus crosses the Sun at different latitudes of the Sun. By sending observers such as Captain Cook on his first Pacific voyage astronomers were able to measure the Astronomical Unit. 3. Now we can use our knowledge of the Astronomical Unit to measure the parallax to nearby stars. If we measure positions of stars on the sky at two opposite points of the year (let's say, summer and winter) we do so from two positions in space that are 2 x AU away from each other. A nearby star will appear to shift positions relative to more distant stars in the background. If we measure the shift in position as an angle in arcseconds (all stars have a parallax of less than 1 arcsecond as seen from Earth) then the distance in parsec is 1/angle. Thus a star with a parallax angle of 0.1 arcseconds is 10 parsecs away from us....
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