How did astronomers originally calculate the distance between the earth and the sun?

Question

Alexander Acosta · Accepted Answer

What we actually measure is the distance from the Earth to some other body, such as Venus. Then we use what we know about the relations between interplanetary distances to scale that to the Earth-Sun distance.

Prior to radar, astronomers had to rely on other (less direct) geometric methods. Since 1961, we have been able to measure interplanetary distances using radar: we send a radar signal at another planet (or moon or asteroid) and measure how long it takes for the radar echo to return.

Claire Anctil · Answer

undefined Astronomers originally calculated the distance between the Earth and the Sun using a method called the "astronomical unit" (AU). This method relies on observations of the transit of Venus across the Sun and triangulation.

Christopher Ales · Answer

undefined Astronomers originally calculated the distance between the Earth and the Sun using a method called the transit of Venus. This method involves observing the planet Venus as it passes between the Earth and the Sun, appearing as a small black dot moving across the Sun's surface. By carefully timing the duration of Venus's transit from different locations on Earth, astronomers were able to use trigonometry to calculate the distance to Venus. Once the distance to Venus was known, astronomers could then use Kepler's laws of planetary motion to determine the distance between the Earth and the Sun. This method was first successfully employed by the astronomer Edmund Halley in the 18th century, and subsequent observations of the transit of Venus helped to refine and improve the accuracy of the calculation.