How do we know our Sun is 93 million miles (150 million km) away^{1}?

The ancient Greek astronomer and mathematician Aristarchus of Samos, who lived around 2,300 years ago, was probably the first person who made a reasonable attempt to determine the distance to the Sun.

Using a method of geometric analysis developed by Euclid (trigonometry had not yet been invented), Aristarchus measured the angle between the half-lit Moon and the Sun and determined that the Sun is 18 to 20 times farther away than the Moon. Though he fell far short of the actual value of 389 due to the extreme difficulty of making accurate measurements using the instruments and methods available to him, Aristarchus showed the way for future generations of astronomers to determine the true distance to the Sun.

Determining the actual distance (and not the relative distance) to the Sun had to wait for Kepler’s Third Law of planetary motion that relates a planet’s orbital period to its distance from the Sun, the invention of the telescope, and Isaac Newton’s laws of motion and gravitation.

Distances within the solar system can be determined using trigonometry and parallax, which is the apparent shift of an object against the distant background stars as seen from different locations.

Measuring the parallax to a Sun-orbiting object (such as Mars) from two different locations on the Earth’s surface allows us to measure its distance and, thanks to Kepler and Newton, sets the scale for the entire solar system. The true distance of each planet from the Sun can then be mathematically determined. This was first accomplished in 1672, and has been done many times since, with ever-improving accuracy.

Today, we have even better methods to determine the scale of the solar system: timing radar reflections off of solar system objects, and measuring travel time for radio communications between Earth and spacecraft. Both radar and radio signals travel at the speed of light, which is very well determined.

^{1}Approximate average distance