All of the planets, comets, and asteroids in the solar system orbit the Sun. The average distance between the Earth and the Sun is 92,955,807 miles (149,597,870 km). Most people just round it up to 93 million miles. This distance is called an astronomical unit or AU and is used to measure and compare other distances in space.
Closest and Farthest Distance to the Sun
Because the Earth’s orbit is elliptical or oval, sometimes the Earth is closer than 1 AU to the Sun and sometimes it is further away. The closest approach to the Sun is called perihelion. This occurs in early January, when the Earth is only about 91 million miles (146 million kilometers) away from the Sun. The Earth is farthest from the Sun at aphelion. Aphelion happens in early July when the Earth is approximately 94.5 million miles (152 million kilometers) from the Sun. The change in distance is fairly dramatic. On NASA’s Earth overview site, the distance updates in real-time. The Earth moves a mile closer or further from the Sun about every four seconds!
Measuring the Distance
Obviously, you can’t just whip out a tape measure to find the distance between the Earth and Sun. It has to be calculated. The first person to find the distance to the Sun was the Greek astronomer Aristarchus around 250 B.C. Aristarchus used geometry to find the distance. He figured the Earth, Sun, and Moon should form a right angle when the Moon was half full. He measured the sizes of the Sun and the Moon and the angles between them and found the Sun to be 19 times further from Earth than the Moon. Since the Sun and the Moon are about the same size in the sky (which is why we get total solar eclipses), Aristarchus thought the Sun was also 19 times larger than the Moon. His measurements contained a lot of error, mainly because he couldn’t precisely determine the centers of the Sun or Moon or find the exact instant the Moon was half full. While his math was off, Aristarchus did conclude the Earth orbits around the Sun a whole 1700 years before Copernicus proposed the heliocentric theory.
Christiaan Huygens calculated the distance between the Earth and Sun in 1653. His method was similar to that used by Aristarchus, but he used the angles formed between Venus, Earth, and the Sun. When Venus was half full, the planet, Earth, and the Sun form a right angle. Huygens estimated the size of Venus to measure the distance. His guess wasn’t too far off, so his number was close to the true distance to the Sun.
Giovanni Cassini used parallax to find the distance to the Sun and to Mars in 1672. He measured the position of Mars against background stars in Paris, while a colleague did the same in French Guiana. Cassini triangulated these measurements with the known distance between Paris and French Guiana. From the distance to Mars, Cassini calculated the distance to the Sun.
Cassini’s measurement was close to the true distance, but scientists use a more direct approach today. A signal sent from a spacecraft travels at the speed of light, so if the time between sending and receiving the signal is known, the distance can be calculated. Another option is to bounce a radar signal off a remote object. We know how long it took between sending the signal and receiving the echo, so distance can be determined.
As of 2012, the definition of the astronomical unit is based on the speed of light. While the true distance between the Earth and the Sun changes, the AU is set at 149,597,870,700 meters or about 92.956 million miles.