Earth travels around the sun in an orbit that is slightly oval-shaped, known as an ellipse. Therefore, the planet’s distance from the sun changes throughout the year.
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.
While his math was off, Aristarchus did conclude the Earth orbits around the Sun a whole 1700 years before Copernicus proposed the heliocentric theory.
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. 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.
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.
The planet follows an elliptical orbit around the sun, rather than a circular orbit. This means over the course of the roughly 365 days it takes Earth to complete one loop around its parent star, the planet swings through its closest point to the sun early in the year and then its farthest point months later.
In early January, Earth reaches its closest position to the star. Astronomers call this point perihelion, and at this time Earth is about 91.4 million miles (147.1 million km) away from the sun, according to NASA.
However, the average distance from Earth to the sun is about 93 million miles (150 million kilometers). Scientists also call this distance one astronomical unit (AU).
This year, Earth will be furthest from the sun on July 4. That moment, called aphelion, will occur when the distance between the two celestial bodies stretches to more than 94.5 million miles.
Keep in mind that Earth’s distance from the sun does not determine the seasons we experience; the seasons are determined by the tilt of the planet’s axis.
Half a year after perihelion, Earth reaches its farthest distance from the star, which is called aphelion. At that moment, the planet is approximately 94.5 million miles (152.1 million km) from the sun. Aphelion occurs in early July.
The term perihelion comes from the ancient Greek “peri,” which means “close,” and “helios,” meaning the sun. Conversely, aphelion comes from combining “apo,” meaning “away from,” with “helios.” Each term is sometimes called an apsis, which refers to the nearest or farthest point between a celestial body and its host.
Though perihelion occurs during winter in the Northern Hemisphere and aphelion occurs during summer, Earth’s elliptical orbit around the sun does not cause the seasons. Rather, the planet’s tilt as it rotates on its axis drives the changes between winter, spring, summer and fall.
An astronomical unit is now more precisely defined as “a conventional unit of length equal to 149,597,870,700 meters exactly.” That translates to roughly 92,955,807 miles (149,597,871 km).
Einstein’s theory of general relativity also throws a wrench in the evaluation of an AU because it argues that space-time is relative depending on the observer’s location in the solar system.
The sun is at the heart of the solar system. All of the bodies in the solar system — planets, asteroids, comets, etc. — revolve around it at various distances.
Everything else falls in between. For example, Neptune is 30.07 AU from the sun. The distance to the nearest star, Proxima Centauri, is about 268,770 AU, according to NASA. However, to measure longer distances, astronomers use light-years.