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.
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).
The universe is a big place, and sometimes researchers use the astronomical units to communicate how far celestial objects are separated from one another. For example, Jupiter orbits about 5 AU from the sun.
Related: How big is Earth?
Related: What's the speed of Earth around the sun?
Earth's distance from the sun changes
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.
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. This is why the season occurring in Earth's Southern Hemisphere is always in opposition to the season in the Northern Hemisphere.
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.
Perihelion and aphelion average out to about 93 million miles (150 million km).
A new, more precise astronomical unit
The International Astronomical Union (IAU) is an international nonprofit organization that is tasked with, among many other things, defining astronomical constants. In August 2012, IAU members voted to approve a more exact measurement of 1 AU.
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).
Why was this decision necessary? The equation that had previously determined the value of an AU depended on information including the mass of the sun. But that value changes because the star is constantly transforming its mass into energy, according to 2012 reporting by Nature.
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. This complication made it difficult for planetary scientists working on models of the solar system.
The IAU's recently-adopted value is measured using the speed of light in the vacuum of space, which is constant.
The original calculation
The first-known person to measure the distance to the sun was the Greek astronomer Aristarchus of Samos, who lived from about 310 B.C. to 230 B.C. He used the phases of the moon to measure the sizes and distances of the sun and moon.
He postulated that when the half moon appears in Earth's sky, the center of our planet and the center of the moon create a line in space that forms a 90 degree angle with another line that could be drawn through space from the moon's center all the way to the sun's center. Using trigonometry, Aristarchus could determine the hypotenuse of a triangle based on those two imaginary lines. The value of the hypotenuse would provide the distance between the sun and the Earth.
Although imprecise, Aristarchus provided a simple understanding of the sizes and distances of the three bodies, which led him to conclude that the Earth goes around the sun, about 1,700 years before Nicolaus Copernicus proposed his heliocentric model of the solar system.
In 1653, astronomer Christiaan Huygens calculated the distance from Earth to the sun. Much like Aristarchus, he used the phases of Venus to find the angles in a Venus-Earth-sun triangle. His more precise measurements for what exactly constitutes an AU were possible thanks to the existence of the telescope.
Guessing (correctly, by chance) the size of Venus, Huygens was able to determine the distance from Venus to Earth. Knowing that distance, plus the angles made by the triangle, he was able to measure the distance from Earth to the sun. However, because Huygens' method was partly guesswork and not completely scientifically grounded, he usually doesn't get the credit.
In 1672, Giovanni Cassini used a method involving parallax, or angular difference, to find the distance to Mars and at the same time figured out the distance to the sun. He sent a colleague, Jean Richer, to Cayenne, French Guiana (located just northwest of the modern-day Guiana Space Center near Kourou) while he stayed in Paris. At the same time, they both took measurements of the position of Mars relative to background stars, and triangulated those measurements with the known distance between Paris and French Guiana. Once they had the distance to Mars, they could also calculate the distance from Earth to the sun. Since his methods were more scientific, Cassini usually gets the credit.
These techniques are also why astronomers continue to use the distance from Earth to the sun as a scale for interpreting the solar system.
"Expressing distances in the astronomical unit allowed astronomers to overcome the difficulty of measuring distances in some physical unit," astronomer Nicole Capitaine of Paris University told Space.com. "Such a practice was useful for many years, because astronomers were not able to make distance measurements in the solar system as precisely as they could measure angles."
Across 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.
Mercury, the planet closest to the sun, gets as close as 29 million miles (47 million km) in its elliptical orbit, while objects in the Oort Cloud, the solar system's icy shell, are thought to lie as far as 9.3 trillion miles (15 trillion km).
Everything else falls in between. Jupiter, for example, is 5.2 AU from the sun. 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, or the distance that light travels in a single Earth-year, which is equal to 63,239 AU. So Proxima Centauri is about 4.25 light-years away.
Additional resources and reading
Watch a video explaining Aristarchus' approach to calculating the distance from Earth to the sun. NASA's sun fact sheet provides basic statistics about our star and its solar system exploration page offers details about solar science and missions studying the sun. You can also explore cosmic distances, within the solar system and beyond, with NASA.
Bibliography
- Brumfiel, G. "The astronomical unit gets fixed." Nature (2012). https://www.nature.com/articles/nature.2012.11416
- International Astronomical Union, "Measuring the Universe," accessed Jan. 21, 2022. https://www.iau.org/public/themes/measuring/
- Kish, G. "A Source Book in Geography," accessed via Google Books. Harvard University Press, 1978.
- Luque, B. and Ballesteros, F. "To the Sun and beyond." Nature Physics (2019). https://www.nature.com/articles/s41567-019-0685-3
- NASA, "What Causes the Seasons?" July 22, 2021. https://spaceplace.nasa.gov/seasons/en/
