We are independent & ad-supported. We may earn a commission for purchases made through our links.
Advertiser Disclosure
Our website is an independent, advertising-supported platform. We provide our content free of charge to our readers, and to keep it that way, we rely on revenue generated through advertisements and affiliate partnerships. This means that when you click on certain links on our site and make a purchase, we may earn a commission. Learn more.
How We Make Money
We sustain our operations through affiliate commissions and advertising. If you click on an affiliate link and make a purchase, we may receive a commission from the merchant at no additional cost to you. We also display advertisements on our website, which help generate revenue to support our work and keep our content free for readers. Our editorial team operates independently of our advertising and affiliate partnerships to ensure that our content remains unbiased and focused on providing you with the best information and recommendations based on thorough research and honest evaluations. To remain transparent, we’ve provided a list of our current affiliate partners here.
Astronomy

Our Promise to you

Founded in 2002, our company has been a trusted resource for readers seeking informative and engaging content. Our dedication to quality remains unwavering—and will never change. We follow a strict editorial policy, ensuring that our content is authored by highly qualified professionals and edited by subject matter experts. This guarantees that everything we publish is objective, accurate, and trustworthy.

Over the years, we've refined our approach to cover a wide range of topics, providing readers with reliable and practical advice to enhance their knowledge and skills. That's why millions of readers turn to us each year. Join us in celebrating the joy of learning, guided by standards you can trust.

What is an Astronomical Unit (AU)?

By Brendan McGuigan
Updated: May 21, 2024
Views: 36,274
Share

An astronomical unit (AU) is a measure of distance often used in astronomy, equal to the distance between the Earth and the Sun. In terms of more common units of measurement, an AU is equal to about 93 million miles (150 million km), or the distance light travels in a little over eight minutes. The symbol AU is most often used to represent the astronomical unit, though less commonly you may see UA used instead.

People have been estimating the distance between the Earth and the Sun for quite a long time. Many Greeks came up with measurements, often wrong by fairly enormous margins. The Greek Eusebius came up with a measurement that is surprisingly close to the modern measure of an astronomical unit. In one of his works he estimated it at 804 million stadia. The stadia, a Greek unit of measure, is roughly 605 to 625 feet (85-90m), making his estimate somewhere between 92 and 95 million miles (149-153 million km).

At the end of the 17th century, the AU was officially estimated to be roughly 87 million miles (140 million km) by using the location of Mars at two different points in the Earth’s orbit. At the end of the 18th century a method was conceived of by using Venus as the measuring point, during its transit across the face of the sun. This method produced a much more accurate figure. At the beginning of the 20th century an asteroid passed near the Earth, and an even more accurate figure for the astronomical unit was calculated.

By the mid-20th century, and on into the 21st century, improvements in various measurement technologies allowed for much more precise measurements to be carried out, further refining the AU. By using space probes and satellites, modern definitions have been created, with a far greater degree of accuracy than any in the past. In 1976 the actual definition of the astronomical unit was updated, to get a more sophisticated measure. Perhaps the most precise definition could be given as the distance from the exact center of the Sun at which a particle would take a Gaussian year (365.2568983 days) to complete its orbit. If that seems confusing, just think of it as roughly the same as saying an astronomical unit is the distance from the center of the Earth to the center of the Sun.

The actual figure for an AU yielded by this definition, and by the most modern measurements, is approximately 92,955,807 miles (149,597,870.691km). This figure was adopted in 1996, and is considered accurate to within about 10 feet (approximately 3m).

The astronomical unit can be useful not only for astronomers, but also for normal people trying to get a handle on the relative distances involved within our own solar systems. While distances between planets can seem too large to ever get a handle on when they’re given in miles or kilometers, when given in astronomical units it becomes much easier to see the relationships between them. For example, while the Earth is of course 1AU away from the Sun, the Moon is only 0.0025AU away from the Earth. And while Jupiter, which we think of as being quite far away, is just over 5AU from the Sun, Pluto is a whopping 40 to 50AU away. And if that seems like a long way, consider that the nearest star to our own solar system is 268,000AU away.

Share
All The Science is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Discussion Comments
By vogueknit17 — On Feb 23, 2011

@sherlock87, I think that the type of thing you study in astronomy depends on the teacher. I had a similar experience, because my astronomy teacher was also a physicist and taught many advanced physics classes. On the other hand, I had a friend who studied astronomy elsewhere, and they barely even discussed the concept of things like a unit of astronomical distance, let alone things like planet size and matter. They spent far more time on things like stars and constellations, like you thought your class would be.

By sherlock87 — On Feb 23, 2011

When I took astronomy in college several years ago, we spent a lot more time than I expected doing formulas to determine the astronomical units of planets' distances from one another, the sun, and other similar things. I thought we would spend more time talking about things like stars and constellations, but it actually turned out to be sort of like a "light" physics class.

Share
https://www.allthescience.org/what-is-an-astronomical-unit-au.htm
Copy this link
All The Science, in your inbox

Our latest articles, guides, and more, delivered daily.

All The Science, in your inbox

Our latest articles, guides, and more, delivered daily.