Astronomical unit
The astronomical unit, abbreviated as AU (or AU for astronomical unit), is a unit of length used in astronomy to conveniently express distances on the scale of the solar system. It is defined as being approximately equal to the average distance between Earth and the Sun, specifically 149,597,870,700 meters, or about 150 million kilometers.
A practical unit for the solar system
Expressing distances in the solar system in kilometers quickly leads to numbers that are difficult to handle. Jupiter is 778 million km from the Sun—but 5.2 AU is much more meaningful. Neptune is 4.5 billion km away, or 30 AU. This unit allows us to intuitively grasp the relative scale of the planetary orbits: Mercury at 0.39 AU, Venus at 0.72 AU, Mars at 1.52 AU, and so on.
A precise definition
Historically, the AU was defined as the average distance between the Earth and the Sun, but this distance varies slightly throughout the year because theEarth’s orbit is elliptical. In 2012, the International Astronomical Union set the exact value of theAU at 149,597,870,700 meters, making it a defined constant rather than a variable measurement. This definition considerably simplifies calculations in celestial mechanics.
When should the AU be used?
The AU is used primarily for distances less than a few hundred AU, that is, within the solar system and its immediate vicinity. For stellar distances, the light-year or parsec is preferred: Proxima Centauri is 268,000 AU away, which is equivalent to 4.24 light-years or 1.3 parsecs.
Importance in Astronomy
The AU is essential in celestial mechanics, planetary science, and all aspects of solar system astronomy. It allows us to calculate orbital periods using Kepler’s third law, to express the orbital parameters of asteroids, comets, and trans-Neptunian objects, and to precisely plan the trajectories of space probes.
Practical example
The Voyager 1 probe, launched in 1977, is currently the human-made object farthest from Earth, at approximately 165 AU. This means it is 165 times the distance between Earth and the Sun. The signals it sends take about 23 hours to reach us, traveling at the speed of light.
Frequently Asked Questions
Why 150 million kilometers?
This average distance results from the dynamic equilibrium between the Earth and the Sun, which depends on both the Sun’s mass and the Earth’s orbital velocity. This value has been measured with increasing precision over the centuries, from Cassini’s first estimates in the 17th century to modern radar measurements.
Does the UA vary over time?
Very slightly, yes. The Sun loses mass through nuclear fusion and the solar wind, which very gradually pushes the Earth farther away—about 1.5 cm per year. But this effect is so small that it is ignored in practice: the value of the AU is fixed by definition.
What is the difference between an AU and a light-year?
One light-year is approximately 63,241 AU. The AU is suitable for distances within a planetary system; the light-year for distances between stars; and the parsec and its multiples for galactic and cosmological distances.