The International Astronomical Union, the authority on astronomical constants, has voted unanimously to redefine the astronomical unit, the conventional unit of length based on the distance between the Earth and the sun.
“The new definition is much simpler than the old one,” says Sergei Klioner of the Technical University of Dresden in Germany, one of a group of scientists who worked decades toward the change, which took effect last month during an IAU meeting
Under the new definition, the astronomical unit (or AU) — the measurement used for the Earth-sun distance — is no longer always in flux, depending on the length of a day and other changing factors. It is now a fixed number: 149,597,870,700 meters, which is the equivalent of almost 92.956 million miles.
Klioner explained the simpler definition is helpful, for instance, for scientists who formulate ephemerides — tables that give the precise position of astronomical objects in the sky. They utilize the astronomical unit to calculate the motion of bodies in the solar system. …
Lacking precise instrumentation, early astronomers relied heavily on angles to calculate the size of the universe. By studying Mars from two separate points on Earth, 17th-century Italian astronomer Giovanni Cassini was able to use trigonometry to calculate the distance from Earth to the sun with only about a 6 percent error.
“Expressing distances in the astronomical unit allowed astronomers to overcome the difficulty of measuring distances in some physical unit,” Capitaine told SPACE.com by email. “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.”
Modern instruments can come within a few meters of exactly determining distances of nearly150 billion meters (150 million kilometers), or some 93 million miles.
The astronomical unit eventually came to be defined by a mathematical expression that involved the mass of the sun, the length of a day, and a fixed number known as the Gaussian gravitational constant. Because the Earth orbits its star in an ellipse rather than a circle, the length of a day shifts over the course of a year. At the same time, the sun is constantly transforming mass into energy.
In the 20th century, famed scientist Albert Einstein added general relativity to the mix. According to the famous theory, space-time is relative depending on one’s frame of reference.
The new fixed number is the best estimate of the original expression, Klioner said.
“If we would decide to continue with the old definition, we would have to add several additional conventions to make the latter meaningful in the framework of general relativity,” he explained. “A better way was to change the definition completely — and this is what we succeeded in doing.” …