After Hundreds of Years, Astronomers Finally Agree: This Is the Distance From the Earth to the Sun

The International Astronomical Union drops the mic.

How far away from Earth is the sun? Not just, you know, very, very far, but in terms of an actual, measurable distance? When you're calculating, how do you decide which location on Earth to measure from? How do you decide which spot on the path of Earth's orbit will serve as the focal point for the measurement? How do you account for the sheer size of the sun, for the lengthy reach of its fumes and flames? 

The measurable, mean distance -- also known as the astronomical number -- has been a subject of debate among astronomers since the 17th century. The first precise measurement of the Earth/sun divide, Nature notes, was made by the astronomer and engineer Giovanni Cassini in 1672. Cassini, from Paris, compared his measurements of Mars against observations recorded by his colleague Jean Richer, working from French Guiana. Combining their calculations, the astronomers were able to determine a third measurement: the distance between the Earth and the sun. The pair estimated a stretch of 87 million miles -- which is actually pretty close to the value astronomers assume today.

But their measurement wasn't, actually, a number. It was a parallax measurement, a combination of constants used to transform angular measurements into distance. Until the second half of the twentieth century -- until innovations like spacecraft, radar, and lasers gave us the tools to catch up with our ambition -- that approach to measuring the cosmos was the best we had. Until quite recently, if you were to ask an astronomer, "What's the distance between Earth and the sun?" that astronomer would be compelled to reply: "Oh, it's the radius of an unperturbed circular orbit a massless body would revolve about the sun in 2*(pi)/k days (i.e., 365.2568983.... days), where k is defined as the Gaussian constant exactly equal to 0.01720209895."

Oh, right. Of course. 

But rocket science just got a little more straightforward. With little fanfare, Nature reports, the International Astronomical Union has redefined the astronomical number, once and for all -- or, at least, once and for now. According to the Union's unanimous vote, here is Earth's official, scientific, and fixed distance from the sun: 149,597,870,700 meters. Approximately 93,000,000 miles. 

For astronomers, the change from complexity to fixity will mean a new convenience when they're calculating distances (not to mention explaining those distances to students and non-rocket scientists). It will mean the ability to ditch ad hoc numbers in favor of more uniform calculations. It will mean a measurement that more properly accounts for the general theory of relativity. (A meter in this case is defined as "the distance traveled by light in a vacuum in 1/299,792,458 of a second" -- and since the speed of light is constant, the astronomical unit will no longer depend on an observer's location with the solar system.) The new unit will also more accurately account for the state of the sun, which is slowly losing mass as it radiates energy. (The Gaussian constant is based on solar mass.) 

So why did it take so long for the astronomy community to agree on a standard measurement? For, among other things, the same reason this story mentions both meters and miles. Tradition can be its own powerful force, and the widespread use of the old unit -- which has been in place since 1976 -- means that a new one will require changes both minor and sweeping. Calculations are based on the old unit. Computer programs are based on the old unit. Straightforwardness is not without its inconveniences. 

But it's also not without its benefits. The astronomical unit serves as a basis for many of the other measures astronomers make as they attempt to understand the universe. The moon, for example, is 0.0026 ± 0.0001 AU from Earth. Venus is 0.72 ± 0.01 AU from the sun. Mars is 1.52 ± 0.04 AU from our host star. Descriptions like that -- particularly for amateurs who want to understand our world as astronomers do -- just got a little more comprehensible. And thus a little more meaningful.