## Astronomers Fix the Astronomical Unit 182

Posted
by
Soulskill

from the and-changed-the-oil dept.

from the and-changed-the-oil dept.

gbrumfiel writes

*"The Astronomical Unit (AU) is known to most as the distance between the Earth and the Sun. In fact, the official definition was a much more complex mathematical calculation involving angular measurements, hypothetical bodies, and the Sun's mass. That old definition created problems: due to general relativity, the length of the AU changed depending on an observer's position in the solar system. And the mass of the Sun changes over time, so the AU was changing as well. At the International Astronomical Union's latest meeting, astronomers unanimously voted on a new simplified definition: exactly 149,597,870,700 meters. Nobody need panic, the earth's distance from the sun remains just as it was, regardless of whether it's in AUs, meters, or smoots."*
## I'd have gone for 149,896,229,000m (Score:5, Interesting)

## Re:let's not waste significant digits! (Score:5, Interesting)

you'd think they could have rounded up to 150 gigameters.

if politicians can be SD-conservative, why can't astronomers? we all know that significance is precious and rare...

Interesting point.

If you are going to pic arbitrary number, why not pick an easy one?

I suspect there is a desire to keep all past references to AU meaningful within a small margin of error, so as to not have to translate any written works.

The difference between the new arbitrary number and the prior imprecise one is probably infinitesimally small for the scale of reference AUs are use for.

Rounding it up almost half a million kilometers (quarter million miles) maybe not so much.

I suspect that since it was imprecise in the first place, and used for almost nothing except astronomical reference, preserving existing references in the literature was more important than the ease of writing it down.

## Re:Distance remains the same? (Score:5, Interesting)

The center is actually the center of mass for the Earth-Sun. Actually, I believe it's the center of mass for the whole solar system, but if we treat it as a 2-body problem it's just the Earth-Sun. If only the Earth was affecting the Sun with it's gravity, the distance would be right twice a year (assuming the major or minor axis) or 4 times (if you use some other axis), since the Sun would be traveling in an ellipse identical to the Earth's but proportionately smaller, so it would be on the fall on the axis at the same time as the Earth would every single year.

In reality the Sun is also moved by the other planets, so the distance will never be correct, since it isn't moving on a pure ellipse at all. Also the Earth isn't either. That's why we use the average distance over a few years, since that will always be the average. Except for the fact that the Sun is losing mass, and therefore gravity, so Earth gets further away every year, so the average is itself changing.

## Re:They should mesure it in miles. (Score:5, Interesting)

How can mass units be "orders of magnitude out of scale" with dimensional units?

That's not even an apples-to-oranges comparison - at least those would both be fruits. Comparing mass and distance is literally nonsensical. What? Are you 3 kg away from me?

Mass relates directly to distance, since 1 liter of water (volume of a cube 0.1m on each side) is approximately a kilogram. Alternately, 1 gram is approximately the mass of a cube of water 0.01m on each side; this was, in fact, the original definition [wikipedia.org] as decreed by the French government.

If the French had chosen the mass of 1m^3 of water as the standard then the unit of mass would be in-scale with the units of distance and volume. In a system like that I could estimate my volume by simply stepping on a scale and reading my mass; the same number would be both my mass and volume, just change the unit label. Instead they chose a system where the volume of the definitive unit mass was 6 orders of magnitude away from the unit volume. As if to confuse matters more, the standard volume unit (liter) is 10^3 smaller than the cube of the unit length and (if holding water) has 10^3 larger mass than the unit mass.

If you don't care about this, that's fine; neither did the French. They cared more about the units being useful on their own in day-to-day life, and were happy that there was an even factor of 10 difference between the scales. The historical fact remains, though, that the French knowingly chose not to unify their units when creating the system, presenting modern geeks with the first-world problem of needing a conversion factor between mass and volume rather than the units being strictly 1-to-1, and affording them the opportunity to complain about it. Just because the complaint is pointless doesn't make it wrong ;^)