An inconstant constant

So on a slightly more serious note, it turns out that the Astronomical Unit may need to be redefined. Not really such a problem for me, but all those planetary astronomers out there might have to find a new benchmark.

To give a little background, the astronomical unit (or AU for short) is the average distance between the Earth and the Sun. It’s based on calculations by Carl Gauss and is the basis for the “Gaussian year”, or the length of 1 year if the Earth was in a perfectly circular orbit. Being as the Gaussian year is the definition, the AU is no longer based on the actual distance from Earth to Sun (which varies, as we’re in an elliptical orbit), or a sidereal year, which is slightly shorter than a Gaussian year.

So where does all of this fall down?
Well, it’s assuming that the Sun’s mass stays constant. The trouble is that the Sun’s mass is steadily decreasing. It’s actually losing about 6 billion kilos every second! Just to put that into perspective, that’s more than the weight of the entire population of the British Isles. Every second. That’s a lot. Not enough to be a problem in the immediate future — the Sun weights around 1.99 Octillion metric tonnes (yes, that is a real number). Though it will put us out of sync by about a kilometre and a half in a hundred years or so. So not a very big change then.

Understandably, the IAU’s official stance is that redefining the AU “is a relatively lengthy procedure that could take many years before the present definition is changed.”

About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
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