Original URL: http://www.theregister.co.uk/2008/02/07/astronomical_unit_inaccurate_shrinking_sun/

Boffin says Astronomical Unit should be binned

Fails to allow for shrinky sun, spiralling Earth

By Lewis Page

Posted in Science, 7th February 2008 12:38 GMT

An astronomer based in Canada has published a paper arguing that distances within the solar system should no longer be measured using Astronomical Units (AU), which is currently standard practice.

An AU is approximately equal to the distance between Earth and Sun, but the International Astronomical Union actually defines it differently. According to the IAU, an AU's value is based on the Gaussian gravitational constant - which itself assumes that the Sun has a constant mass.

But Peter Noerdlinger of St Mary's University points out that in fact the Sun loses an ultra-minuscule proportion of itself every second due to the continual conversion of mass into solar energy. (About three millionths of a quadrillionth goes every second if we've got our sums right; or three billionths of a trillionth if you prefer. This equates to around six million tonnes, which is 60 US Navy aircraft carriers or 80-million-odd international adjusted Sarah Beenys completely annihilated.)

The Sun's shrinkage means that Earth gets further away and slows down in its progress all the time, gradually spiralling outwards into darkness and deviating from where it should be under IAU rules; which makes Noerdlinger mad.

"Units are not supposed to change," he told New Scientist, describing the situation as "quite a nuisance" and insisting that "for scientific and engineering usage, it is essential to get it right".

The testy boffin reckons variance in the AU could account for the so-called "Pioneer anomaly", in which NASA's Pioneer space probes have wandered off their calculated courses since heading out into interstellar space. He proposes using metres instead. ®