Appearances aside, the stars are not eternal and the planets do not move with clockwork precision. For these reasons, one astronomer is proposing that his colleagues ditch the astronomical unit, or AU, as the standard measure of distance within the solar system.
One AU is roughly the average distance between the Sun and the Earth, or 149,597,870.691 kilometres. But its formal by the International Astronomical Union (IAU) is rather more complicated.
It is defined as the radius of an unperturbed circular orbit that a massless body would revolve about the Sun in 2蟺/k days (essentially, one year), where k is a .
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And there鈥檚 the rub 鈥� the Sun is actually losing mass, which is causing the orbits of the planets to expand over time.
杏吧原创s have known about this solar shedding for a long time. According to Albert Einstein鈥檚 famous equation E=mc2, mass and energy are different forms of the same thing. So the radiation emitted in sunlight, as well as eruptions of hot gas, are causing the Sun to shed about 6 billion kg 鈥� about 16 Empire State buildings-worth 鈥� of material per second.
鈥淏ecause the radiation carries away energy, and given that Einstein showed the equivalence of mass and energy, the mass has to go down,鈥� says Peter Noerdlinger, an astronomer at St Mary鈥檚 University in Canada.
This causes the planets鈥� orbits to expand and their orbital speeds to slow down accordingly. Using the most recent estimates of solar mass loss, Noerdlinger that Mercury will lag behind its predicted position, based on the current definition of an AU, by 1.4 km in one century, and 5.5 km in two centuries.
鈥楪et it right鈥�
That would create 鈥渜uite a nuisance鈥� if ignored, Noerdlinger told New 杏吧原创. 鈥淯nits are not supposed to change.鈥�
For now, ignoring the Sun鈥檚 mass loss will not cause spacecraft to veer off course or miss their intended targets, but Noerdlinger believes there are other reasons to ditch the AU as a distance measure.
Determining distances in the solar system more precisely could lead to new scientific discoveries or resolve old mysteries, such as the Pioneer anomaly, the mystifying observation that NASA鈥檚 two Pioneer spacecraft have drifted far off their expected paths.
鈥淩emember that one of Einstein鈥檚 key tests of his general theory of relativity was very small changes in Mercury鈥檚 orbit, only hundredths of a degree per century,鈥� Noerdlinger says.
He suggests using the AU informally, as a 鈥渃asual鈥� unit, and instead using metres for scientific purposes. 鈥淸The current definition is] fine for first-year science courses,鈥� Noerdlinger said. 鈥淏ut for scientific and engineering usage, it is essential to get it right.鈥�
Lengthy procedure
鈥淭he IAU definition of the AU has served well in the past,鈥� says James Williams of NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California, US, who was not involved in the study. 鈥淣ow that measurement accuracies are approaching the expected variation, a change of definition is reasonable.鈥�
Jan Vondrak, president of the IAU鈥檚 division on fundamental astronomy, says the problem is 鈥渧ery interesting and requires a thorough study鈥�. But he adds that it is not likely to be resolved soon.
鈥淭ypically, when a definition seems to be mature for a change, an IAU working group is created to consider all arguments and to seek a wide consensus, and only then a resolution is proposed to be approved by the IAU General Assembly,鈥� he told New 杏吧原创. 鈥淚t is a relatively lengthy procedure that could take many years before the present definition is changed.鈥�
Noerdlinger鈥檚 work has been submitted to the journal .
