What Is an Astronomical Unit? The Ruler the Solar System Is Measured In
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Distances within the solar system are quoted in units of the Earth's distance from the sun, and for most of history that was a ratio rather than a length, since the proportions of the system were known long before anybody could state any of them in kilometres.
Why a relative unit came first
Kepler's laws relate the orbital period of a planet to the size of its orbit, and periods are easy to measure by simply watching. That means the relative sizes of all the planetary orbits could be calculated accurately in the seventeenth century without knowing any distance in absolute terms, so a complete and correct scale model of the solar system existed while the model's scale factor was unknown. Everything was expressed in units of the Earth's own orbital radius, which made that quantity the single measurement needed to convert the whole system into real distances. That is why so much effort went into measuring it and why the transits of Venus in the eighteenth century prompted international expeditions. The unit has remained convenient since, because solar system distances expressed in it are small comprehensible numbers, with Mars at about one and a half, Jupiter at about five and Neptune at about thirty, which is far easier to hold in mind than the equivalent in kilometres.
How it was measured
The absolute value was pinned down through a sequence of methods of increasing precision:
- •Ancient attempts by Aristarchus and others using lunar geometry, which were correct in method and produced values far too small because the angles could not be measured accurately
- •Parallax of Mars observed from widely separated locations in the seventeenth century, which gave the first estimate of the right order of magnitude
- •Transits of Venus in 1761, 1769, 1874 and 1882, timed from expeditions worldwide, which brought the value within a few percent
- •Parallax of asteroids passing close to Earth, notably Eros in the 1930s, which improved precision further and was less affected by the optical difficulties that limited transit timings
- •Radar ranging to Venus from the 1960s, bouncing a signal off the planet and timing the return, which measured the distance directly and to enormous precision
- •Spacecraft tracking, which now provides the most accurate solar system distances of all
From measurement to definition
The unit was redefined in 2012 in a way that reflects a general trend in metrology. Previously it was defined in terms of the gravitational constant and the mass of the sun through a quantity called the Gaussian gravitational constant, which meant that the unit's value depended on measurements of the sun's mass and on the fact that the sun is slowly losing mass, so the unit itself was drifting. The International Astronomical Union replaced that with a fixed conventional value, defining the astronomical unit as exactly 149,597,870,700 metres, which detaches it from any physical measurement and makes it simply a named length. That mirrors what happened to the metre, defined by fixing the speed of light, and to the kilogram, defined by fixing Planck's constant. The pattern is consistent: once a quantity can be measured more precisely than the artefact or phenomenon defining it, the definition is replaced by a fixed number and the measurement problem moves elsewhere.
The other distance units
Several units coexist for different ranges and each has a rationale. The light year is the distance light travels in a year and is intuitive for a general audience, being a length despite sounding like a time. The parsec is the distance at which a star shows a parallax of one arcsecond and is preferred by professional astronomers because it falls directly out of the measurement rather than requiring conversion, and it equals about three and a quarter light years. Kiloparsecs and megaparsecs scale it for galactic and cosmological distances. Redshift is used as a distance proxy at cosmological scales and depends on a model rather than being a direct measurement. The astronomical unit remains standard within the solar system and appears in exoplanet work, since an orbit's size in these units immediately indicates how the planet compares with our own, which is more informative than any absolute figure.
The takeaway
Kepler's laws gave the relative sizes of every orbit from periods alone, so a correct scale model existed with an unknown scale factor, and measuring the Earth's own distance converted all of it at once. That is why the Venus transits prompted worldwide expeditions. Radar ranging settled it in the 1960s, and in 2012 the unit was redefined as a fixed number of metres rather than a measured quantity.