astronomical_unit conversion factor is imprecise

[meawoppl]

Summary

The astronomical_unit in src/si/length.rs uses the conversion factor 1.495_979_E11 (149,597,900,000 m), but the IAU 2012 Resolution B2 defines the astronomical unit as exactly 149,597,870,700 m (1.495_978_707_E11).

Current value (line 46 of src/si/length.rs)

@astronomical_unit: 1.495_979_E11; "ua", "astronomical unit", "astronomical units";

Proposed value

@astronomical_unit: 1.495_978_707_E11; "au", "astronomical unit", "astronomical units";

Note: The IAU also recommends the symbol "au" (lowercase) per IAU Style Manual (2006) and IAU 2012 Resolution B2, rather than "ua".

Reference

• IAU 2012 Resolution B2: "the astronomical unit of length [...] is a conventional unit of length equal to 149 597 870 700 m exactly"
• NIST SP 811: Lists the astronomical unit as 149 597 870 700 m

The current value introduces ~29,300 m (~29.3 km) of error per AU, which is significant for high-precision astrometry.