Nominal Solar Radius
| Value | 695,700,000 m |
| Status | Exact by definition — no uncertainty |
| Source | IAU 2015 Resolution B3 |
| Categories | Astronomicalsolar-systemstellar |
| femtometer | 6.9570000e+23 fm |
| picometer | 6.9570000e+20 pm |
| nanometer | 6.9570000e+17 nm |
| micrometer | 695,700,000,000,000 μm |
| millimeter | 695,700,000,000 mm |
| centimeter | 69,570,000,000 cm |
| decimeter | 6,957,000,000 dm |
| meter | 695,700,000 m |
| kilometer | 695,700 km |
| inch | 27,389,764,000 in |
| foot | 2,282,480,300 ft |
| yard | 760,826,770 yd |
| mile | 432,287.94 mi |
| nautical mile | 375,647.95 nmi |
| astronomical unit | 0.0046504673 AU |
| light-year | 7.3535548e-08 ly |
| parsec | 2.2546102e-08 pc |
Learning zone
The Sun has no surface. What we call the photosphere is the depth at which the gas finally becomes transparent, and where you place that boundary depends on the wavelength you observe and the model you fit — recent determinations from solar oscillations and from eclipse timings differ by a few hundred kilometres. Rather than let every published stellar radius shift whenever solar measurements improve, IAU 2015 Resolution B3 fixed a nominal value of exactly 695 700 km and told astronomers to express stellar radii as multiples of it.
The convention pays off in consistency: two papers written a decade apart mean exactly the same thing by "1.4 R⊙". The nominal radius is intentionally close to, but formally independent of, the measured photospheric radius of about 695 700 ± 100 km.