606 APPENDIX A
Astronomy Units
and ConstantsAstronomy, and science in general, is a way of learning
about nature and understanding the universe. To test hypotheses
about how nature works, scientists use observations of nature.
Th e tables that follow contain some of the basic observations that
support science’s best understanding of the astronomical uni-
verse. Of course, these data are expressed in the form of numbers,
not because science reduces all understanding to mere numbers
but because the struggle to understand nature is so demanding
that science must use every valid means available. Quantitative
thinking—reasoning mathematically—is one of the most power-
ful techniques ever invented by the human brain. Th us, these
tables are not nature reduced to mere numbers but numbers sup-
porting humanity’s growing understanding of the natural world
around us.
■ Table A-5 ❙ Astronomical ConstantsVelocity of light (c)
Gravitational constant (G)
Mass of H atom
Mass of Earth (M⊕)
Earth equatorial radius (R ⊕)
Mass of sun (M⊙)
Radius of sun (R⊙)
Solar luminosity (L⊙)
Mass of moon
Radius of moon 3.00 108 m/s
6.67 10 ^11 m^3 /s^2 kg
1.67 10 ^27 kg
5.98 1024 kg
6.38 103 km
1.99 1030 kg
6.96 108 m
3.83 1026 J/s
7.35 1022 kg
1.74 103 km■ Table A-6 ❙ Units Used in Astronomy1 Angstrom (Å)1 astronomical unit (AU)1 light-year (ly)1 parsec (pc)1 kiloparsec (kpc)
1 megaparsec (Mpc) 10 ^8 cm
10 ^10 m
10 nm
1.50 1011 m
93.0 106 mi
6.32 104 AU
9.46 1015 m
5.88 1012 mi
2.06 105 AU
3.09 1016 m
3.26 ly
1000 pc
1,000,000 pc