1. astronomical unit (AU) ≡ 1.49597870700*10**11 m ≅ 1.496*10**11 m for general planetary systems. Something about planetary-system formation and star-star interactions makes this a good natural unit.

   The value since 2012 has been exact by definition and it is very close to exactly the mean Earth-Sun distance AKA the semi-major axis of the Earth's orbit.

2. light-year (ly) = 0.94607304725808*10**16 m = 63241.0770 ... AU = 0.306601393 ... pc ≅ 0.3066 pc for interstellar distances. This is of order typical nearest neighbor interstellar distances (NOT counting distances among stars in multiple star systems), and hence is a good natural unit.

   The light-year is the distance light at the vacuum light speed c = 2.99792458*10**8 m/s ≅ 3*10**8 m/s = 3*10**5 km/s ≅ 1 ft/ns travels in 1 Julian year (Jyr) ≡ 365.25 days ≅ π*10**7 s.

3. parsec (pc) = 3.08567758 ... *10**16 m = 206264.806 ... AU = 3.26156377 ... ly ≅ 3.26 ly for interstellar distances also. This is also of order the typical nearest neighbor interstellar distance NOT counting stars in multiple star systems, and hence is a good natural unit.

   It is the distance to an astronomical object (usually a star) that has a parallax (a stellar parallax for star) of 1 arcsecond ('') for a baseline of 1 astronomical unit (AU). For a complete explication of how the parsec is arrived at, see parallax_stellar.html.

4. kiloparsecs (kpc) and kilolight-years (kly) are natural units for INTRAGALACTIC distances since large galaxies are typically tens to hundreds of kiloparsecs in size scale (Wikipedia: Parsec: Parsecs and kiloparsecs).

   For example, the Milky Way disk diameter (by some conventional metric) is ∼ 30 kpc. However, dwarf galaxies have a much smaller size scale. Currently, the smallest known are ultra-compact dwarf galaxies with a size scale of order 0.1 kpc. One should also consider galactic halos for non-dwarf galaxies: stellar galactic halo have size scales ∼ 50--100 kpc (e.g., FK-566); dark matter halos (which are set by dark matter) have size scales of ∼ 100--200 kpc (e.g., FK-566).

5. megaparsecs (Mpc) and megalight-years (Mly) are natural units for INTERGALACTIC distances since nearest neighbor galaxies are typically of order 1 to a few megaparsecs apart (see Wikipedia: Megaparsecs and gigaparsecs). Typical INTERGALACTIC distances in cosmic voids are probably larger: maybe 10 or more megaparsecs.????

   For example, the Andromeda galaxy (M31, NGC 224) (the closest non-dwarf galaxy) is at 0.778(33) Mpc (see Wikipedia: Andromeda galaxy).

6. gigaparsecs (Gpc) and gigalight-years (Gly) are natural units for cosmology since the comoving radius of the observable universe = 14.25 Gpc = 46.48 Gly current value and the Hubble length = L_H = c/H_0 = 4.2827 Gpc/h_70 = 13.968 Gly/h_70, where h_70 = H_0/(70 Mpc/(km/s)) is the reduced value of the Hubble constant reduced using Hubble constant = 70 (km/s)/Mpc (fiducial value) which is within a few percent of the true Hubble constant which is still uncertain by a few percent.

   Note the value of comoving radius of the observable universe is model-dependent result. It depends on the currently favored Λ-CDM model which is holding up remarkably well circa 2021 (see, e.g., Scott 2018), but may NOT last forever. Note also that one definition of the local universe (but certainly NOT the only one), and thus also the modern observable universe, gives it a radius of ∼ 5 Gly ≅ 1.5 Gpc and a lookback time from cosmic present of ∼ 5 Gyr which is roughly the age of the Solar System---and remember "humankind is the measure of all things" as said Protagoras (c.490--c.420 BCE).

7. Note 1 Gpc = 10**3 Mpc = 10**6 kpc = 10**9 pc.

Local file: local link: astronomical_distances_larger.html.
File: Cosmology file: astronomical_distances_larger.html.