[wddkltib] Some solar system distances in light units

July 05, 2019

The speed of light gets used in the unit the light year to express distances between stars.  The speed of light can also similarly be used to express distances within the solar system.  Using the speed of light is more compact than SI's typical millions, billions, or trillions of meters, and is especially useful if lightspeed communication is happening, e.g., to a space probe.

Astronomical units (AU) (which are not SI) are currently used to express distances within the solar system.  Historically, au were useful when the absolute scale of the solar system was not yet known, but sizes relative to the size of earth's orbit were known.  I think we now know absolute distances within the solar system as precisely as ratios.  (But not masses.)  So the au ought to be obsolete.

earth diameter = 0.04244 - 0.04255 light second

earth half-circumference = 0.067 light second

earth circumference = 0.13 light second

moon diameter = 0.0116 light second

Jupiter diameter = 0.45 - 0.48 light second

sun diameter = 4.6 light second

geostationary orbit altitude = 0.12 light second

earth moon distance = 1.21 - 1.35 light second

Jupiter Io = 1.4 light second

Jupiter Europa = 2.2 light second

Jupiter Ganymede = 3.6 light second

Jupiter Callisto = 6.3 light second

(Distances are, I think, center to center.)

sun Mercury = 2.6 - 3.9 light minute

sun earth = 8.18 - 8.46 light minute

sun Mars = 11.4 - 14 light minute

sun Jupiter = 41 - 45 light minute

sun Saturn = 75 - 84 light minute

sun Neptune = 4.1 - 4.2 light hour

earth 2014 MU69 at the moment of New Horizons closest approach = 6 light hour

Planetary orbits are quite elliptical.

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