Saturday, July 06, 2019

[wddkltib] Some solar system distances in light units

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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