Caption: A calculation of the Earth's mean orbital speed = 29.78 km/s in the inertial frame of the Solar System (i.e., the center-of-mass free-fall inertial frame (COMFFI frame) of the Solar System). Note all (true) inertial frames are NOT rotating relative to the observable universe.

By the by, the kilometer per second is a good natural unit for astrophysical velocities since these are often of order a few or a few 10s, 100s, 1000s, or 10,000s of kilometers per second.

By the by also, the Julian year = 365.25 days = 3.1557600*10**7 s ≅ π*10**7 s (underestimate by -0.4489 ... %). So π*10**7 s is a good and very memorable approximation to the length of the Julian year measured in seconds. That π*10**7 s is a good approximation is just a coincidence: there is NO profound reason why it is so.

Credit/Permission: © David Jeffery, 2003 / Own work.
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