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mikedilger / astrotime

Time for astronomy
Apache License 2.0
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TCB is not defined correctly #2

Open spenczar opened 1 year ago

spenczar commented 1 year ago

TCB is a dynamical coordinate time scale which does not proceed at a fixed rate relative to TT. This implementation is not correct.

See, for example, the description provided by SOFA:

image

In fact, because of this, most people use TDB in practice, since it has a quasi-periodic behavior that can be approximated. See Fairhead and Bretagnon for a useful model. TDB can only be specified at a particular location, though. You could assume the geocenter but you'd be off by a significant amount (microseconds).

mikedilger commented 1 year ago

Great info, thanks. This repo has long been idle, I never finished what I started here, but if I come back to it, I'll take this into consideration.

spenczar commented 1 year ago

Yep, makes sense. I came across it because I'm doing some work on orbit determination and ephemeris generation in Rust, and needed an astronomy-quality time library. You're the first thing that shows up in crates.io when you look in that direction.