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I'm looking at the BMW book and see that equation 7.5-7 gives $\alpha_0 = \theta = \theta_g + \lambda_E$ as the equation for right ascension (for some launch site at east longitude $\lambda_E$). $\theta_g$ is Greenwich sidereal time at launch, presumably as an angle.

I know that SPICE contains a routine et2lst which can be used with longitude 0 to get Greenwich sidereal time in h/m/s, but how can I use SPICE to calculate GST in radians instead?

It looks like there's an equation for it, but if SPICE has already implemented this, I'd rather not reinvent the wheel.


Fundamentals of Astrodynamics, Roger R. Bate, Donald D. Mueller, Jerry E. White

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First things first: If the planet of interest is the Earth, I strongly suggest you consider using the Standards of Fundamental Astronomy library rather than SPICE.

That said, hour angle is a measure of angle rather than time. To convert the sexagesimal hour angle to a decimal hour angle, use $h{:}m{:}s \to h+(m + s/60)/60$. This decimal hour angle can readily be converted to degrees by multiplying by 15 (there are 360 degrees or 24 hours in one revolution), or to radians by multiplying by $\pi/12$.

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  • $\begingroup$ I figured it would be that simple. Thank you, Dave. Good to hear from you; I hope you're well. $\endgroup$
    – Translunar
    Dec 4 '18 at 14:51
  • $\begingroup$ Do you happen to know if there's a simple way to find the equinox in SPICE? I find myself wanting to check the results, and finding right ascension of Greenwich at the equinox seems to be the best spot-check. $\endgroup$
    – Translunar
    Dec 4 '18 at 16:11

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