# How does one use SPICE to compute the Greenwich sidereal time as an angle?

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

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 $$\pi/12$$.