I am working radioastronomy and have a formula for sun/earth calculation.

The formula for sun begin with $20(\cos270 \cos 30 \ldots\ldots+$


The first part is $\cos270=0$ and is that correct in the formulas attached.


Vr(sun) = 20.0 [cos 270° cos 30° cos α cos δ + sin 270° cos 30° sin α cos δ + sin 30° sin δ] (km/s)

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    $\begingroup$ @GrapefruitIsAwesome it looks like JamesK has understood what the actual question is sufficiently to answer it, and I've added the expression in question back into the question itself. $\endgroup$
    – uhoh
    Commented Feb 20, 2022 at 1:25
  • $\begingroup$ This question might be related or useful :-) $\endgroup$
    – nuwe
    Commented Feb 21, 2022 at 12:16
  • $\begingroup$ I know the formula and james have the answer - thank you :-) $\endgroup$ Commented Feb 21, 2022 at 22:36

1 Answer 1


Yes, for a direction with $α = 18 \mathrm{hr}$, or $270^◦$ and $δ = 30^◦$ it is correct to have a term in the dot product with $\cos 270$. It is merely a coincidence that that is zero.

The true value of the direction isn't exactly 270, so you can easily adjust the formula to a more accurate value later. A more precise value is 277 or 18hr 28min.

  • $\begingroup$ Hi james me again - yes from Denmark. I already have been trying to put in 271 and so on - but I felt it was not the right thing to do. And it dident make mutch chance in result. Sorry for my typing error. again thank you james $\endgroup$ Commented Feb 16, 2022 at 8:32
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    $\begingroup$ If this solves your problem, you may mark it as accepted by clicking on the green "tick". $\endgroup$
    – James K
    Commented Feb 16, 2022 at 8:38
  • $\begingroup$ Where do I find the green tick? $\endgroup$ Commented Feb 21, 2022 at 22:16

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