# A Compound Angle

Tags:
1. Jul 24, 2017

### Philosophaie

A certain planet has a vector (Recl, L, B). The Earth and Sun vector exists on ecliptic. I need to know how to calculate the compound angle between the planet vector and the Earth-Sun vector.

2. Jul 24, 2017

### andrewkirk

There are a number of different possible approaches. Which is easiest will depend on the form in which the vectors are given.

One approach is to express each of the two vectors in Cartesian coordinates, normalise them both to length 1 (ie divide each by its length), take the dot product (sum of componentwise product) , then apply the arcos function.