1. The problem statement, all variables and given/known data Using a radio telescope we are recording data from the galactic plane at the 1420.4MHz frequency of neutral Hydrogen - we are aiming to create a polar map of the Hydrogen in the Milky Way. When looking through a cloud of Hydrogen (i.e. a spiral arm) the frequency observed can be doppler shifted depending upon the relative velocity of the cloud, this allows us to calculate the velocity of the cloud along our line of sight. Now all we need is to calculate the distances of these observed clouds. We know the distance from us to the centre of the galaxy, we know the longitude of the cloud (angle between the galactic centre, us and the cloud), we know the velocity of the Sun around the Milky Way and we know the observed velocity of the cloud along the line of sight. How can we calculate the distance to the cloud? We think this is just a geometry problem but we may be wrong. 2. Relevant equations Trig equations 3. The attempt at a solution We can calculate the distance from the centre of the galaxy to the closest point of the line that runs through the Sun and this cloud somewhere else in the galaxy (so where this line is perpendicular the centre of the galaxy). We have tried using all the tricks in the book geometry wise and have come up short to how to find the solution. Any ideas?