if you have two circles, how do you define the two tangents which are common to both?

To all,

I am new and am studying Aircraft Engineering at university.

I have a problem i am trying to work out for a project I’m working on currently.

As in the title; if you have two circles, how do you define the two tangents which are common to both?

The situation is relative to an aircraft flying full 360 degree circles. So i have calculated the co-ordinates and headings of the aircraft every second through two circles.

I know i can draw out the circles and simply draw both tangents of the circles with a ruler and measure it like that. But i need something i can put into excel or a formula i can show will work for any 2 circles of the same size give the common tangents.

I think that’s everything. Both circles are the same size but can be anywhere relative to each other. I know the tangents have a common link that is the heading out of one circle will equal the reciprocal heading of the second circle.

Thanks in advance

Alphecca

Progress but not complete

I have made some progress into this problem. It seems the key lies in knowing the bearing of angle between the centres of the two circles!

This bearing compared with the headings will give only two matches. They are your tangents

However; this doesn’t solve the tangents labelled in red which cross over, only the ones labelled in blue.

Is anyone willing to help me out in this problem knowing this new information?

I’m you if i ponder for more hours over diagrams i might get it but some help would be greatly appreciated.

Thank you