Compute great-circle track from longitude
@James Turner: considering the date of the question, you've surely moved on. If you found a solution elsewhere, you can post it here and get some bonus points for answering your own question. Anyhow, the Aviation Formulary has the answer to your question: it's not Clairaut's Formula. You can find the latitude of ownship position by entering its longitude and the coordinates of the GC endpoints into the &quot;Latitude of point on GC&quot; formula. Then, knowing lat and long of both current position and destination, you can determine the heading to use leaving that point from the &quot;Course between points&quot; formula.