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Thanks for that Geoffrey - I've used your method to try and replicate the prediction for Aldebaran on 29Oct2015 at the Orwell Observatory near me in Suffolk (oasi.org.uk). I get close with 215130 immersion 215900 emersion compared to their prediction of 214949 and 224824UT. It became easier when I remembered that I could get all this calculated for me on the usno site. I did use a slightly different AP of 52N and 1E for ease which has complicated things but should only be about 1 minute difference in time. Hopefully it won't be cloudy on the 29th so I'll be able to a real test soon.

When I set out to look at this I thought that there should be a fairly easy analytical solution but have concluded that this is a hard way to find Longitude. However, the investigation has uncovered new depths in my ignorance and proved very interesting. I'll give it a rest now until the end of October.

Regards

Bob Crawley