NavList:

Haha, yes sorry Frank I am still learning and my definitions are a bit shaky by chart datum I meant the mathematical model forgetting chart datum is measured from the or a lowest astronomical tide... 

When I said calculated the positions by formula I wondered whether it's had mathematical formula for calculating individual objects positions or if it works like an electronic version of one of those old planetarium projectors that have the like lamp shade with the holes in it to project the stars... Ie it is rotating a picture rather than mapping the passage of individual objects.

Ahhh... So parallax is how leading lights work? If you approach the harbour at anything other than the correct bearing they appear separated but on the correct approach they are line... 

That's really interesting information about stellarium... it's Iis a Solas requirement that we use celestial navigation to check our compass error every shift but it's funny how many ships you go on who don't know where their sextant is!!! I have a lot of questions... I've downloaded the free version of your app but I wil definitely take the full version to sea in august!