NavList:

Hello Sean and David,

Thank you for your replies.

I realised as you correct a Moon sight for HP to determine its True Altitude there would be little point if the Calculated Altitudes given in the Sight Reduction Tables had not also being corrected to give True Altitude. This was the reason for my question. We need to compare cheese with cheese.

I also note there are small corrections in altitude correction tables which I understand are for HP for Venus and Mars when they are at their closest point to Earth.

I realise HP is at its greatest the closer the heavenly body and the lower the altitude.

I decided to compare HP for Sun, Moon and Planets all at their closest point. The Earth radius remains constant at 3,963 miles, altitude is zero.

     HB                           Closest                            Maximum error if HP ignored

Saturn                       746,000,000                        about 31 metres  (1 arc second)

Jupiter                       365,000,000                       about 62 metres   (2 arc seconds)      

Sun                             92,960,000                       about 279 metres  (9 arc seconds)

Mars                           34,600,000                       about 744 metres  (24 arc seconds)

Venus                         24,000,000                       about 1054 metres   (34 arc seconds)

And the winner is..................

Moon                               225,623                       about 111,806 metres/60.37 miles   (3,623 arc seconds)

Mike