I've used the previous years' almanac, adding the corrections, but that's only good for Sun and Stars, no Moon and Planets. If you're seriously navigating by sextant
, you want all the options open. If your sextant is only for "backup", chances are the math of an ordinary reduction will be tough enough without additional corrections to splice in. If you are a complete math genius (which I am not), Bowditch has a Perpetual Almanac from which you can extrapolate all the information necessary, further into the future than you could ever need. The reason most of us use almanacs and tables is that all the toughest math has been done for us by smarter heads (bless them!). I, too, have heard of a periodic repetition of some almanac info, but I think it's limited: precession, nutation, and tidal friction would preclude it's being identical in all respects.