I would like to offer an amplification to Gregory Dill’s correspondence piece on using stars to check the accuracy of a sextant (“Testing a sextant using stellar distance,” Issue No. 77). Although (because Earth wobbles) the SHA and declination of stars do vary over time, the relative interstellar angles remain the same over our lifetimes. This is due to the vast distances between the stars.
The angle that you calculate between the stars Dubhe and Alkaid will always be the same no matter which date in the almanac you choose.
Yuri Mykolayevych lives in New York and is a sailor and a member of the New York Amateur Astronomy Association.