Sextant...

[kefroeschner]

btrayfors is quite right. It is called parallax error and results from the fact that the two lines of sight to the object (from the index and horizon mirrors) are not quite parallel unless the object is at infinity. Since one second of arc is about 5 microradians and the separation of these mirrors and LOS's is a few inches, the distance to the object should be greater than 3,333 feet (a km or so) for this error to be less than ten seconds of arc, usually regarded as negligible. (It is pre-coffee early so someone ought to check my arithmetic.)