Graph of altitude against local hour angle.
A fixing method that dispenses with the transfer of position lines, and so facilitates plotting, is a sandwich fix. The observed altitude of a body varies linearly with time, except near or during meridian passage time. Hence, if two sights are taken on a body, the mean of the two values of an hour angle is at the mean of the midtimes of the two sights. Hence, a pair of sights of a body equally separated from a planned fix time (i.e., one before and one after the fix time) will give a position line without having to take a sight at that time. Thus, by taking a pair of sights on two or more stars so that the mean times coincide with a planned fix time, a simultaneous fix can be obtained. Such fixes are referred to as a sandwich fix
. Generally, in this method, four position lines are taken from two sources in such a way that the two pairs of lines are approximately at 90° to one another and the times of the observations are symmetrically spaced at a central time. Meridian passage time is the time at which a celestial body crosses the observer's celestial meridian.