## Double Stars

These then are the techniques of lunar occultation observation, where the star being occulted is a single star. If the star is in fact a double or a binary system, the intrinsic spatial resolution of the technique can be exploited to determine several useful parameters, depending upon the observing method. If the times of occultation of each of the components are measured by one of the techniques discussed above, then the Figure 18.1.

Schematic of an occultation of a double star; the components are separated by angular distance p in position angle d. The projected separation % may be estimated from the time difference between the two occultation events.

### Figure 18.1.

Schematic of an occultation of a double star; the components are separated by angular distance p in position angle d. The projected separation % may be estimated from the time difference between the two occultation events.

separation of the components, projected onto the apparent direction of motion of the Moon, can be determined simply from | = At.r, where At is the difference in time between the two events, and r is the rate of motion of the Moon. Now also | = p.cos (Q -p) where Q and p are respectively the true angular separation and position angle of the double star components, and pis the position angle of the occultation event on the lunar limb. The situation is illustrated in Figure 18.1, where the star A is about to be occulted by the limb of the Moon. Component B of the double star is separated from component A by distance p in position angle Q. The distance measured by the observed time difference At is the projected separation Provided that the personal equation effects discussed above are the same for each of the two events, then the accuracy of determination of is limited only by the resolution of the timing technique. Of course, from observations of a single event it is not possible to determine p and Q. However, if a series of observations of the same double star is obtained either from different locations on the Earth or over a period of time, such that a range of values of p is achieved, statistical methods can be used to estimate the values of p and Q from the deduced values of Most prediction packages that may be obtained from the references given in the Resources Section of this chapter include for each event the values of position angle Q and the rate of motion r of the lunar limb. 