One of the difficulties of observing and accurately measuring the relative positions and magnitudes of components of double stars is their mutual interference. Either one component is much brighter than the other, or the apparent separation between them is too small to be resolved by the optical system, particularly in the presence of distortion by the Earth's atmosphere. Ideally the components could be obscured one after the other to allow unambiguous observation of the companion as well as an estimate of the separation between them. This in essence is the principle behind application of the occultation technique to the observation of double stars.

Now, the Moon in its orbit around the Earth-Moon barycentre frequently obscures (occults) stars. As a consequence both of the inclination of the Moon's orbit with respect to the ecliptic and the precession along the ecliptic of the nodes of the Moon's orbit, all the stars in a belt of some 10° around that plane are occulted at some time during a period of about nine years. Among these are the bright stars Aldebaran, Regulus, Spica and Antares, and the star clusters Pleiades, Hyades and Praesepe. Since the Moon always moves eastward, an occulted star disappears at the Moon's eastern limb and reappears at its western limb. The phenomena can be best observed at the dark limb of the Moon, so in general disappearances are observed each month during the two weeks between New and

Full Moon, and reappearances during the following two weeks. Since the invention of the telescope, professional and amateur observers using a variety of techniques and instrumentation have recorded many thousands of timed observations of lunar occultations. Analyses of these observations have addressed such problems as improving the dynamical theory of the motion of the Moon, investigating the variable rate of rotation of the Earth, determining stellar reference frame anomalies, and measuring apparent stellar diameters and parameters in multiple star systems. It is the last two items that are of particular relevance to the subject of this chapter, but in the following sections the power of the occultation technique will be examined with reference to all of these applications.

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