Observation

The scientific observation of an occultation involves accurately recording the instant at which the star disappears behind or reappears from behind the lunar limb. In all but occultations of the brightest stars, telescopic or binocular aid is essential for making an accurate measurement; as the Moon approaches the star the glare from the sunlit part of the disk totally overwhelms the light from the star. By using optical aid to restrict the field of view, in most cases the star can clearly be seen at the moment of occultation.

The Moon orbits the Earth in approximately 28 days, which leads to an average easterly motion against the background of stars at a rate of 0.5'' per second of time. If the instant of occultation can be estimated to a precision of 0.1 s, then the relative position of the lunar limb and the star is known at that instant to a precision of 0.05''. The analysis of such observations proceeds by the computation both of the position of the centre of the Moon at that instant by interpolation in a lunar ephemeris and a precise knowledge of the position of the observer on the Earth's surface, and the position of the star taken from an appropriate star catalogue. Also, the lunar limb is not smooth; it has roughness of apparent angular extent ±2'', caused by variations in the level of the lunar terrain along the line of sight from star to observer. From this information, the apparent distance of the star from the lunar limb at the instant of recorded occultation may be calculated.

Almost certainly, the computation will imply that the star should have been occulted at a slightly different time than that recorded by the observer. The reasons for the discrepancy will include errors in all the assumptions made to compute the circumstances of the occultation, such as errors in the position of the star given in the catalogue, errors in the lunar ephemeris and in the charts used to derive the level of the lunar terrain. A further correction will be attributable to the method used to make the observation. No matter how well prepared and experienced the observer, there is inevitably a time delay between the instant that the observer perceives and then records the event. If a stopwatch is used to record the event, it has been estimated1 that this delay, or personal equation, is on average about 0.3 seconds for a disappearance and 0.5 seconds for reappearance, the larger value for the latter being due to the intrinsic "surprise" element of this type of event. Another recording technique in common use is the so-called eye-and-ear method; the observer listens to an audible one-second time signal whilst concentrating on making the observation, then mentally estimates the time of the event as a fractional part of a second. Results of analyses1 suggest that this method is essentially free from personal equation effects, with observers achieving measurement precisions of about 0.1 seconds. A far more accurate technique is to record the occultation events electronically. A photomultiplier is used to count individual photons reaching the telescope from the star, and the counts are integrated over contiguous, short time intervals, of duration typically one millisecond. The resulting light curve can then be analysed to determine among other quantities the instant of occultation with precision close to one millisecond.

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