Lunar Occultations

Imagine that the Moon is due to cross a particular well-known star. What useful information can be obtained about our natural satellite?

First, because we can measure and catalog the coordinates of the stars with great precision, by timing the instant at which the star disappears behind the Moon one may determine the lunar position at that instant with similar accuracy. It is relatively easy to ascertain the locations of objects that effectively stand still, like the stars. Because they are moving in concert around the sky, a telescope can continuously track them if it is rotated at just the right rate to compensate for the turn of the Earth.Yet this is not so with the Moon or other members of the Solar System, which are in constant but variable motion relative to the static background of stars. Timing an occultation to a fraction of a second allows the observed location to be referenced against the predicted position from the computed ephemeris, perhaps leading to an update.

Nineteenth-century astronomers argued over what they saw through their telescopes when the Moon occulted a star. To many observers it seemed that the image of the star was projected onto the dark lunar disk, seeming to remain visible even after it was obvious that the star must be hidden. Some claimed that this image seemed to be colored even though the star may have been white.

Debates over this phenomenon raged for years, various hypotheses being advanced for its origin. In those days the nature of light was still a mystery. Some argued that the Moon was partially translucent, acting like a cloud whose periphery lets some light through (the "every cloud has a silver lining" effect).

Eventually it was realized that the apparition is simply an artifact of the human eye. It is similar to staring at a light globe for a few seconds, and then looking towards a dark background, resulting in a residual colored image: your retina takes a short while to recover from the bright light it had been sensing. Shakespeare knew all about this, having Katherina in The Taming of the Shrew say this:

Pardon, old father, my mistaking eyes, That have been so bedazzled with the Sun That everything I look on seemeth green . . .

Precisely the same thing happens if you follow a star with a telescope as it slips behind the Moon: paradoxically the stellar image seems to creep over the lunar landscape for a second or two, even though the source has already disappeared from view.

The second sort of quantitative information about the Moon that may be obtained from modern-day occultation observations pertains to its surface contours. Suppose that a particular occultation was timed by a string of observers spread over some hundreds of miles. If the Moon were exactly spherical, then there would be a simple arithmetical relationship between the times they recorded. But we know that the Moon is not spherical: rather, it is mountainous in some regions, deep canyons and rills permeating the surface elsewhere, and it is pockmarked with craters, too. Imaginary straight lines from the star to each of the observers,just touching the lunar limb, will variously strike crater rim, mountaintop, or slip through a deep valley. Because of this, some watchers will record the occultation as occurring a split second early, others a similar time late, compared to a perfectly even curve.

With a concerted effort, and accurate knowledge of the observers' positions and timings, a contour of the lunar limb may be drawn up. In addition, because the Moon vacillates slightly, not presenting a completely constant face to us, each occultation presents the opportunity to study a different arc drawn across the Moon's surface. An especially valuable opportunity occurs when a star passes virtually parallel to the lunar limb—a grazing occultation—because then observers at critical locations on the Earth see it being successively hidden and then briefly revealed as it skims along the serrated edge of the Moon. Observers separated by just a mile will see different aspects of the Moon's crinkled fringe.

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