Occultations by the Moon
The most common object causing an occultation is indeed the moon. This is due to its relatively large apparent size compared to planets and also to its motion right round the sky in about 27 days so that, in the course of a month, it moves across a considerable area of sky. Even so only a limited number of stars can ever be occulted by the moon, they need to be within a few degrees of the ecliptic. Occultations by the moon can be seen, using a medium size telescope, on almost any clear night the moon is in the sky. The exception is near full moon when the glare from the moon makes it difficult to see any but the brightest stars.
Occultations can only be successfully observed at the dark limb of the moon, unlit by sunlight. At the sunlit limb a star will be swamped by the brightness of the moon so that it can only be seen when some distance from the limb. This applies to all but the very brightest stars.
Between New Moon and Full Moon the occultations at the “dark” limb, will be disappearances of the star behind the Moon. In fact the “dark” limb is lit by Earth light, that is sunlight reflected from the Earth onto the Moon. The Earth lit limb can be quite bright particularly before the first and after the last lunar quarters. Before full moon the reappearance of the star occurs at the sunlit moon limb. Between Full Moon and New Moon the opposite applies, it is the reappearances which take place at the unlit limb of the moon, the disappearances taking place at the sunlit limb.
Disappearances are easier to observe than reappearances. They mostly occur in the evening and it is usually easy to see the star before it disappears. At a reappearance (observed after full moon), the star is not visible before the event. So it is necessary to have a good idea of where it will appear, that is where to look, round the limb of the moon. Added to that problem, most reappearances from occultation occur after midnight, a less convenient time.
In either case accurate predictions of when the disappearance or reappearance will occur are needed. Fortunately such predictions are readily available.
In effect, during an occultation a shadow of the moon (or planet) is cast onto the Earth by the occulted star. The size of the “shadow” is the size of the moon or planet as projected onto the Earth. Since the moon is smaller than the Earth, only about the size of Australia, a particular occultation is only visible from a limited region of the Earth.