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Intro to Occultations
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Introduction

An occultation occurs when a solar-system body passes in front of a more distant object (e.g. a star or another solar system body), partially or totally hiding the more distant object and momentarily blocking its light. Each occultation can be seen only at the right time and from a limited part of the Earth. There are two main areas of occultation astronomy: lunar occultations and asteroid occultations.

Lunar Occultations

Observing lunar occultations tends to be fun and easy to learn. These events do provide novice observers with a training tool from which they can make timings using visual means or more advanced methods such as with video. Lunar occultations are more easily observed by those with small telescopes or less experience. The Moon makes a very convenient guide to identifying the area in the sky in which to find the star which is to be occulted. Lunar occultations can be classified into two groups: total lunar occultations and grazing lunar occultations. In a total lunar occultation the observer will see only one occultation event, either a disappearance of the star as the moon passes in front of the star or a reappearance of the star as the moon moves away from in front of the star. In a grazing lunar occultation, the very edge of the moon passes in front of the star and the star may disappear and reappear many times as the mountains on the edge of the moon individually occult the star.

Asteroid Occultations

For asteroid occultations the star is usually the brightest component of the occultation. The asteroid is usually several magnitudes fainter than the star and often too faint to be detected in a small telescope. In an asteroid occultation, the observer must find the star to be occulted and monitor the star to watch for any drop in brightness that would signal an occultation. Asteroid occultation events typically last several seconds but observers may record much shorter or much longer events in rare cases. An observer will only see an event (drop in the brightness of the star) if they are located inside the path of the asteroid’s shadow. Since asteroids are generally much smaller than the moon, choosing a location for observing an asteroid occultation is more important than location in lunar occultations. In addition, asteroids subtend a much smaller angular size on the sky and this leads to more uncertainty in the actual location of the asteroid’s shadow. Asteroid occultation predictions provide information on the expected location of the shadow path, expected time of the occultation, the level of drop in the star’s light and the expected duration of the occultation event. An observer can expect to see a single disappearance (or drop in starlight) and a single reappearance though it is possible to see step events.

Some asteroids actually have one or more satellites (also known as moons) orbiting them. As the asteroid moves in its orbit, it projects a shadow across Earth. Rarely, an unknown asteroid moon moving with it creates opportunity for discovery. Such secondary occultations should be expected to occur within +/- one minute of the actual asteroid occultation and be of very short duration, perhaps a fraction of a second and may be quite distant from the predicted asteroid shadow line. For this reason, if you cannot travel you should consider observing an asteroid occultation even if you are some distance from the predicted shadow line. You may be the person who discovers a new moon of the asteroid!