Occultation

An occultation is an astronomical event where one object moves in front of another. This movement blocks the background object from view.

While the term is used in many fields, it is most common in astronomy. If a foreground object only partially covers the object behind it, the event is called a transit. If the two objects result in a shadow falling on the observer, it is called an eclipse. A total solar eclipse is actually a type of occultation where the Moon occults the Sun.

Lunar occultations are the most frequent type of this event. They occur when the Moon passes in front of a star during its orbit around Earth. Because the Moon has a very thin atmosphere, the transition happens very fast. A star can disappear or reappear at the Moon's edge, or limb, in 0.1 seconds or less. Astronomers often prefer to watch these events on the Moon's dark limb. The lack of glare on the dark side makes it much easier to time the event accurately.

Scientists use the timing of these events to gather vital data. By measuring exactly when a star vanishes, they can refine their knowledge of lunar topography, which is the study of the Moon's surface features. Photoelectric analysis of these moments has also revealed binary stars. These are stars that appear as one but are actually two stars orbiting each other. Additionally, timing occultations helps measure the angular diameter of stars. This measurement is useful for determining the effective temperatures of those stars.

Occultations can also involve planets. When a planet hides a star, the event is quite rare. For example, Venus occulted the star Regulus in 1959. The next time Venus will occult Regulus is in 2044. Planets also have significant angular sizes, which differs from stars. Because of this, a lunar occultation of a planet creates a narrow zone on Earth where only a partial occultation is seen. This allows observers in that specific zone to see the planet's disk being slowly blocked by the Moon.

Some of the rarest events involve mutual planetary occultations. This happens when one planet passes in front of another. If the closer planet looks larger than the distant one, it is a mutual occultation. One such event involving Venus and Jupiter occurred in 1818. The next will happen on November 22, 2065. An even rarer event is when Jupiter occults Saturn. This is one of the rarest known events in the sky. The last one occurred in 6857 B.C.E., and the next will not happen until February 10, 7541.

Small bodies like asteroids and dwarf planets also cause occultations. These events are incredibly useful for measuring the size and position of distant objects. By having many observers at different locations watch an occultation, scientists can create a cross-sectional profile of a body's shape. In 1983, the asteroid 2 Pallas was mapped using observations from 130 different locations. In 1998, over 38 observatories watched the asteroid 39 Laetitia. These precise measurements help scientists understand the dimensions of objects like Ixion and Varuna.

Occultations have also led to major scientific discoveries. In 1977, the rings of Uranus were discovered when the planet occulted a star. In the late 20th century, radio astronomers used lunar occultations to find the exact positions of radio sources. This was a key step in identifying the quasar 3C 273. This discovery helped Maarten Schmidt understand the cosmological nature of quasars. Even the atmosphere of Pluto has been studied by watching it occult stars. These moments of darkness provide a window into the structure of our entire solar system.