What Is an Occultation? One Object Passing in Front of Another

The principle is geometric. A star is effectively a point source at these distances, so when an asteroid passes in front of it the star vanishes instantly and reappears instantly, and the duration of the disappearance multiplied by the object's known speed gives the length of the chord the asteroid's shadow traced across the observer. Observers at different locations across the shadow path record chords of different lengths, and assembling many of them reconstructs the object's outline directly, which is an extremely precise measurement of size and shape available in no other way for small distant bodies. If the star does not vanish instantly but fades over a measurable interval, the occulting body has an atmosphere, and the shape of the fade gives its temperature and pressure structure. If the star blinks off and on briefly before the main event, something else is there, which is exactly how rings are found. The technique requires only accurate timing and a telescope, which is why amateur observers contribute substantially.

Several distinct events fall under the heading and each answers a different question:

• •Lunar occultations, where the moon passes in front of a star, which are frequent, easy to observe and were historically used to measure the moon's position and to detect close double stars
• •Asteroid occultations, which measure sizes and shapes and occasionally reveal satellites
• •Planetary occultations of stars, which probe atmospheres and revealed the rings of Uranus in 1977 when a star blinked several times before and after the planet passed
• •Transits, which are the same geometry where the nearer object is much smaller and does not hide the farther one entirely, which is how most exoplanets are found
• •Eclipses, which are occultations of the sun or of a moon entering a shadow, distinguished mainly by convention and by what is doing the hiding
• •Mutual events, where the moons of a planet occult and eclipse each other, which refines their orbits

The technique has a strong record of discoveries that were not being looked for. The rings of Uranus were found because observers watching a star pass behind the planet recorded five brief dips before the occultation and five afterwards, symmetrically arranged, which could only be narrow rings. Rings around the small body Chariklo were found the same way in 2013, which was genuinely unexpected for an object of that size and prompted a search that found evidence for others. Pluto's atmosphere was detected by occultation in 1988 and has been monitored since, revealing pressure changes as the body moves along its orbit. Asteroid shapes measured this way are used to validate radar and spacecraft results. Binary asteroids have been identified from double events. And occultation timings were the method that pinned down the position and shape of the target for the New Horizons flyby of a distant Kuiper belt object, which required campaigns of observers deployed across remote locations to catch a shadow path a few tens of kilometres wide.

This is one of the areas where non-professional observers produce data that professionals use directly, and the reason is logistics rather than generosity. A shadow path is narrow and its predicted location carries uncertainty, so catching an event requires observers spread across a wide region, which no single institution can arrange. Portable equipment and accurate time from satellite navigation make a useful observation achievable with modest telescopes and video cameras, and organised networks coordinate deployments, distribute predictions and collect results. The measurement required is timing rather than imaging, which is exactly what inexpensive equipment does well. Results are submitted to central databases and published, and amateur observers appear as co-authors on professional papers routinely in this field. The same structure supports variable star monitoring and meteor observation, and it reflects a general pattern where a scientific problem requiring many simultaneous observations from many places is better solved by a distributed network than by a large facility.

A star vanishes instantly behind a body with no atmosphere, so the duration of the disappearance gives a chord across the object, and many chords from many observers reconstruct its outline precisely. A gradual fade reveals an atmosphere and brief blinks either side reveal rings, which is how the rings of Uranus were found in 1977. Catching a narrow shadow path needs many observers, which is why amateurs contribute directly.