Dark side of astronomy, or what is “occultation”

Astronomy, with all the desire, cannot be attributed to the occult sciences, which, unfortunately, have recently begun to gain popularity in society along with irrational beliefs such as the “Flat Earth theory”. But astronomers also have their own “occult knowledge” that helps them study the Universe. And, oddly enough, everyone can join them — even novice observers.

The most beautiful occultation

The astronomical term “occultation” comes from the Latin word “occultus”, which means “hidden”. This word is commonly used to call a complex of phenomena during which one celestial body briefly hides another from the observer.At the same time, two more conditions must be met: first, the body closest to the observer must have a larger visible size than the far one, and cover it completely. And secondly, it should have a lower gloss or at least a surface brightness.

The largest visible diameters in our sky are the disks of the Moon and the Sun. They are constantly changing depending on where the Earth (perihelion or aphelion) and its natural satellite are located in the orbit. When the lunar disk has a larger size than the solar one, and at the same time completely covers the last, a total solar eclipse occurs — one of the most beautiful astronomical phenomena. It allows us to briefly see the layers of its atmosphere closest to the surface of our luminary. Until recently, no spacecraft could look there, and only the PSP and Solar Orbiter probes were able to study this area of space in more detail.

Total solar eclipse. Details of the Moon’s disk are visible due to the fact that they are illuminated by the Sun’s rays reflected from the Earth

Unfortunately, no total solar eclipses have been observed on the territory of Ukraine since 1961, and this situation will continue until April 20, 2061. Therefore, to see them, you need to go abroad. But if you have such an opportunity, make sure to use it. The celestial show accompanying the closing of the Sun by the Moon is worth seeing at least once in your life. Partial eclipses, even with large phases (over 90%), cannot be affected by anything like this. 

Behind the Moon’s disk

The sun is one in our sky, and there are many more stars there, and it is clear that the Moon covers them more often. However, near the shining lunar disk with the naked eye, we can see only the brightest of them. When our moon is in phases between the last and first quarter, that is, it looks like a narrow sickle, people with normal vision manage to notice occultations of stars brighter than the 3rd magnitude. There are less than two dozen such objects in the strip along the ecliptic along which the Moon moves. And there are only four luminaries of the 1st magnitude — Aldebaran, Regulus, Spica and Antares. Our ancestors observed the closing of this four with a lunar sickle long before the invention of telescopes. Such observations, made from different points of the earth’s surface, made it possible to make the first estimates of the distance to the nearest celestial body.

Further progress in observations of lunar occultations was ensured not even by the invention of a telescope, which made it possible to see fainter stars near the Moon, but by the appearance of an accurate clock. This deepened our knowledge of celestial mechanics, and also contributed to the emergence of the first effective way to calculate geographical longitude. If it is easy enough to find out the latitude by measuring the height above the horizon of the Polar Star (in the Southern Hemisphere everything is a little more complicated), then it was not possible to measure latitude with acceptable accuracy until the middle of the XIX century. The moment of lunar occultation can be registered with an error of up to tenths of a second, and then, having its prediction for certain fixed points on the earth’s surface, the result obtained can be recalculated into a deviation from the nearest of such points in latitude. This method remained the best until the appearance of radio signals of the exact time. It was limited only by the fact that there are not so many “candidates for coverage” of a sufficiently high gloss in the sky, and their closure by the Moon is not observed every night, even in cloudless weather. 

But even now, observations of star coverings by the Moon have a certain scientific value. Firstly, modern sensitive electronic photometers make it possible to register diffraction effects arising from the interaction of starlight with the edge of the lunar disk. This allows us to estimate the apparent diameter of the eclipsed luminary, and also sometimes leads to the discovery of its moons. Secondly, the most interesting observations of occultations in areas near the edge of the coverage line of sight are the so-called “tangent occultations”. During them, the stars may disappear and appear several times when they are briefly obscured by the irregularities of the lunar surface. The obtained data are used to build a high-altitude profile and then can be compared with the information transmitted by the altimeters of artificial satellites of the Moon.

Planets and bright asteroids hide behind the lunar disk much less often, if only because there are not so many of them in the sky. But occultations of one or two large planets usually occur in a year. For example, on September 14 and December 5, 2022, from the territory of Ukraine, it will be possible to see the closure of Uranus by the Moon, and on December 8, the occultation of Mars will coincide with its opposition. In the era of space flight, the scientific value of such phenomena is small, and they attract mainly astrophotographers.

The Moon’s closure of the planet Saturn. Its appearance was due to the dark edge of the lunar disk

At the end of this section, it should be noted that the easiest to observe are the covering of objects with the dark edge of our natural moon, occurring in the period from new moon to full moon. Then the lunar disk moves forward across the sky with its illuminated side, and it becomes impossible to register the exact moment when even sufficiently bright stars disappear behind it. But they appear already because of the dark edge. To see this, you need to know as accurately as possible from which point they will “emerge”. Despite the apparent complexity of such observations, there is nothing impossible in them and they are quite accessible even to amateur astronomers.

Occultations occurring during lunar eclipses should be singled out separately. Only a very limited number of stars can participate in them, and the “dark moon” covers of planets are among the rarest astronomical phenomena. On November 8, 2022, the fully eclipsed Moon will cover Uranus. Unfortunately, only residents of Northeast Asia will be able to see this. The next similar phenomenon is expected in the XXII century.

Stars hide behind planets

Planets are another category of celestial bodies that have disks visible, at least through telescopes. Moving across the firmament, they also close the stars from time to time, although the probability of such an event is much less than the closure of a luminary of similar brightness by the Moon. Due to the relatively large average angular size and rapid movement across the sky, Venus most often becomes a “participant” in such events. In addition, most of the time it looks like an incomplete disk with a lighted and dark edge. Analysis of changes in the spectra of stars and optical phenomena during such occultations allowed astronomers to evaluate the properties and composition of the upper layers of the Venusian atmosphere long before the flights of the first interplanetary probes.

The same observations were made for other planets. They helped to clarify the elements of their orbits, the density and composition of the atmospheres, and also contributed to the discovery of the rings of Uranus, made during the occultation of the star SAO 158687, which took place on March 10, 1977.

Occultation by planets of bright luminaries is an extremely rare phenomenon. The nearest such event will occur on October 1, 2044, when Venus closes Regulus (alpha Leonis). The star will disappear behind the illuminated edge of the planet’s disk and appear from behind the dark edge. It will be possible to see this in Central, South and East Asia.

Can a planet cover a planet? Yes, but it happens even less often. The first event of this series in almost 250 years — the transit of Venus through Jupiter on November 22, 2065 — cannot even be fully called an occultation, since the apparent size of the “closing” body will be smaller than the more distant one. In addition, both planets will be located only 7° from the Sun, which will greatly complicate observations.

“Shadows” of asteroids and Kuiper Belt Objects

Asteroids, comets, Kuiper belt objects are classified as small bodies of the Solar System. But, no matter how “small” they are, the stars are much further away from us and have a significantly smaller angular size. Consequently, covering distant luminaries with asteroids is a classic occultation, and even their amateur observations can be quite useful.

But such an event can be observed only when it can be predicted. And there were problems with this until the 70s of the last century: firstly, astronomers did not know with acceptable accuracy the parameters of the orbits of the vast majority of small bodies, secondly, it is not difficult to understand that occultation will be observed only in a narrow band on the earth’s surface, the width of which is not much larger than the size of an asteroid or comet nucleus.

Oddly enough, the first asteroid which occultation could be predicted and successfully registered was a relatively small Eros (433 Eros), belonging to the class of near-Earth objects. In January 1975, it was just passing near the Earth and at some point found itself between it and the bright star of the κ Geminorum. The asteroid’s “shadow” was supposed to pass through North America, but the exact position of the coverage band remained unknown, so numerous observers in the United States and Canada were involved, scattered over an area of more than a million square kilometers. Many of them actually managed to see how the star faded out for a few seconds at the predicted time. According to the results of observations, the size of Eros was estimated at 23 km. Now we know that it has an irregular shape and a length of almost 33 km.

As the orbital elements were refined and computer technology improved, asteroid occultations began to be observed more often. On May 29, 1978, a fairly bright star in the constellation Vulpecula was closed by Pallas (2 Pallas) — the largest object of the Main Asteroid Belt, to which automatic spacecraft have not yet flown. Later, based on data on the duration of the “disappearance” of the star, measured at 130 different points, astronomers were able to reproduce the shape of a small body resembling an ellipsoid with axes 532 and 558 km.

Pallas contours based on the results of observations of two occultations in 1978 and 1983. Each chord corresponds to the duration of the “disappearance” of the star at a particular observation point. Source: www.semanticscholar.org

Covering stars with objects outside the Main Belt also brought astronomers a lot of surprises. During such an event involving the dwarf planet Pluto (134340 Pluto), held in 1988, its nitrogen-methane atmosphere was discovered, and in subsequent years scientists were even able to register changes in its pressure. The researchers were even more surprised by centaurs — small icy bodies that spend most of their time in the gap between the orbits of Saturn and Uranus. The passage of Chiron (2060 Chiron) against the background of a distant star in November 1993 and March 1994 was accompanied by its additional short “fades” before and after the main one. The most logical explanation was the presence of a thin dark ring in the object. During occultation on March 26, 2014, rings were found at the centaur Chariklo (10199 Chariklo), and on January 21, 2017 — at the Kuiper object Haumea (136108 Haumea).

Also in 2017, astronomers used the covering of the star by the Kuiper Belt Object 2014 MU69, later named “Arrokoth”, to check whether there were no invisible moons or rings in its immediate vicinity. The fact is that the New Horizons spacecraft was just heading to this object, and the accompanying group wanted to make sure that it was as safe as possible during the flyby.

Global visibility map of asteroid coverage of a star from the site www.asteroidoccultation.com/

Cometary occultations are much more difficult to observe, since comet nuclei are surrounded by a coma — a gas-dust shell that emits quite intensively and prevents seeing the disappearance or appearance of a star. But the spectral analysis of the starlight that passed through the coma, at the beginning of the last century, allowed us to draw the correct conclusion that the substance in it is extremely sparse.

Information about current predictions of asteroid occultations can be found on specialized websites (for example, https://www.asteroidoccultation.com/). Almost every day there is one or even more coverage of stars brighter than the 11th magnitude, which can be seen in small amateur telescopes in urban illumination conditions. Even information about the presence or absence of coverage in some cases may have scientific value. The results obtained will help scientists refine the orbits of asteroids, determine their size and shape, and sometimes even discover their moons.

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