Decrease font size Increase font size Print this page Français

Understanding > Fundamental concepts > Phenomena IV


In astronomy , especially regarding the solar system , the notion of occultation or eclipse is of particular importance . This concept refers to the phenomena that occur frequently and which can provide valuable information. 

I - Definition  : 

An occultation occurs when a planet passes in front of a celestial body, hiding totally or partialy this body. This is purely apparent and related to the observer. No interaction takes place between the bodies involved. Example: a solar eclipse is an occultation : the Moon masks all or part of the solar disk (without, of course, any modification of the Sun itself) . This phenomenon is related to the observer: changing position on the surface of the Earth, the phenomenon may disappear and no longer be visible. Note that we made a mistake by calling eclipse which is in fact an occultation .

Click here for a figure showing the difference between occultation and eclipse (case of Jupiter's satellites , similar to all other bodies).

An eclipse occurs when a planet passes into the shadow of another celestial body and disappears at least partially in the eyes of all observers, regardless of their position. Example: a lunar eclipse is a real eclipse when the Moon disappears in the shadow of the Earth ( the Moon is affected by the sudden absence of light). This phenomenon is visible to any observer of the Moon, regardless of its position on the surface of the Earth or in space (seeing the bright side of the Moon , of course ... ) . Note that a solar eclipse is a partial eclipse of the Earth for an observer in space.

II - Description of the phenomenon, examples: 

Eclipses of the Moon and the Sun are perfect examples of eclipses and occultations . It is thus found that the main characteristic of this type of phenomenon is the observation by the observerof, a decrease in brightness of the observed body. The observation of such a phenomenon is a photometric observation measuring the variation over time of the light received from the body occulted or eclipsed .
Furthermore lunar eclipses and Sun, there are many examples :

a). The phenomena of the satellites of Jupiter (and Saturn)  : 

Jupiter's satellites rotate in the same plane and are close to the planet Jupiter. By its large size, the planet makes a major shadow behind it and also presents a large apparent disk. This will provide us with occultations and eclipses :
- When a satellite is in the shadow of the planet, it will be an eclipse of the satellite: this phenomenon , sometimes called "immersion" at the beginning and "emersion" at the end, is spectacular and observable even in a small telescope (predictions are available here) ;
- When a satellite passes in front of or behind the apparent disk of the planet, there is occultation of the satellite or of a small part of the disk of the planet (occultation called "transit");
- When the shadow of a satellite is projected on the planet, there is a " shadow transit "
- When the satellites pass in the shadow of each other , there is mutual eclipse ;
- When a satellite passes in front of or behind another satellite, there is mutual occultation .
Click here for more details on the phenomena of Jupiter's satellites .

b). Occultations of stars by asteroids  : 

In this case , replace the Sun by a star and the Moon by an asteroid in a solar eclipse. When an asteroid passes in front of a star, so there is an area on Earth as large as the asteroid itself where there is "Total Eclipse" and where the star will disappear in the eyes of observers. You obviously need a telescope to observe this phenomenon , but it is spectacular since star, usually faint, disappears for a moment behind the asteroid, generally much less bright than the star. The large number of stars and asteroids in the sky will lead to a significant number of such phenomena, about one a week being observable. Click here for an example of a star occultation by an asteroid .

c). Occultations of stars by planets  : 

This is the same case as above, but the asteroid must be replaced by a planet. In the case of a planet with an atmosphere , the passage of light through the atmosphere makes possible to know the composition. It should be a bright star to have a good contrast with the planet.
Click here for an example of a star occultation by the planet Uranus.

d). The transits of Mercury and Venus across the Sun  : 

The proximity of Mercury and Venus from the Sun makes that, for a terrestrial observer, Mercury and Venus will move from time to time in front of the solar disk. This is an eclipse but it is obvious that these planets do not have a sufficient size to reduce the sunlight.Tthese phenomena are called "passages" or "transits" .
Click here for more details on the passages to the Sun.

e). The stars called "eclipsing binary stars "

In this case, we leave the solar system, but this is the same type, some stars have a fast change in brightness explained by the presence of a very close second star (and not discernible in a telescope). The second star regularly passes in front of or behind the first star, the couple being called "binary star". At that time their lights are no more added since the hidden star is no morevisible. We have a succession of occultations. The observed variation of the brightness shows the occultation as seen below.

Credit : All right reserved

III - Usefulness of the observation of occultations and eclipses

The observation of occultations provides various information. - Information on the precise position of the body. The turbulence of the atmosphere and the limited resolving power of a telescope make not possible observing positions beyond a certain precision . Very close bodies are not detectable separately and only a large point is observable. If both bodies pass one before the other, the drop of light shows exactly when they are in contact and thus their precise relative position .
- Information on the size and shape of the body: the speed of a moving body in the solar system is better known than its position. Thus, measuring the duration of the occultation gives the size of the occulting body. This is the only way to know the size of asteroids: no telescope can distinguish their diameter for the reasons mentioned above.
- Information on the nature of the surface of the occulted body. If the brightness of the occulted body is not uniform over its entire surface , the observed light curve present small variations. These variations due to the nature of the surface are not always directly observable for the same reasons .
The position, the size and the surface of the Moon are now well known thanks to spacecraft observations or laser ranging. Solar eclipses, as also those of the Moon, are no longer used for this purpose. The same is true for the transits of Mercury and Venus across the Sun.
Contrarily, occultations of stars by asteroids or planets and phenomena of Jupiter's satellites are still one of the best sources of accurate information about their position, shape and nature of the surface (see pages specific) .

Credit : J.E. Arlot/IMCCE