Syzygy

In astronomy, objects can always cross in front of another object, if the alignments are right. They are usually referred to one of three terms: an eclipse, a transit, or an occultation. These three terms are part of a broader definition called a syzygy*.


   *Syzygy in astronomy is when three bodies are in a line.





Let's define the three of them.





Previously, I posted about eclipses of the Moon. These occur when the Moon, Sun, and the Earth are lined up in such a way that the Moon eclipses the Sun (a total solar eclipse), or the Moon goes through the Earth's shadow (a lunar eclipse). An eclipse can also happen in multiple star systems when one companion star passes in front of the other. What occurs is that the obscured body is either completed blocked out temporarily as it passes through the shadow of the eclipsing body (lunar eclipse) or the eclipsing body passes between the observer and the eclipsed object (total solar eclipse).





A transit is when a smaller body passes in front of a larger body, mostly a planet crossing in front of a star, but can also occur when a moon crosses in front of a planet, partially blocking out the Sun. Transits of extra-solar planets can be used to help astronomers find the planet and determine its size based on the light-curve of the star. Exoplanet transits are discussed more here. Transits occur in the inner solar system when Mercury and Venus cross in front of the Sun as seen from Earth. On my post about opposition and conjunction, what configuration(s) are Mercury and Venus in when they transit the Sun? Comment below if you know the answer. Also, the moons of Jupiter and Saturn can also transit across the face of their parent planets.

Released with Image The Galilean satellite Io floats above the cloudtops of Jupiter in this image captured on the dawn of the new millennium, January 1, 2001 10:00 UTC (spacecraft time), two days after Cassini's closest approach. The image is deceiving: there are 350,000 kilometers -- roughly 2.5 Jupiters -- between Io and Jupiter's clouds. Io is the size of our Moon, and Jupiter is very big.

Via NASA/JPL/University of Arizona

 

The last example of syzygy in astronomy is called occultation. In this case, the body that crosses between the observer and the more distant object appears much larger. These occur when the Moon, the Sun, or a planet pass in front of distant star, when the Moon passes in front of a planet, or when the satellite of a planet passes in front of an apparently smaller satellite.

Dione occulting Rhea (two moons of Saturn)

via NASA/Cassini-Huygens Mission

 

In picture form, this is what the three types of syzygy look like: