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