Home > Astronomical transit

The word transit has two meanings in astronomy:

• A transit is the astronomical event that occurs when one celestial body appears to move across the face of another celestial body, as seen by an observer at some particular vantage point.
• A transit occurs when a celestial body crosses the meridian due to the Earth's rotation, about halfway between rising and setting. For instance, the Sun transits the meridian at solar noon. Observation of meridian transits was once very important for timekeeping purposes.

The rest of this article refers to the first kind of transit.

The word "transit" refers to cases where the nearer object appears considerably smaller in apparent size than the more distant object. Cases where the objects are comparable sizes are known as eclipses; cases where the nearer object appears larger and completely hides the more distant object are known as occultations.

One example of a transit involves the motion of a planet between a terrestrial observer and the Sun. This can happen only with inferior planets, namely Mercury and VenusVenus is the second planet from the Sun, named after the Roman goddess Venus. It is a terrestrial planet, very similar in size and bulk composition to Earth; it is sometimes called Earth's "sister planet" as a result of this similarity. Although all plane (see transit of MercuryA transit of Mercury across the Sun takes place when the planet Mercury comes between the Sun and the Earth, and Mercury is seen as a small black dot moving across the face of the Sun. Transits of Mercury with respect to Earth are much more frequent than and transit of VenusThis article is about the astronomical phenomenon. For other meanings, see Transit of Venus (disambiguation). 2004 transit of Venus The_transit_of_venus A transit of Venus across the Sun takes place when the planet Venus passes directly between the Sun an). However, as seen from outer planets such as MarsMars is the fourth planet from the Sun in the solar system, named after the Roman god of war (the counterpart of the Greek Ares), on account of its blood red color as viewed in the night sky. Mars has two small moons, Phobos and Deimos, both small and odd, the Earth itself transits the SunMars Global Surveyor A transit of Earth across the Sun as seen from Mars takes place when the planet Earth passes directly between the Sun and Mars, obscuring a small part of the Sun's disc for an observer on Mars. During a transit, Earth can be seen from on occasion.

thumb IoIo is the innermost of the four Galilean moons of Jupiter. It is named after the Greek mythological figure Io, one of the many lovers of Zeus (who is also known as Jupiter in the Roman mythology). Although the name "Io" was suggested by Simon Marius soon transits across Jupiter as seen by Cassini spacecraft

The term can also be used to describe the motion of a satellite across its parent planet, for instance one of the Galilean satellites ( Io, Europa, Ganymede, Callisto) across Jupiter, as seen from Earth.

A transit requires three bodies to be lined up in a single line. More rare are cases where four bodies are lined up. One such case occurred on March 21 1894 at around 23:00 UTC, when Mercury transited the Sun as seen from Venus, and Mercury and Venus both simultaneously transited the Sun as seen from Saturn (see Transit of Mercury from Saturn and Transit of Venus from Saturn).

In recent years the discovery of extrasolar planets has excited interest in the possibility of detecting their transits across their own stellar primaries. HD 209458b is the first such transiting planet to be discovered.