Transit (astronomy)

A solar transit of the Moon captured during calibration of the STEREO B spacecraft's ultraviolet imaging. The Moon appears much smaller than it does when seen from Earth, because the spacecraft–Moon separation was several times greater than the Earth–Moon distance.

In astronomy, a transit (or astronomical transit) is a phenomenon when a celestial body passes directly between a larger body and the observer. As viewed from a particular vantage point, the transiting body appears to move across the face of the larger body, covering a small portion of it.

Definition[edit]

A simulation of Io transiting Jupiter as seen from the Earth in February 2009. Io's shadow is seen on the surface of Jupiter, leading Io slightly due to the sun and Earth not being in the same line.

The word "transit" refers to cases where the nearer object appears considerably smaller than the more distant object. 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 Venus (see transit of Mercury and transit of Venus). However, as seen from outer planets such as Mars, the Earth itself transits the Sun on occasion.

Io transits across Jupiter as seen by Cassini spacecraft

Dione transits Titan, as seen by the Cassini probe; in the background, little Prometheus is occulted by the rings of Saturn

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. The one closest to the present occurred on 27 June 1586, when Mercury transited the Sun as seen from Venus at the same time as a transit of Mercury from Saturn and a transit of Venus from Saturn.^{[citation needed]}

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.

Mutual planetary transits[edit]

Main article: Planetary transits and occultations

In rare cases, one planet can pass in front of another. If the nearer planet appears smaller than the more distant one, the event is called a mutual planetary transit.

Contacts[edit]

Mercury transiting the Sun as viewed by the Curiosity rover on Mars(June 3, 2014)^[1]

During a transit there are four "contacts", when the circumference of the small circle (small body disk) touches the circumference of the large circle (large body disk) at a single point. Historically, measuring the precise time of each point of contact was one of the most accurate ways to determine the positions of astronomical bodies. The contacts happen in the following order:

Artistic animation of a planet transitting its star

• First contact: the smaller body is entirely outside the larger body, moving inward ("exterior ingress")
• Second contact: the smaller body is entirely inside the larger body, moving further inward ("interior ingress")
• Third contact: the smaller body is entirely inside the larger body, moving outward ("interior egress")
• Fourth contact: the smaller body is entirely outside the larger body, moving outward ("exterior egress")^[2]

A fifth named point is that of greatest transit, when the apparent centers of the two bodies are nearest to each other, halfway through the transit.^[2]

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