With this term, astronomers indicate the passage of a celestial body through the meridian, that is, through that maximum arc that ideally joins the North cardinal point with the South through the zenith.
The measures of the transit of the stars are important to solve problems of Astrometry and time measurement.
The word transit also indicates the passage of the inner planets to Earth's orbit (Mercury and Venus) over the Sun's disk.
In 1639 a transit of Venus was registered for the first time. In 1679 the English astronomer Edmund Halley indicated that these transits could be used to determine the distance to the Sun. Normally, for Venus two transits occur eight years apart and after an interval of 105 to 122 years another two take place every eight years. For Mercury there are about 13 transits in a century.
The moment at which an object crosses the local meridian is called transit and is observed with the transit instrument.
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