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Transiting, more commonly called an astronomical transit, has three meanings: 1. the event that occurs when one celestial body appears to move across the face of another celestial body, as seen from a particular vantage point, 2. occurs when a celestial body crosses the meridian due to the Earth’s rotation, about halfway between rising and setting, 3. is used for the passage of a star through the eyepiece of a telescope.
Transiting often refers to instances where the nearer object appears to be considerably smaller than the more distant object. It can refer to an inferior planet(Mercury and Venus) coming between an observer and the Sun. It can also refer to an instance where a natural satellite is between an observer and it main body(a moon in front of Jupiter). Whichever the case, transiting requires three celestial bodies to be in a single line.
During a transit there are four “contacts”, when the circumference of the small circle touches the circumference of the large circle at a single point. The contacts happen in the following order: first contact is when the smaller body is entirely outside the larger body and moving inward, second contact is when the smaller body is entirely inside the larger body and still moving inward, third contact happens when the smaller body is entirely inside the larger body and begins to move outward, fourth contact takes place when the smaller body is entirely outside the larger body and continues moving outward.
When transiting is referring to the passage of a star across a telescope’s eyepiece, the measurements can be done in several ways. Visually with an accuracy of 0,1″ to 2″ (depending on the instrument); timing with digital clocks about 0,05-0,2 seconds, by CCD, semi-automatic instruments, and scanning. Scanning is usually considered the most precise method.
We have written many articles about astronomical transiting for Universe Today. Here’s an article about the transit of Mercury, and here’s an article about the transit of Venus.
We’ve also recorded related episodes of Astronomy Cast about the Eclipse. Listen here, Episode 160: Eclipses.