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A syzygy happens when three celestial bodies are in a straight line(usually the Earth, Moon, and Sun) in a gravitational system. A planet can be in place of the Moon when it is in conjunction or opposition. Eclipses, transits, and occultations occur at times of syzygy. It is also used when talking about a new or full moon.
A syzygy occurs during conjunction. When this happens it means that two celestial bodies appear near one another in the sky. The event is also sometimes known as an appulse. During opposition, a syzygy indicates when two celestial bodies are on opposite sides of the sky when viewed from a particular place.
A transit occurs when a celestial body appears to move across the face of another. It can also occur when a celestial body crosses the meridian due to the Earth’s rotation, about halfway between rising and setting. At one time meridian transits were important for timekeeping purposes. Occultations occur when one object is hidden by another object that passes between it and the observer.
Syzygy has other meanings in mathematics, medicine, art,wine, and several other fields. This article is only meant to convey an understanding of the term in relation to astronomy.
We have written many articles about Syzygy for Universe Today. Here’s an article about the 8 phases of the Moon, and here’s an article about annular eclipse.
We’ve also recorded a series of episodes of Astronomy Cast about every planet in the Solar System. Start here, Episode 49: Mercury.