Elongation

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This diagram shows the elongations (or angle) of the Earth's position from the Sun.
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This diagram shows the elongations (or angle) of the Earth's position from the Sun.

Elongation is an astronomical term that refers to the angle between the Sun and a planet, as viewed from Earth.

When an inferior planet is visible after sunset, it is near its greatest eastern elongation. When an inferior planet is visible before sunrise, it is near its greatest western elongation. The value of the greatest elongation (west or east), for Mercury, is between 18° and 28°; and for Venus between 45° and 47°.

Unfortunately for observers in the Northern hemishere, the greatest elongations of Mercury take place when the ecliptic is near the horizon, so northwards of approximately 40° it is not possible to see Mercury at greatest elongation with the naked eye.

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