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The Sky Tonight: Mercury at Greatest Western Elongation

mercury at greatest western elongation

The planet Mercury reaches greatest western elongation of 27 degrees from the Sun. This is the best time to view Mercury since it will be at its highest point above the horizon in the morning sky. Look for the planet low in the eastern sky just before sunrise.

In astronomy, a planet’s elongation is the angular separation between the Sun and the planet, with Earth as the reference point. The greatest elongation of a given inferior planet occurs when this planet’s position, in its orbital path around the Sun, is at tangent to the observer on Earth. Since the planets able to elongate are well within the area of Earth’s orbit around the Sun, observation of such a phenomenon should not pose that much a challenge (compared to deep-sky objects). When a planet is at its greatest elongation, it is farthest from the Sun as viewed from Earth, so its apparition is also best at that point.

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.

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