How It Works
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Mercury’s orbit

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Mercury_Globe-MESSENGER_mosaic_centered_at_0degN-0degE

Of all the Solar System’s planets, Mercury has the most eccentric orbit. Moving in an ellipse its distance from the Sun varies from 46 million kilometres (28.6 million miles) to 70 million kilometres (43.5 million miles) across its orbital cycle.

Not only does Mercury travel in an ellipse, but the planet’s closest approach to the Sun is not always in the same place. Mercury’s orbit drifts, with each ellipse around the Sun seeing it move along slightly, tracing a shape similar to the petals of a daisy (see picture).

This drifting is partially caused by the gravitational pull of local bodies; the Sun, of course, has the most influence, but other planets and asteroid belts also have an effect, dictating its path.

However only part of the drift is accounted for by other objects’ gravity near Mercury. The orbit can only be fully explained by Einstein’s general theory of relativity. The Sun’s gravitational field distorts the fabric of space and time, forming a curvature. This distorted space geometry also affects the route Mercury takes around the Sun.