By Chris Deziel
Updated Mar 24, 2022
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Mercury, the innermost planet, circles the Sun at an average distance of 57 million km (35 million mi). This is less than 40 % of the distance between Earth and the Sun. Because Mercury’s orbit is highly elliptical, the planet’s separation from the Sun fluctuates by 24 million km (15 million mi).
Unlike Earth’s nearly circular path, Mercury follows an ellipse with an eccentricity of 0.2056—over ten times the 0.0167 eccentricity of our home planet. In fact, Mercury’s orbit is the most eccentric of all eight planets in the Solar System.
Mercury’s closest approach to the Sun, called perihelion, brings it to 46 million km (29 million mi). At the farthest point, aphelion, the distance extends to 70 million km (43 million mi). These extremes occur because the Sun sits at one of the two foci of the elliptical path.
With negligible axial tilt, Mercury has virtually no seasons. The planet’s temperature swings are driven almost entirely by the changing solar distance, making its climate one of the most extreme in the Solar System.
