Mercury is the closest planet to the sun, and on average, it is 57 million kilometers (35 million miles) away. That's less than 40 percent of the distance from Earth to the sun. Mercury's orbit is elliptical, though, and its distance from the sun varies by 24 million kilometers (15 million miles).

Unlike Earth, which circles the sun in a nearly circular orbit, Mercury orbits in an ellipse. The eccentricity of Mercury's orbit, which is a measure of how much it differs from a circular orbit, is 0.2056. That's more than 10 times greater than the eccentricity of Earth's orbit, which is 0.0167. In fact, it's the most eccentric orbit of any of the eight planets in the solar system.

Unlike a circle, an ellipse doesn't have a center; instead, it has two foci, and in the case of Mercury's orbit, the sun occupies one of them. When Mercury is closest to the sun, it is only 46 million kilometers (29 million miles) distant, but when the planet is rounding the opposite focus of its orbit, it is 70 million kilometers (43 million miles) away from the sun. Because Mercury's poles aren't tilted relative to its orbit, temperature differences caused by its changing distance to the sun are the closest the planet has to experiencing seasons.