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Mercury’s orbit is somewhat eccentric: the eccentricity is 0.10. (The standard value assumed by modern ephemerides is 0.097657336.) The orbital eccentricity causes the inclination of the planet’s orbit relative to Earth to oscillate around its mean value of 85.2°; this gives rise to the slight oscillating effects that dim the planet to varying degrees across its orbit relative to the Sun when it is seen from Earth. The aurorae in the southern hemisphere exhibit both a slow drift towards the south and an oscillation with the same period, which causes Earthshine to appear from the poles. The perihelion distance of Mercury is 0.38 astronomical units (5.29×1015 m)—the closest planet in the Solar System—but because its orbital period of 83 days is so much shorter than that of Earth at 365.24 days, Mercury is significantly closer to Earth than any other planet.
The deepest part of Mercury’s Hill sphere is 71.5 Earth radii at perihelion; at aphelion it is 75.8 Earth radii. The planet’s eccentricity means that its orbital distance varies by up to 8.5 percent.
When viewed from Earth, Mercury as seen in transit is backlit by the Sun as it is slowly occulted by the dark side of the Moon. From Earth, both the Sun and the Moon appear distinctly brighter than Mercury, and Mercury appears as a crescent, as if seen from an angle. On average the leading limb of Mercury is about 0.8 of the diameter of the Sun; the bottom limb is 0.7 times the Sun’s diameter. In modern ephemerides, Mercury normally has a phase of about 0.38° wide, and a duration lasting up to about 14.8 hours. The brightness of the Sun varies throughout the day because the Sun is visible from over the entire Earth-Sun distance for periods of time. (NOTE: The following “Sun distance” and “Mercury distance” figures are given in AU. Actual figures are in the hundreds of millions of kilometres.)