Mercury Reaches Aphelion

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The little planet Mercury reaches aphelion, it’s furthest point from the Sun, on March 6, 2015 at 20:10 UT.

Mercury is a planet of extremes. It’s the smallest of the major planets, with a diameter of just under 4,900 km, smaller than Jupiter’s moon Ganymede.

It’s the fastest moving of the eight major planets and the closest to the Sun.

Mercury also has the most elliptical orbit of the eight major planets. At aphelion, it’s about 0.47 astronomical units (AU) from the Sun. At perihelion, it’s just 0.31 AU from the Sun. This means it receives more than twice as much sunlight at perihelion that at aphelion.

While Mercury is farthest from the Sun this week, it remains visible from Earth– barely– low in the eastern sky before sunrise in the constellation Capricornus. The planet appears to move back toward the Sun later this month and becomes impossible to see visually or in a telescope. It reaches superior conjunction on the far side of the Sun relative to Earth on April 10.

But it’s not gone for long. The speedy planet then quickly returns to the evening sky in April and passes just 1.3 degrees north of Mars low over the western horizon after sunset on April 22, 2015.

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