For months we’ve been able to enjoy bright Venus in the western sky after sunset. Venus is now out of view as It has moved in between us and the Sun, but we are graced with the other interior planet this week, Mercury.
Mercury won’t stick around for as long as Venus though, it will move between the Earth and the sun on June 30. Mercury’s stay is so brief because of its short orbital period. It takes the Earth 365 days to go around the sun, while it takes Mercury only 88 days.
Mercury is a lot closer to the sun than we are, orbiting at an average distance of 36 million miles, compared to the Earth-sun distance of 93 million miles.
It’s also a lot faster. Mercury moves around the sun at about 106,000 miles per hour, which is 40,000 mph faster than the Earth.
The orbit of Mercury is also historically significant and served as an early proof of a revolutionary theory, and as an illustration of the self-correcting nature of science.
To understand the importance of Mercury, we must first look at the discovery of Neptune in 1846. Neptune was first hypothesized after irregularities in the orbit of Uranus were observed.
Scientists concluded from these perturbations that another massive object had to be causing this disturbance. A couple of people independently came to this conclusion, including the French astronomer and mathematician Urbain Le Verrier.
Le Verrier not only predicted the existence of Neptune, but he calculated its position to within 1 degree of where it was discovered! He had no computers, no calculators, not even a good telescope, but by relying on known physics and mathematics, he achieved one of the greatest triumphs of 19th century science.
Le Verrier also observed that the orbit of Mercury did not agree with how we understood gravity to work at the time. Mercury’s orbit was seen to precess around the sun.
All planets orbit in ellipsis, which you can think of as elongated circles or ovals. Each planet has a point in its orbit where it is closest to the sun, called perihelion. Mercury’s perihelion moves from orbit to orbit in a way that cannot be explained by the gravitational theories of Isaac Newton.
Le Verrier proposed the existence of another planet in close orbit around the sun to explain this precession. This theoretical planet was called Vulcan.
Observation did not support this idea, and Vulcan was never found. Mercury’s motion was instead explained by Albert Einstein and his "General Theory of Relativity" in 1915.
The Morning Sky
Jupiter now rises before midnight, with Saturn close behind. These two gas giants are well placed for viewing high in the south before dawn. A pair of binoculars may reveal several of Jupiter’s four Galilean moons. Mars is low in the east before sunrise. To the left of Mars sits distant Neptune, visible only with telescopic aid.
The Evening Sky
Mercury is at its farthest point from the sun on June 4. Look for it low in the western sky after sunset. Spot it while you can. Mercury sinks quickly and will be lost in the glare of the sun by the Summer Solstice on the June 20. The moon is full on the June 5.
Dan Price is a NASA/JPL Solar System Ambassador and informal educator. He leads the Night Sky Tours at Josephine Sculpture Park. Have a question about astronomy or space science? Send an email to firstname.lastname@example.org and it might be featured in a future column.