Mercury: Location and Orbit

Facts in Brief

Aphelion	69.7 million km (0.31 A.U.)
Perihelion	45.9 million km (0.47 A.U.)
Minimum Distance from Earth	45 million km
Rotational Period (Day)	0.24 Years (58.6 Days)
Orbital Period (Year)	87.9 Days
Orbital Inclination	7.0^o
Eccentricity	0.21
Axial Tilt	0^o 0"

Relative Location

Along with the planet Venus, Mercury is one of only two inferior planets, those with orbits encompassed by Earth's. It is the planet that moves closet to the sun, coming within 46 million kilometers of it, less than a third of the distance between the Earth and the Sun. This accounts for the extreme temperatures on its surface. Mercury is also the third closest planet to the Earth, located roughly midway between it and the Sun.

Orbit

Mercury has an extremely eccentric orbit, 0.21 from the circular. That is, there is a twenty-four million kilometer difference between the closest and farthest points from the sun, respectively its perihelion and aphelion. More, it moves so quickly around the sun that it rotates exactly three times around its axis for every two orbits around the Sun--there are three days in two years! This may sound incredible, but some scientists speculate that in the past the rotation was even faster, even as rapid as 8 hours (as compared to its current period of 58.6 hours). Over one billion years, however, scientists believe that solar tides de-spun the planet, slowing its rotation considerably and incidentally raising the interior temperature by 100 Kelvin.
During the 1880's, Giovanni Schiaparelli sketched a map showing faint surface markings on Mercury. He believed that Mercury must be tidally locked to the Sun, just as the Moon is tidally locked to Earth. Eighty years later, however, in 1962, radio astronomers determined

that the dark side of the planet was too warm to be tidally locked. In 1965, radio astronomers Pettengill and Dyce determined Mercury's period of rotation to be 59ą5 days. In 1971, Goldstein refined the rotation period to be 58.65ą.25 days. Most recently, the Mariner 10 spacecraft determined the orbit to be 58.646ą.005 days.
Though Mercury is not tidally locked to the Sun, its rotational period is tidally coupled to its orbital period. Mercury rotates one and a half times during each orbit. Because of this 3:2 resonance, a day on Mercury (sunrise to sunrise) is 176 Earth days. A strange effect of Mercury's highly elliptical orbit and rotational period is that the Sun appears to rise briefly, set, and rise again before it travels westward across the sky. At sunset, the Sun appears to set, rise briefly, and then set again. A day on Mercury has three sunrises and three sunsets!

Seeing Mercury from Earth

Because Mercury is so near the Sun, it is difficult to observe. One of the major causes for the retardation of studies of this planet (it wasn't until the 1960's that scientists could determine the correct rotational period), this close proximity still frustrates the amateur astronomer. If you wish to locate it, however, there are a few conditions when Mercury is visible, even to the naked eye. Mercury is always within 28 degrees of the Sun. From temperate latitudes it can only be seen in twilight, and then only when it is near elongation. In the northern hemisphere that is from January to April as an evening object, and from July to October as a morning object. In the southern hemisphere these times are reversed. To find Mercury, look about ten degrees above the Sun's place forty-five minutes before sunrise or -set, low in the horizon; a sky diary can give the best times to look. Mercury is white but often looks pink in the evening.
When observed at dawn or dusk, however, Mercury is so low on the horizon that the light must pass through 10 times the amount of Earth's atmosphere that it would were Mercury directly overhead. For this reason it may be advisable to use a small telescope to assist viewing. Only with a telescope can the small planet be seen away from its greatest elongations. Mercury can be seen (even in the day-time) to have a small disc between 5 and 15 arcseconds across. The disc reveals phases, much like the Moon's, with a superior conjunction (when Mercury is at its farthest from the Earth, behind the Sun) and an inferior conjunction (when it is between the Earth and the Sun).