Mercury: A planet of extremes
by Quamrul Haider
Out of the eight planets in the Solar System, Mercury is a planet of extremes. Being an inner planet closest to the Sun, most of the time Mercury is lost in the Suns glare, thereby making it hard to find in the sky. That is why it is often called an elusive planet. Nevertheless, the planet can be seen very close to the horizon, 30 minutes before or after sunset. We can also see the planet transiting across the face of the Sun 13 to14 times per century in May and November. The next transit will occur on May 9 this year.
The ancient Greek astronomers believed Mercury to be two different stars. They named the evening star Hermes, the winged messenger of the Gods, and the morning star Apollo, a Sun god of great antiquity. By about 350 BC, they acknowledged the morning and evening stars as being one planet.
Mercury is the smallest, has the shortest year and the most eccentric (elongated) orbit of any planet. Its average distance from the Sun is 58 million kilometres. At perihelion, point on its orbit closest to the Sun, it is 46 million kilometers away; at aphelion, point farthest from the Sun, it lies 70 million kilometers out. Sunlight at perihelion is 2-3 times more intense than at aphelion.
The rotation of Mercury is a little strange, as viewed from the Earth. It rotates on its axis - the line joining the North and South Poles, very slowly compared to its orbital period. The planet takes 88 Earth days to complete one full orbit around the Sun, while the time it takes to rotate once about its axis is 59 Earth days. In other words, a full rotation of Mercury takes exactly two-third of a full orbit. The 3:2 ratio of the orbital to rotational periods of Mercury, known as the 3-2 spin-orbit coupling, did not occur by chance. It is a consequence of the tidal effect of the Suns gravity and Mercurys eccentric orbit. The differences in strength and direction of the attractive gravitational forces exerted by one body at different points on another are called tidal forces.
Since Mercurys orbit is highly elliptic, the tidal forces exerted by the Sun at perihelion is three times as strong as at aphelion, Also as Mercury moves around the Sun, its orbital speed increases from 38.9 km per second at aphelion to 59 km per second at perihelion. Owing to the highly elongated orbit and changing orbital speed, the Sun couldnt lock Mercury into a purely synchronous rotation, always presenting the same face to the Sun as our Moon does because of equal rotational and orbital periods. However, like the Moon, Mercury displays phases as it revolves around the Sun as seen from Earths perspective outside Mercurys orbit.
Mercurys odd rotation gives it an extremely long solar day - the time between successive sunrises. It is 176 days which is equal to two Mercurian years! Thus, Mercury experiences 88 days of scorching sunlight with an average equatorial temperature of 425 degrees Celsius, high enough to melt lead. It is followed by 88 days of frigid night with an average temperature of minus 180 C, cold enough to freeze carbon dioxide.
The rotational axis of most of the planets is tilted relative to their orbital plane due to the tidal forces exerted by their satellite(s). Consequently, their North and South Poles are not straight up and down. For Earth, the 23.5 degrees tilt caused by the tidal effect of the Moon is responsible for the seasons we experience.
In the absence of natural satellites, the Sun influences the tilt of Mercurys rotational axis. It is 2.11 degrees. Because of the small tilt, its axis is almost perpendicular to the orbital plane. Hence, Mercury has no seasons and the Sun is always on top of the equator. As viewed from Mercurys poles, the Sun would always be on the horizon, never rising above or dipping below. In addition, the Suns movement across Mercurys sky is not as regular and steady as that across the Earths sky. The small axial tilt makes the Sun wobble up and down slightly, as it travels all the way around the horizon.
The Sun also rises on Mercury, but unlike Earth where the Sun always rises in the east and sets in the west, on Mercury it rises twice in a Mercury-day. Why? A few days prior to reaching the perihelion, when Mercurys orbital speed becomes equal to its rotational speed, the Sun slowly rises in the east, stops about half-way, and moves backward across the sky, from west to east. Once Mercury is at perihelion, its orbital speed exceeds the rotational speed and the Sun resumes its westward motion moving downward in the sky.
The writer is Professor of Physics at Fordham University, New York.
Photos: Google Image
