Tiny Mercury is an airless rocky world not dissimilar in appearance to our own Moon. At 4,878 kilometers in diameter, it is 40% larger than our nearest neighbour but still smaller than Jupiter's largest Moon, Ganymede, and Saturn's largest Moon, Titan. Jupiter's Moon Callisto, at 4,806 kilometers is its nearest match.
Mercury orbits the Sun every 88 days, but rotates on its axis every 58.6 days, a bizarre sidereal day, two-thirds of the year in duration. It overtakes the Earth (on the inside track so to speak) every 116 days, at which time it can come within 80 million kilometers of the Earth and appear 12.9 arc-seconds in diameter. Of course, when this happens it is invariably only visible as a hair-thin crescent too close to the Sun for observation. Thirteen or fourteen times per century, as it nears the Earth, it actually crosses the face of the Sun (a transit), as occurred on May 7, 2003. The best time to observe Mercury is when it is at its greatest angular distance, or elongation, from the Sun. At such times the phase will be close to 50% (so it will look like a half Moon), its diameter will be 7 or 8 arc-seconds and it will be between 18 and 27 degrees from the Sun. Yes, it really does stay close to the Sun in the sky. Unfortunately, Mercury's angular elongation is not the only consideration. As well as having a good elongation, Mercury needs to be well above the horizon after sunset/before sunrise to be observed easily. Like the Sun and Moon, Mercury's declination varies throughout the year, because of the Earth's axial tilt of 23V2°. Not surprisingly, it sticks close to the Sun, but to be at a reasonable altitude in twilight at a good elongation it needs to be spring in the northern hemisphere, for an eastern elongation/evening apparition, or autumn in the northern hemisphere, for a western elongation/morning apparition. Why? Well it is all because the Earth's axial tilt will be pointing favorably to raise Mercury's altitude above the twilight horizon. Put another way, the angle the ecliptic makes with the twilight horizon is favorable. For the southern hemisphere the same rule applies, except remember that spring and autumn are six calendar months away from the corresponding northern hemisphere seasons.
Let's have another look at Mercury's bizarre day and how it rotates with respect to the Earth. What features might the webcam user be able to image? As we have seen, Mercury's sidereal (with respect to the stars) rotation period is 58.6 days. Also, it has a highly eccentric orbit around the Sun: 45.9 million kilometers at perihelion and 69.7 million kilometers at aphelion. From the viewpoint of an inhabitant of Mercury (not that anyone would want to live on a world capable of peak daytime temperatures of 430° Celsius) this creates an extraordinary situation. Because the planet moves fastest in its orbit at perihelion and the Mercurian year and sidereal day are of a similar size, the angular rotation speed of the planet in its orbit can exceed the planet's axial rotation. This means that for an observer sweltering in the daytime Mercurian heat, the Sun would actually stop moving from east to west and crawl backwards in the sky for eight days before resuming its westward travel!
As far as the earthbound observer is concerned, the northern and southern hemisphere observers tend to see the same features on the planet over many apparitions. This led the early observers of Mercury to imagine that the planet was not rotating at all with respect to the Sun. However, as recently as 1962, astronomers realized that Mercury's dark side was far too warm to be permanently turned away from the Sun; in other words, it was not a "dark side" at all. A few years later, radar echoes bounced off the planet from the Arecibo radio telescope in Puerto Rico produced the actual rotation period—58.6 days. So it is only 40 years since Mercury's rotation period was established: a testimony to how tricky it has been to record its surface features! So why should we tend to see the same surface features? Well, if an observer on Earth is observing Mercury at a favorable time (e.g., in the evening sky in a northern hemisphere spring), this favorable situation will next occur roughly three Mercury synodic (Mercury catch up time) periods later. Mercury catches Earth up every 116 days, and three synodic periods is just two weeks short of an Earth year. In addition, with the Earth in roughly the same part of its orbit, an Earth-based observer will be looking at a planet that has rotated roughly six times on its axis (6 x 58 days). The two "coincidences" here are that 3 x 116 is not far short of 365 days and 58 is half of 116.
Incidentally, there are various coincidences or, rather, orbital periodicities in the solar system, most of which are caused by gravitational/tidal forces. Eight Earth years equal approximately 13 Venusian years and two Uranian years equal approximately one Neptunian year. We shall see shortly that the moons of Jupiter are similarly linked.
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