Hermograph Press presents...

The Planet Mercury

The 88-day rotation period seemed quite secure, both observationally and theoretically. Tidal friction from the sun's pull caused Mercury to have one face eternally sunward, a situation similar to the Earth-Moon system (so it was thought). It, therefore, was a considerable jolt when Pettengill and Dyce (1965), using radar doppler techniques, found that they couuld fit their measures only to a 59 +/- 5 day Mercurian "day." Peale and Gold (1965) proposed immediately that Mercury was locked into a exact 2/3 resonance with the orbital period. Other astronomers quickly proved the strong resonance theoretically (Columbo 1965, Columbo and Shapiro 1966), which immediately created new consequences. The motion of the Sun in the Mercurian sky caused the formation of "hot poles" 180 degrees apart (Soter and Ulrichs (1967)) and reinterpretation of the infrared data was necessary to evaluate light and dark side temperatures (Morrison 1970, Section 7.1). *Our* most important concern would be how much of the old visual observations are still accurate and useful. Indeed the question "Why was the 59-day period not discovered visually?" immediately begs an answer.

The answer lies in several interesting coincidences. A point on Mercury at a noon position will be at the noon position at the same orbital position exactly 3 Mercurian days later, which is also exactly 2 Mercurian years. Now, the synodic (phases) period is 116 days, which is 4/3 the orbital period, twice the Mercurian day, and about one-third of an Earth year. The synodic period means that there are 116 days between similar apparitions but not all of these are favorable in mid-northern latitudes, where the observers were. Favorable apparitions tend to occur at every third apparition.

This situation, for a particular face of Mercury, will exist for six years after which the favorable apparitions will shift by 1 synodic period, for the synodic period is slightly out of phase with the calendar year.

Thus, if an observer examined Mercury for just the favorable apparitions for a few years, he would always see the same features which leads to asynchronous solution!

There is one more coincidence to the puzzle. Maps made by the best observers, computer digitized and placed upon a map based on a 59 day rotation, still show the famous "Figure 5", a set of dark markings that resemble the numeral. The problem is....the "5" is replicated in three places! Though one is by far the most prominent, the three together, each 120 degrees in hermographic longitude apart mean that once more, an observer could be fooled into believing he has seen the same feature!