Planetary Phenomena

The planets undergo changes in brightness as well as position and angular speed across the sky. The configurations of the planets, however, provide the phenomena in which ancient astronomers, especially in Mesopotamia, seemed most interested. The conjunctions of one planet with another, or the location of a planet in a particular zodiacal constellation, were interpreted in astrological ways as propitious or calamitous signs for kingdoms and sometimes for individuals (see §15.6).

Some configurations are of particular astronomical interest. These include the transits of Venus and Mercury across the face of the Sun. A transit requires an interior planet to be near a node of its orbit at the time of inferior conjunction, in other words, nearly in line with the Earth and the Sun. The transit limits—analogous to ecliptic limits—are only a few degrees. Those for Venus are narrower than are those for Mercury, and inferior conjunctions are fewer also, so that transits of Venus are more rare than are transits of Mercury, as Table 5.6 reveals. This table, based on Meeus (1989), is a continuous record of transit events but over a much different range of dates for each planet. Notice that Venus transits occur in pairs separated by eight years, and the pairs recur in alternate intervals of 121.5 years and 105.5 years, but the cyclicity is complicated and the intervals vary. The Venus transit seasons—roughly analogous to solar/lunar eclipse seasons—are currently in June and December. The Mercury transit seasons currently occur in May and Novem ber. Most seasons, however, are without a transit. There are 13 to 14 Mercury transits per century. The highly eccentric orbit of Mercury (cf. §2) makes the limits and transit conditions different for the two node crossings;29 the ratio of the number of November to May events is 9 : 4. It is possible for such phenomena to have been observed in ancient times with optical aids in, for example, Mesoamerica.

In more recent times, the transits of Venus were studied in the 18th century to determine the astronomical unit. By observing the event from widely separated regions of the Earth, the times of contact could be compared. From them, the corresponding angle between the sites subtended at Venus could be computed. These angles, plus the knowledge of the distance between the sites permit the calculation of the distance from Venus in the same units, viz., miles or kilometers. Because the distance of Venus at the instant of transit is known in astronomical units (from orbital theory), the A.U. is determined in kilometers. Fernie (1976) describes the tribulations that were faced by astronomers attempting to carry out those observations.

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