Planet and Star Observations

Planetary observations were carried out in Mesopotamia as early as the reign of King Ammisaduqa. Venus observations are recorded in the 63rd tablet of an extensive series entitled Enuma Anu Enlil, a list of omens, which Neugebauer (1969, p. 101) likens to papal bulls of the Middle Ages. A more detailed list of observations of the Moon, planets, and stars from Babylonia from ~700 b.c. (the mulAPIN) is discussed in §7.1. These observations were probably for royal astrological purposes—to study and prognosticate the impact of planetary influences on kings and kingdoms. The quantities that were historically measured were a celestial longitude and latitude. The equivalent of celestial longitude (see §2.3.3) was expressed in degrees of the zodiacal signs; e.g., $ 12° ft indicates that Mars is 12° east of the beginning of the sign of ft, Taurus, or 42° from the first point of Aries, See §7.1.4 for the usage of this notation on the baked brick cuneiform tablets of ancient Babylon. The celestial latitude was measured more directly, with an armillary sphere or similar device (see §3.3).

The rise and set points of stars do not change periodically during the year like the risings of the Sun, nor on other short time scales like those of the Moon or planets. But, precession (see §3.1.6) changes the right ascensions and declinations of the stars, and the changing declinations cause secular changes (actually very long-term periodic changes) in their rise and set azimuths. The purely secular variation of the stars' proper motions change the right ascension and declination and, thus, the rise and set azimuths. The usefulness of pointing to the locations of star positions arises only because the visibility of the stars varies with season. One type of important date marker is the heliacal rising, when an object is visible for the first time in the dawn sky following (solar) conjunction. Others are heliacal setting, marking the last visibility before conjunction; acronychal rising, when an object is seen to rise at sunset; and acronychal setting, when an object is seen to set at dusk for the last time (thereafter it will be in conjunction with the Sun, and, later, rise helia-cally). Some of these phenomena are illustrated in Figure 3.18.

Such phenomena apply to any object in the sky, of course, but early skywatchers may have used a more convenient stellar marker in case of hazy or cloudy conditions or generally when the object is intrinsically difficult to observe. A paranatellon is a bright star undergoing the same phenomenon (literally, to rise or appear beside) as a fainter star or asterism. An example of such use is suggested in §14, in relation to a celestial marker (Vega) for a "dark constellation," the "Dark Cloud Llama."

heliacal/cosmical rising cosmical setting

\ ecliptic (setting at dawr^

ecliptic,-*

.first visibility tcr conjunction)

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