19. More About Line 8
A Babylonian month has either 29 or 30 days. Line 8 records that the 1st day of Month 2 was on the 30th of the preceding month, which means Month 1 had only 29 days.
Around the beginning of a month, Babylonian astronomers watched at sunset to catch a glimpse of the moon, which signaled the first day of the month. If bad weather got in the way, they had methods for accurately predicting the first visibility of the lunar crescent. A full understanding of their methods eludes historians of astronomy.
The astronomy of Line 8, sunset on May 22, -56720. Lagtime
The data recorded about the first day of the month includes lagtime, a measure of the time between sunset and moonset. Diary No. -567 Line 12 notes, it (the crescent) was thick; sunset to moonset: 20°.
The 20° lunar lagtime corresponds to 80 minutes (1° is equivalent to 4 minutes, 15° to an hour, 360° to a day.) Thus, according to the Diaries, the moon set 80 minutes after the sun disappeared over the horizon.
The astronomy of Line 8, sunset on June 20, -56721. Characteristic Phenomena: Superior Planet
In addition to simply logging the location of a planet, Babylonian astronomers watched for and recorded distinctive patterns of the planet's motion relative to the fixed stars. The events that interested the Babylonians are today called characteristic phenomena.
For superior planets Saturn, Jupiter and Mars, the characteristic phenomena are: 1) First appearance in the East 2) First stationary point 3) Opposition 4) Second stationary point 5) Last appearance in the West.
The characteristic phenomena of the superior planet Jupiter (5 pages)22. Characteristic Phenomena: Inferior Planet
For inferior planets Venus and Mercury, the characteristic phenomena are: 1) First appearance in the East 2) Last Appearance in the East 3) First Appearance in the West 4) Last appearance in the West.
The characteristic phenomena of the inferior planet Mercury (3 pages)