1. Barleycorn (sze): Defined as 1/180 of a cubit (kusz3). Originally taken from weight metrology, where 180 sze = 1 gin2.
2. GAR: This equals 1/60 of a degree.
3. Finger (szu-szi): In the OB period, there are 30 fingers in a cubit, while in the NB period there were 24.
4. Degree (usz): This is a fundamental unit of measurement in astronomy. There are 12 fingers in a degree. One degree also corresponds to 4 minutes rotation time.
5. Cubit (kusz3): This is a fundamental unit in length metrology. In Babylonian astronomy, it is given as either 30 fingers (OB) or 24 (NB).
6. Double hour (danna): This corresponds to 30 degrees, or 12 cubits. It also equals 2 hours, hence the name.

1. Hour: This is half of a double hour, or 15 degrees.
2. Large hour: Twice of a double hour.
3. Day: Equal to 24 hours.

In addition, two intercalary months were used as needed. One was placed after month six, and written kin-2-kam (denoted here VI2), and the other after month twelve, written dirig or dir-sze (denoted XII2).

Years were denoted by whole numbers starting from…

The most commonly recorded observations of the Seleucid Period were the appearances of the new moon and full moon (during which we say the moon and sun are in conjunction and opposition, respectively). These sightings determined the lengths of the months. In addition, several other statistics were measured such as the velocity, longitude, and latitude of the moon at time of syzygy, and the length of daylight on that day. Such data was used to predict solar and lunar eclipses whose occurrence held deep significance in ancient Mesopotamian cultures.

In Neugebauer's system of classification, there are two methods of computing ephemerides, which he names System A and System B.

System A

In this scheme the sun is presumed to move at one of two constant speeds on different arcs of the ecliptic. From 13 degrees Virgo to 27 degrees Pisces the sun is presumed to move one whole zodiac sign (30 degrees) in a month. For the rest of the year it moves slightly slower, namely 28;7,30 degrees per month.

By way of illustration, the contents of ACT 5 will be explained column by column. The labels of the columns follow Neugebauer's notation.

The first column on the left side of the obverse gives the year and the months within, each one of which records a syzygy. Neugebauer calls this Column T. Here the records begin with year 146 of the Seleucid period, which began in 311 BC. Thus the year is 166 BC. The first month is Nisan, or April. Note the addition of an intercalary month at the end of 164 BC, in Rev. 19.

This column records the longitude of the moon at the time of the new moon, measured relative to the ecliptic. These values are calculated sequentially based on the measurement of an initial value. The numbers on the left hand side are the degrees within the zodiac sign on the right.

Neugebauer has drawn up the table so that the two periods of the year with different velocities of the sun are separated by dashed lines. At a new moon the longitude of the moon at syzygy equals the longitude of the sun when it sets on the horizon. Thus the values of column III, which record the progress of the moon through the zodiac, also measure the movement of the sun. For example in obv. 6-12 the difference in successive entries is one whole zodiac sign, i.e. 30 degrees. In the next interval obv. 13-18 the differences of successive entries is 28;7,30 degrees. The change between the two months at the boundaries is an interpolated value between the two velocities.

This column provides the length of daylight on the day of the given observation. In Neugebauer's rendering of the tablet the units are expressed as large-hours (that is, the largest number on the left denotes integral multiples of a large hour, the next to the right represents 1/60's of a large hour, then 1/3600's of a large hour, etc). Thus e.g. in obv. 13 the length of daylight is (3 + 4x1/60 + 32x1/3600 + 30x1/216000)x4 = 12.3016664 hours, or approximately 12 hours 18 minutes and 6 seconds. On the tablet, the quantities are expressed as usz, or degrees.

The entries for this column were calculated based on a trigonometric formula dependent on the latitude of the observer and the part of the ecliptic that the sun is in at that time, i.e. the time of the year. Rather than measuring their latitude directly, however, the Babylonians, used the ratio between the length of the longest day of the year (at the summer solstice) to that of the shortest day (at the winter solstice), which is a parameter giving the same information as latitude. At Babylon the ratio was approximated by 3:2.

For the increments in daylight based on the sun's position λ along the ecliptic, calculations were made starting from 10 degrees into the sign of Aries (or hun is Sumerian) whose beginning point marks the vernal equinox where the daylight equals 12 hours, and from then on in increments of 30 degrees. Within these increments linear interpolation was used.