The lunar eclipse reckoning algorithm used in the book Risālah az-Zain by Ibn Ya’qūb al-Batāwy uses the Jean Meus Algorithm, but there are some formulas that are different such as calculating the value of K (estimated year of the eclipse), simplification of the formula F (argument for the latitude of the Moon), formula for calculating the value of A (additional correction to get the value of C), correction formula to determine the value of C (time of maximum eclipse), and determination of Pasaran when the eclipse occurs.
This research is a type of library research. The analysis process uses descriptive analysis. The primary data used is the book Risālah az-Zain and direct interviews with the author of the book, namely Ibn Ya’qūb al-Batāwy. Secondary data is in the form of documents related to lunar eclipses. The purpose of this study was to determine the lunar eclipse reckoning algorithm in the book Risālah az-Zain and to determine the accuracy of the reckoning by analyzing the eclipse reckoning data of Jean Meeus in Astronomical Algorithms and NASA.
This research has 2 results: First, the lunar eclipse reckoning algorithm in the Risālah az-Zain book is classified as hisāb haqiqi contemporary, this can be seen from the astronomical data used using the latest data and using the latest algorithms. This lunar eclipse reckoning algorithm is based on Jean Meeus’s Algorithm in the book Astronomical Algorithms and uses Julian day, the new epoch of 2000, and Danjon’s rules.
Second, the lunar eclipse reckoning algorithm has good accuracy because the result of data from the calculation of the lunar eclipse of Risālah az-Zain and Jean Meeus differs slightly with the largest average difference of -0:01:21 in the phase of U1, maximum of the eclipse, and P4. The smallest difference is -0:01:12 in the phase of P1, U1, U2, U4 and P4. The result of data from the calculation of Risālah az-Zain’s lunar eclipse with NASA is slightly different with the largest average difference of -00:05:48 in U1 phase. The smallest difference is 00:00:00 / has no difference in U4 phase.
Lunar eclipse calculation algorithm in book Risālah az-Zain needs some additions, such as: calculations to find out the area and state of a place during a lunar eclipse as done by NASA and the inclusion of a formula for calculating the value of E (eccentricity value of the Earth’s orbit around the Sun), JDE WD (Julian date adjusted to the time of the area), and Z (value to determine the days and markets when the eclipse occurs) in chapter al-Kusūfaini (Two Eclipses).