Obtaining the longitude of the moon through mnemonics – An example from a medieval Kerala text, Dṛkkaraṇa

Abstract

In modern astronomy, the expression for the longitude of the Moon has a large number of terms to include the effects of the Sun, planets etc. apart from the Earth. If the desired accuracy is an arc minute or so, it is sufficient to include only about 5 to 6 terms. The Moon-mnemonics (Candravākyas) are based on a simple epicycle model for the Moon (with only one correction to the mean longitude to obtain the true longitude). Moon’s longitude can be computed in a few simple steps using these vākyas This paper presents the details of the actual method for computing Moon’s longitude using vākyas in one of the texts, namely, Dṛkkaraṇa from medieval Kerala. It highlights the simplicity and ingenuity of the method in taking various corrections into account.