Ibn al-Shatir
To calculate the true longitude of the moon, Ibn al-Shatir assigned two variables, η, which represented the moon's mean elongation from the sun, and γ, which represented its mean anomaly. To any pair of these values was a corresponding e, or equation which was added to the mean longitude to calculate the true longitude. Ibn al-Shatir used the same mathematical scheme when finding the true longitudes of the planets, except for the planets the variables became α, the mean longitude measured from apogee (or the mean center) and γ which was the mean anomaly as for the moon. A correcting function c3' was tabulated and added to the mean anomaly γ to determine the true anomaly γ'. As shown in Shatir's model, it was later discovered that Shatir's lunar model had a very similar concept as Copernicus.