At seventeen he wrote his Vernal Walk in imitation of Thomson.
Briefly, it may be explained here that Easter day is the first Sunday after the full moon following the vernal equinox.
At the close of evening twilight the angle is greatest about three weeks before the vernal equinox.
Before the departure of the Israelites from Egypt their year commenced at the autumnal equinox; but in order to solemnize the memory of their deliverance, the month of Nisan or Abib, in which that event took place, and which falls about the time of the vernal equinox, was afterwards regarded as the beginning of the ecclesiastical or legal year.
At Tyre the year was counted from the 19th of our October, at Gaza from the 28th of the same month, and at Damascus from the vernal equinox.