The Egyptians’ failure to account for the extra fraction of a day, however, caused their calendar to drift gradually into error.
If the position of the Earth in its orbit around the Sun is reckoned with respect to the Equinox, the point at which the orbit crosses the celestial equator, then its dates accurately indicate the seasons, that is, they are synchronized with the declination of the Sun.
The moment of the vernal equinox in the northern hemisphere is determined using the location of Tehran "by means of astronomical computations from reliable sources".
[1] If the position of the Earth (see above) is reckoned with respect to the fixed stars, then the dates indicate the zodiacal constellation near which the Sun can be found.
They are calculated on the basis of the apparent motion of the Sun through the twelve zodiacal signs rather than the tropical movement of the Earth.