0 the time in the spring when the sun crosses the equator, and when night and day are of equal length
This would mean that, certainly, those collateral-adjective constructions are lexical where the adjective collocates with a single noun only (vernal equinox).
These are, firstly, those where the adjective occurs only with a restricted set of heads - recall the extreme case of vernal equinox.
The time between periastron and vernal equinox is a variable quantity, however, because orbital speed depends on orbital longitude as well as eccentricity.
Since the calendar uses astronomical calculation for determining the vernal equinox, it has no intrinsic error, but this makes it an observation based calendar.
They differ by the precession of the vernal equinox in right ascension relative to the stars.
In one version of this definition, winter begins at the winter solstice and ends at the vernal equinox.
The vernal equinox point is one of the two where the ecliptic intersects the celestial equator.
It is a solar calendar that is designed to maintain synchrony with the vernal equinox tropical year.
