Noun
the time for the earth to make one revolution around the sun, measured between two vernal equinoxes
Source: WordNetBecause the number of days in the tropical year is not a whole number, a solar calendar must have a different number of days in different years. Source: Internet
So a "tropical year" measured from one passage of the northward ("vernal") equinox to the next, differs from the one measured between passages of the southward ("autumnal") equinox. Source: Internet
Given a tropical year of main days from Simon et al. Source: Internet
He also compared the lengths of the tropical year (the time it takes the Sun to return to an equinox) and the sidereal year (the time it takes the Sun to return to a fixed star), and found a slight discrepancy. Source: Internet
If managed correctly this system could have allowed the Roman year to stay roughly aligned to a tropical year. Source: Internet
It is not clear if this would be a value for the sidereal year (actual value at his time (modern estimate) about 365.2565 days), but the difference with Hipparchus's value for the tropical year is consistent with his rate of precession (see below). Source: Internet