Are there any observations of the vernal equinox around the Nicaea council or before? For this we have to rely on the oldest texts available which acknowledge the Julian calendar.
Anatolius of Alexandria (dead 283) relates in his
"Paschal Canon" to the Julian calendar and says:
"... with respect to the day of Easter, that attention must be given not only to the course of the moon and the transit of the equinox, but also to the passage (transcensum) of the sun ...
There is, then, in the first year, the new moon of the first month, which is the beginning of every cycle of nineteen years, on the six and twentieth day of the month called by the Egyptians Phamenoth. But, according to the months of the Macedonians, it is on the two-and-twentieth day of Dystrus. And, as the Romans would say, it is on the eleventh day before the Kalends of April. Now the sun is found on the said six-and-twentieth day of Phamenoth, not only as having mounted to the first segment, but as already passing the fourth day in it. And this segment they are accustomed to call the first dodecatemorion (twelfth part), and the equinox, and the beginning of months, and the head of the cycle, and the starting-point of the course of the planets. And the segment before this they call the last of the months, and the twelfth segment, and the last dodecatemorion, and the end of the circuit of the planets. And for this reason, also, we maintain that those who place the first month in it, and who determine the fourteenth day of the Paschal season by it, make no trivial or common blunder."
Alden Mosshammer's (ref.1) and our interpretation of what Anatolius says is that Easter may not be celebrated before the vernal equinox has passed, and that the nominal day for the vernal equinox is March 22. And that at this date the sun already has passed the equinox astronomically with four days,
meaning that the astronomical equinox was on March 19 in the late 3rd century AD.
This interpretation of Anatolius' statements presupposes that somebody living near his time, or he himself, has determined the date for the vernal equinox experimentally. This is not at all impossible, as Anatolius had been a scholar in Alexandria and had expert knowledge in astronomy (ref.1, p.130):
"Eusebius says that Anatolius was an Alexandrian by birth and one of the most eminent scholars of the time. He excelled in mathematics, astronomy, physical science, philosophy, and rhetoric. At the invitation of his fellow citizens, he founded a school of Aristotelian philosophy at Alexandria."
If the vernal equinox was astronomically on March 19 in the late 3rd century AD, it was on March 21 or 22 when the Julian calendar was introduced about 300 years before. Several scientists have noted the brisance of this fact and concluded accordingly that Anatolius must have erred. There is also a latin version of Anatolius' Paschal Canon, mentioning March 25 instead of March 22 for the vernal equinox.
This version is regarded as a 7th century forgery.
If we assume that Anatolius has not erred, the vernal equinox was astronomically on March 19 also at the Nicaea Council in 325, only a few years after Anatolius' death. As the vernal equinox was astronomically on March 11 in 1582, there are only eight days offset between the two observations. This equals 1026 years if we multiply 8 with 128.2 which is the number of years per one day offset (see above).
However, the nominal difference between the date for the Gregorian reform and Nicaea is instead 1257 years (see above). This means that the historical time line between 325 and 1582 counts 231 "phantom" years too much.
Note: The calculation of the size of the calendar error is only a very rough estimation as the offset increases with one day every 128 years. So the close compliance of the 231 phantom years with our 232 postulated years is only a chimera.