Ready now: The Earth moves. Everyone knows that. Denying that is just denying reality. The Sun also moves. As does our entire Solar System, as does the entire Milky Way galaxy.
And Zahavasdad isn’t wrong. If we still had Kiddush Hachodesh Al Pi R’iyah and had Beis Din be M’chasheiv the Ibbur we would have to correct our system by a few days. Because while 365.25 days is very close to the amount of time it takes the Earth to circle the Sun (hmm, I can see a problem developing here), it is not precise. And therefore a Shanah, as you call it, will move farther away from the actual seasons by a negligible amount each year. But after 2000 years, those amounts add up and we end up off by a few days. The T’kufah is off by a tiny bit more each year. The Gregorian calendar adjusts for this better than our calendar (not that that’s a problem with our calendar, C”V; it’s just a reality that this long Galus has brought us). Thus, the equinoxes and solstices are not determined by the secular calendar. Rather, that calendar took those days and set them at points throughout the year (March 21, June 21, September 21, December 21) and bases its calendar around those days, and not the other way around as you seem to have assumed.