Spring equinox

[Klitos]

Happy springtime! The snow has melted at last and it's getting warm enough to be out without a woolly hat!

Spring equinox was yesterday, 20 March. It occurred 89 days after winter solstice, and 93 days before summer solstice. Does anyone know why it's not exactly half-way between them?

[Rick]

The Earth's orbit isn't a perfect circle, but slightly elliptical, so the Earth moves faster when it's closer to the Sun, and therefore it gets from Northern Winter Solstice to Spring Equinox a bit quicker. The longest gap will be between the Summer Solstice and the Autumn Equinox, as that's when the Earth is furthest from the Sun and therefore moving slowest...

https://www.timeanddate.com/astronomy/perihelion-aphelion-solstice.html

[The Thing]

Also the solar year average duration is 365.256363004 mean solar days and the tropical year 20 minutes less so the equinoxes become earlier every year as we use the tropical year for time. I think that's the right way round. Some time in the future the Vernal Equinox will be on 19th March. Now who knows their synodic from their sidereal?

[Klitos]

This is getting confusing. Brittannica says: "The solar year (365 days 5 hours 48 minutes 46 seconds), also called tropical year, or year of the seasons, is the time between two successive occurrences of the vernal equinox."

|https://www.britannica.com/science/year#ref120288

[Rick]

It gets pretty complicated if you start looking over long timescales.

The dates of perihelion and aphelion are getting slightly later each year (on average). The Earth's axis of rotation processes (which is why you need to know the date if you're using Polaris to align your polar axis). The Moon also helps things along, as its orbit doesn't fit nicely and exactly into years...

[Klitos]

Ah, now I get it. The tropical year differs by 20 minutes from the sidereal year - not the solar year, which is the same thing as the tropical year.