How do they determine when the first day of summer, spring, winter or fall?

Question

How do they determine when the first day of summer, spring, winter or fall?

Answer 1

The first day of a new season is determined by the position of the Sun relative to the latitudes (circles running east-west) around the earth. There is the winter and summer solstice, and the spring(vernal) and fall(autumnal) equinox.

Spring in the northern hemisphere begins when the Sun moves north of the Equator. That is usually on the 21st of March(but could be the 20th or 22nd). Day and night are both equal.

When the Sun moves north of 22.5 degrees north of the Equator(Tropic of Cancer?) around June 21, then that is the official beginning of Summer, the longest day of the year.

The Sun then starts seeming to move south again, and by around Sept. 21, dips south of the Equator again. That is the first day of fall. Day and night both equal.

When the Sun hits 22.5 degrees south of the Equator(Tropic of Capricorn?) that is the beginning of Winter, the shortest day of the year, and the Sun starts moving north again. Astronomers can time all this down to the very second it occurs, so Spring might officially begin at 10:15 am on March 21st.

The Sun doesn't actually move up and down. It's position on Earth makes it appear to, but in reality, it is because the Earth is on a tilt of 22.5 degrees, and the Sun seems to move up and down as the Earth circles the Sun.

Google Summer or Winter Solstice, or Vernal or Autumnal Equinox, and that hopefully will give a good explanation with pictures to help understand better how it all works.