The Equinox: The Sun on Sunday

This year the September equinox ("fall equinox" in the Northern Hemisphere) is at 5:44am Sunday, 22 September. 

Among other things, this means that today, everywhere on the Earth, the Sun is above the horizon for almost exactly 12 hours, sunrise happens almost exactly due East (90 degrees compass course), and sunset happens almost exactly due West (270 degrees compass course). The local times of sunrise and sunset vary from place to place. 

The only exceptions are at the North pole (where the Sun is just setting today, and will stay below the horizon for the next 6 months), and at the South pole (where the Sun is just rising today, and will stay above the horizon for the next    6 months).

The underlying reason for all these phenomena is that the Earth spins (daily) about an axis that's tilted relative to the Earth's orbit around the Sun.

The Spring and Fall Equinoxes are the times each year when the Earth's position in its (yearly) orbit around the Sun places that tilt precisely "sideways" relative to the Sun.

There's a more detailed explanation, with a nice animation, at

https://en.wikipedia.org/wiki/Axial_tilt#Earth

Jonathan T.

(TI’s local astrophysicist)