Why does the sun rise exactly East and West on the equinox?
Quick Answer: On the equinox, the sun rises exactly in the East and sets exactly in the West because the Earth's tilt is perpendicular to the Sun’s rays. This means the Sun crosses the celestial equator, making its path align perfectly with the Earth's equatorial plane.
The Earth is tilted about 23.5 degrees relative to its orbit around the Sun. Most days, this tilt causes the Sun to rise north or south of East. But on the equinox, the tilt is sideways to the Sun, so sunlight hits the equator directly.
Because of this, the Sun appears to travel along the celestial equator, rising exactly where the horizon points due East and setting exactly due West. This creates nearly equal day and night lengths all over the planet on that day.