Astro Bob: Planet seasons and the start of fall

The fall equinox is a significant event in any tipped planet's life. Seasons are caused by a planet's tilted axis. Those which orbit the sun standing nearly straight up and down with little to no axial tilt, like Mercury and Jupiter, don't experience seasonal changes. Earth's axis is inclined 23.5 degrees, which tilts the northern hemisphere toward the sun during summer. That shifts the sun's position to high in the sky, with longer daylight hours and more intense heating as consequences.

In winter, when the planet has moved to the opposite end of its orbit, the northern hemisphere faces away from the sun. The sun follows a low path in the sky, which shortens the number of daylight hours and ushers in colder weather.

The fall and spring equinoxes mark the mid-points between these day-l