Today is the first day of fall. Here’s why
Summer in the northern hemisphere and winter in the southern hemisphere are both coming to an end. That means the days and nights are becoming roughly equal in length, and the path the Sun traces across the sky is changing.
On a winter day, the Sun is low in the sky, whereas on a summer’s day the Sun lies considerably higher. But on a specific day in the spring or autumn, the Sun will be visible directly above the equator, somewhere in the middle of the two arcs traced by the Sun in the summer and winter.
This is what’s called the equinox, and there are two each year. Around March 20 we have the vernal equinox or March equinox, also known as the spring equinox in the northern hemisphere. Then around the September 22 or 23 is the autumnal or September equinox.
The word “equinox” comes from the Latin words aequus meaning equal and nox meaning night. But day and night are not exactly equal length on the equinox. To understand why, we need to know what causes the equinox in the first place.
Earth’s tilt
The reason we on Earth have equinoxes at all is because the Earth’s axis is tilted. The Earth spins around an imaginary line running through it called its axis. If the axis pointed straight from top to bottom, at a right angle to the direction of Earth’s orbit around the sun, the intensity of the light shining on Earth’s hemispheres would be the same all year round, and we wouldn’t have seasons.
Some planets in our solar system are like this – for example, Venus’ axis points nearly straight from top
to bottom.
