The Autumnal Equinox

We live on the surface of a giant spinning ball. From our perspective, everything else appears to move around us as we spin. The sun, the moon, the planets, the stars. As we spin to face the sun, it appears to rise the sky. As we spin to face away is appears to set. It is no coincidence that we spin around exactly once each day. It is our spinning that determines the length of the day.

But we don't just spin in place. As we spin, we move in a circle around the sun. But the way we spin is tilted compared to the way we move around the sun. And the direction we're tilted doesn't change as we move around the sun, which means that sometimes we're tilted toward the sun, and sometimes we're tilted away from it. When you're tilted away from the sun, the days are shorter and the nights are longer, which makes it colder. The winter solstice is the day when you're tilted exactly away from the sun. When you're tilted toward the sun, the days are longer and the nights are shorter, which makes it hotter. The summer solstice is the day when you're tilted exactly toward the sun. It is no coincidence we move around the sun exactly once each year. It is our moving around the sun that determines the length of the year.

Not to scale. Credit: NOAA

Keep in mind that when the northern hemisphere is tilted toward the sun, the southern hemisphere is tilted away, and vice versa. The summer solstice in the northern hemisphere is the winter solstice in southern hemisphere, and the winter solstice in the northern hemisphere is the summer solstice in the southern hemisphere.

But today, we aren't tilted toward the sun, or away from it. Today, we are tilted perpendicular to the sun. Today is the autumnal equinox, the day when the day is equal to the night.

The equinox is a time of change. It marks the midpoint in the transition from the summer solstice to the winter solstice. Beyond that, it is also an inflection point. After the summer solstice, the days get shorter. At first, only a little bit. One day will be only a few seconds shorter than the day before it. But over time, the change increases, until one day will be minutes shorter than the day before it. The solstice is the time of the fastest change. After the solstice, the days will continue to get shorter, but the speed of the change will slow down again.