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.

Today is the First Day of Autumn

Why? Because I say so and I really like autumn. That's why.

There are those who say that solstices and equinoxes mark the beginning of each season. In which case we're still three weeks away from autumn. Others say that solstices and equinoxes mark the midpoint of each season.

Nonsense, I say! Solstices and equinoxes have to do with the alignment of Earth's tilt to the Sun. Seasons have to do with the weather and the temperature. The two are related, but not the same.

If you say solstices mark the beginning of seasons, then a week before Christmas isn't winter yet, which seems kind of ridiculous. If you say equinoxes mark the midpoint, then Valentine's Day is already spring, which is even more ridiculous.

And more importantly, there is no first day of any season. As my brother and I were discussing on Facebook, everything is continuous, and seasons are a prime example of that. It's not like there is one single day where the temperature drops from 30°C to 20°C and all the trees change color. It's a gradual transition, like red turning to orange in a rainbow. Any line of demarcation is going to have to be arbitrary.

And since it has to be arbitrary, it might as well line up with another, well established arbitrary date-point. In this case, the first of the month.

And so, I decree: The first day of autumn is September 1st. By extension, the first day of winter is December 1st, the first day of spring is March 1st and the first day of summer is June 1st. And it lines up much better with the weather that way.