“The day becomes more solemn and serene
When noon is past: there is a harmony
In autumn, and a lustre in its sky,
Which thro’ the summer is not heard or seen,
As if it could not be, as if it had not been!”
—Percy Bysshe Shelley, Hymn to Intellectual Beauty, 1816/7
About three weeks after the meteorological start of fall, today sees the astronomical beginning as well. While last year it felt like we plunged straight from August into November, this year the change is more gradual. There were a few colder days but also days that were still nice and warm.
We owe the seasons to Earth’s axial tilt. As Earth does not orbit the Sun in an upright but in a slightly tilted position, one of Earth’s poles is sometimes tilted more toward the Sun during the orbit and sometimes tilted away. When a pole is tilted toward the Sun, this part of Earth gets more sunlight so that daylight hours are longer and average temperatures higher. In short: it is summer. When a pole is tilted away from the Sun, this part of Earth accordingly gets less sunlight, so that there are fewer hours of daylight and average temperatures are lower; it is winter.
Twice a year there is a time when day and night have the same length as the Sun equally illuminates both the northern and the southern hemisphere. The spring or vernal equinox occurs in March, the fall or autumnal equinox in September. Equi means “equal” in Latin and nox “night”. This year’s autumnal equinox takes place on Sunday, September 23 at 01:54 UTC. Because of time zone differences this is some time today (Saturday, September 22) in the Americas, depending on where you are.