Have you had that feeling recently that the season has not so much shifted to autumn, as snapped?
There is a time each year when we get this feeling and its suddenness not purely psychological, it is because we witness the most dramatic changes in the Earth-Sun relationship at two times in the year: spring and autumn.
Some of the things that we tend to assume change gradually, actually don’t at all. On this blog I have mentioned that the bearing of sunrise and sunset change most dramatically at the equinoxes, in March and September, and briefly stand still at the solstices.
There are other things that change with varying speed over the course of the year and they follow a similar pattern. The length of day hovers at the same length at each solstice, but changes rapidly in spring and autumn. For example, in London (latitude is important), the length of the day changes by only 2 minutes over the course of 10 days in late June. However, in late September the day shortens by 40 minutes in just 10 days.
The plants are very sensitive to this rapid change. The leaves on the trees begin to fall as the days shorten, or to be more precise, when the length of night increases. Since these are not steady, gradual changes, it is that much easier for nature to react at the right time. Autumn arrives with a loud rustle, if not a bang.