"Autumn seemed to arrive suddenly that year. The morning of the first September was crisp and golden as an apple."Earth's tilt relative to the Sun creates our seasons. As we move around the sun, the position of each hemisphere in relation to the Sun changes. Today we in the northern hemisphere are well underway tilting away from the sun, shortening the days and lengthening nights.
― J.K. Rowling, Harry Potter and the Deathly Hallows
|Northern Hemisphere||Southern Hemisphere|
|June Solstice||Tilted towards the Sun → Summer||Tilted away from the Sun → Winter|
|December Solstice||Tilted away from the Sun → Winter||Tilted towards the Sun → Summer|
As parts of Earth are illuminated by fewer hours of sunlight, days are shorter and nights are longer. The temperature drops. Autumn arrives, then in time, winter follows.
Trees respond to fewer hours of sunlight by generating less and less chlorophyll.
|Intercity Bridge / Ford Parkway Bridge / 46th St Bridge over the Mississippi River|
Chlorophyll in leaves enables trees to absorb and convert sunlight energy into sustenance via photosynthesis.
When chlorophyll (green) production stops, carotenoid (yellow, orange, brown) in the leaves become visible. Carotenoids are pigments synthesized by plant life. Reddish leaves are determined by temperature and cloud cover. Many consecutive warm autumn days and cool non-freezing nights produce anthocyanin (red) causing a reddish colors in leaves because of the sugar produced.
Pale amber sunlight falls across
The reddening October trees,
That hardly sway before a breeze
As soft as summer: summer's loss
Seems little, dear! on days like these.
Let misty autumn be our part!
The twilight of the year is sweet:
Where shadow and the darkness meet
Our love, a twilight of the heart
Eludes a little time's deceit.
Are we not better and at home
In dreamful Autumn, we who deem
No harvest joy is worth a dream?
A little while and night shall come,
A little while, then, let us dream.
Beyond the pearled horizons lie
Winter and night: awaiting these
We garner this poor hour of ease,
Until love turn from us and die
Beneath the drear November trees.
― Ernest Dowson, The Poems and Prose of Ernest Dowson
- Earth Rhythms, GrokEarth, 13 October 2012.
- What Causes Seasons, dateandtime.com.
- Why do Leaves Change Color, dnr.wi.gov.