Autumnal

As Earth orbits the Sun, it rotates on an axis tilted at 23.5 degrees. Accordingly, on its yearly journey, parts of Earth receive proportionally fewer hours of sunlight.

"Autumn seemed to arrive suddenly that year. The morning of the first September was crisp and golden as an apple."
― J.K. Rowling, Harry Potter and the Deathly Hallows

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.

	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.

AUTUMNAL

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

REFERENCES

• Earth Rhythms, GrokEarth, 13 October 2012.
• What Causes Seasons, dateandtime.com.
• Why do Leaves Change Color, dnr.wi.gov.

River Run Dance

Gallatin River

If a river is the dancer, then gravity must be the choreographer.

A river's source is a lake, a spring, or a series of small streams or headwaters. All rivers flow downhill from their source. Rivers typically terminate in the ocean.

Would we need to trace a water molecule to know that water flowing through a river is never the same water?

A river's life is significantly longer than a human generation. Seen over the course of a human lifetime, a river swells or shrinks seasonally and temporally, but in the aggregate, seems steady in course and flow.

"I thought how lovely and how strange a river is. A river is a river, always there, and yet the water flowing through it is never the same water and is never still. It’s always changing and is always on the move. And over time the river itself changes too. It widens and deepens as it rubs and scours, gnaws and kneads, eats and bores its way through the land."
― Aidan Chambers, This is All: The Pillow Book of Cordelia Kenn

On a geologic time scale, a river has a dramatic life-cycle.

Youthful

A youthful river has a steep gradient. It has rapid flow and has few tributaries. The steep gradient causes erosion that is deeper rather than wider.

Mature

Mature rivers have a milder gradient and slower flow. It is fed by many tributaries and has a greater volumetric flow rate. The milder gradient causes erosion that is wider rather than deeper.

Old

An old river has a low gradient and low erosive energy. Old rivers are characterized by flood plains.

Mississippi River

Eventually, all things merge into one, and a river runs through it. 
― Norman Maclean, A River Runs Through It