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.

Solar Trails

Yesterday was our December solstice - the shortest day of the year, winter solstice, in the northern hemisphere and the longest day of the year, summer solstice, in the southern hemisphere.

From a warm shelter on the 45th parallel in the northern hemisphere, I observed an orange sunrise above a snow-covered wetland. Later the sun rose to its lowest position in the sky for the year.

"I don't have to have faith, I have experience."
― Joseph Campbell, The Power of Myth

The solar trails of the sun are beautifully realized using solargraphy.

A solargraph is made from a pinhole camera that records the movement of the sun over many months. Over the course of the exposure, the sun leaves a trail every day.

Solar trails over half a year at a location in the Andes Mountains.

Each day the sun's path progressively increases above the horizon from a low on winter solstice day, (December in the northern hemisphere), to a peak on summer solstice day (June in the northern hemisphere). Breaks in the solar trails occur when the sun is obscured by cloud cover.

"The black moment is the moment when the real message of transformation is going to come. At the darkest moment comes the light."
― Joseph Campbell

Cerro Paranal in northern Chile

REFERENCES

• Solargraphs of ESO by Robert Fosbury and Tarja Trygg. Astronomical News.
• A Solargraph taken from APEX at Chajnantor. European Southern Observatory.