Wednesday, Sept. 22 marks what scientists call the fall equinox in the Northern Hemisphere, when night and day are of equal length and cooler weather is on the way -- if it hasn't already arrived.
But what's behind the shift?
A year-long series of pictures by Greek photographer Anthony Ayiomamitis helps put the equinox into perspective, while also illustrating why we have seasons.
The image shows the Sun's position in the sky at the same time each day, every week or so, during a full year. The figure-8 pattern created by the Sun's apparent annual movement is called an analemma.
The high point comes during summer when the days are long, and the low point in winter when the Sun remains frustratingly low on the horizon, days are short, and less radiation is available to warm the ocean, air and land.
Ayiomamitis explains where the Sun is in his picture as we reach the fall equinox:
"The crossover point between the smaller upper loop and the larger lower loop is Aug. 30," he explained. "We are now between the third and fourth solar disks below and to the right of the crossover point."
The Sun appears to move around so much because Earth is tilted on its rotational axis, by about 23.5 degrees. Earth's North Pole always points in roughly the same direction in relation to the plane of the solar system. But as the planet orbits the Sun (once each year) a major change occurs.
In the Northern Hemisphere's summer, the North Pole leans toward the Sun, causing the Sun to be higher in our sky and spreading warmth copiously. In winter, the pole leans away from the Sun and things get chilly (while those south of the equator are treated to longer days). To visualize this, imagine pointing toward someone who is at the center of a room, then walk around to the other side of the room without changing your arm's pointing position in relation to your entire city -- you'll then be pointing away from the person.
The analemma's figure-8 is not symmetrical. That's because Earth's 365-day orbit is not a perfect circle. We are closer to the Sun in January, and this causes Earth to move faster compared to summertime, making the bottom loop of the analemma -- the winter portion -- larger.
(Being slightly closer to the Sun in winter does not provide enough extra warmth to overcome seasonal effects caused by the planet's tilt, however. More about that here.)
For his image, Ayiomamitis patiently made 47 exposures of the Sun between Jan. 7 and Dec. 20 last year, all on one frame of film. Then a second picture that included the foreground -- the temple Hephaisteion, in Athens -- was taken and added to the first.
On his web site, Ayiomamitis has more views of the analemma and details about how he created the photographs.
- Live Sun Cam
- Mysteries of the Sun
- How the Night Sky Changes with the Seasons
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