We are all intimately tied to the rhythm of the seasons. The explosion of fresh life in spring, the long exuberant days of summer, the turning of autumn and the cold silence of winter, remind us year after year of the larger rhythm of all life. An integral part of, and in fact the very driving force behind, this seasonal pageantry is the ever changing dance of the sun across the sky. To fully appreciate the seasons, you need to understand at least a little about the sun's journey.

Though it is convenient to talk of a moving sun, it is really the Earth traveling along its solar orbit and rotating on its axis that causes our shifting sky views. The axis is tilted nearly 23.5 degrees and it remains oriented to the same point in space. This has the effect of leaning the Northern Hemisphere directly towards the sun on the summer solstice, and away from it on the winter solstice. On the first day of spring (vernal equinox) and of fall (autumnal equinox), the axial tilt is perpendicular to the direction of the sun, leaning the Earth sideways.

The illustration above also shows me standing outside my home in Phoenix, Arizona at about 33.5 degrees north latitude. On the summer solstice, Earth's tilt puts the noon sun overhead for someone located at 23.5 degrees north latitude (Tropic of Cancer). I am 10 degrees farther north, so to me the sun appears 10 degrees from overhead in a southerly direction. On the winter solstice, the noon sun passes directly overhead at latitude 23.5 degrees south (Tropic of Capricorn). That is a 57-degree difference from my latitude, so the noon sun in Phoenix rides that distance from overhead. At the equinoxes, Earth's tilt does not change the angle of light. The noon sun is straight up at 0 degrees latitude (the equator), or down an amount equal to your latitude.

You don't really have to figure any of this out for your own location. The important thing is to get an idea of the changing orientation of the Earth's tilted axis and how it changes the angle of the sun in your sky. For as we see next, the angle of sunlight is the key to the seasonal change.
The globes were generated using
Earth and Moon Viewer

Map features are from the Visible Earth Image by NASA

Angle is Everything

The illustration of winter solstice at left shows why sun angle is so important. Equal areas of solar energy are depicted hitting the Earth's surface at two different latitudes. The northern band corresponds to the general latitudes of the Continental United States while the southern one is centered over the Tropic of Capricorn where the winter solstice noontime sun is directly overhead. Sunlight at the more-angled northerly area spreads out more, diminishing energy for any single point. Less heating, coupled with the much shorter winter days, means cooler ambient temperatures. Conversely, summer is hotter due to longer days of more direct sunlight.

Being a stargazer and living in the Arizona desert, I have spent many hot summer days with two big reasons to want the sun to set quickly—light and heat. Yet no amount of wishful thinking ever made it happen any faster. Why are those summer days so long, anyway?

As already mentioned, the sun lies its farthest north of the equator on the summer solstice. I could try to explain how more than half of the solar track lies above the horizon, but there is a much easier picture to understand, and it involves yet another globe illustration.

At left is the same summer scene from the seasons illustration at the top of the page, but this time looking directly down onto the North Pole (red dot). It is noon for Phoenix and the 6:00 AM and 6:00 PM points show where Phoenix would be at those times. A line through the two 6:00 points defines a half-day period of 12 hours. The red arc shows the path Phoenix takes from sunrise to sunset. It is easy to see the time spent in sunshine is greater than 12 hours. In fact, with the sun rising at 5:19 and setting at 7:41 at my location in Phoenix, the first day of summer for me is 14 hours 22 minutes in length.

Equinox the Equalizer

Summer and winter solstice are interesting times of extremes, but what the equinox lacks in such bravado, it adds in its order and symmetry. Equinox means equal night, so it is a time when day and night are basically the same length. It is also a time when the sun rises and set directly east and west, no matter what your geographic location. The one thing that does change with latitude is the angle the sun makes with the horizon. However, it will always be perpendicular to the Earth's axis as illustrated below.

The illustration at left includes a multiple-exposure photo of the setting sun on the autumnal equinox at a large sundial in Carefree, Arizona. The gnomon of such classically-designed sundials always points directly at the pole which aligns it to Earth's axis. The photo shows the track of the setting sun is straight and perpendicular to the gnomon and axis just as it should be at equinox. The illustration shows me standing atop the globe in Carefree. I am facing due west watching the sunset in the same position as when I took the photo. Illustrations like this always suffer distortion due to the different scales involved. The sun is far enough away that the Earth is defined more as a single point. A truer perspective in the illustration would be as if you were viewing from the center of the Earth.

To the right of the illustration I added another photo. This one shows star trails around the North Celestial Pole, which is simply the projection of the Earth's north pole onto the celestial sphere. As we look to the north, the Earth spins us in a circle, making the stars rotate around the North Celestial Pole. In the photo, the pole is marked with a red plus. The short bright streak is Polaris, the North Star. Notice it is not exactly at the pole position. I aligned the star trail photo with the illustration at the left to give a sense of how this all works together. Remember that the Earth's pole always points to the North Celestial Pole, not just at the equinox.

All images are copyrighted by Frank Zullo. Please do not use without written permission.