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 shows me standing outside my home at local noon in Phoenix, Arizona at 33.5° N latitude. Notice how the sun appears higher in my sky at summer than winter. At summer solstice the noon sun is overhead at 23.5° N latitude (Tropic of Cancer). I am 10° farther north, so to me the sun appears 10° from overhead in a southerly direction. On the winter solstice, the noon sun passes directly overhead at latitude 23.5° S (Tropic of Capricorn). That is a 57° 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° latitude (the equator), or down towards the south an amount equal to your latitude.

Globes were generated using
Earth and Moon Viewer

Map features are from the Visible Earth Image by NASA

 

For a description of azimuths and degrees, see my webpage Navigating the Sky

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 to cover a larger area of land. This diminishes the heating energy available 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.

Seasonal Sunrises

The changing tilt of Earth also shifts the sunrise-sunset points north and south throughout the year. Below is a composite image of solar tracks taken from Phoenix at roughly 33.4° N latitude. At this location, summer solstice sunrise is at azimuth 61° towards the ENE while on the first day of winter the sun rises at azimuth 118° in the ESE. Spring and fall equinox sunrises always occur at 90° due East for all locations, with only the slant of the track varying due to changes of latitude. At the equator, where solar tracks are untilted, solstice sunrise occur at roughly 66° and 113°. At 45° N latitude, they are at azimuths of 56° and 123°.

Sunrise times and azimuths are always figured for a perfectly flat horizon depicted above by the horizontal line. Elevated and irregular horizons along with the slanted solar paths cause the sun to rise a bit later and southwardly shifted. The higher the horizon, the more sunrise points will be shifted.


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