Published: April 20, 2008 01:19 am         

Understanding the seasons, Earth’s tilt

Variety versus constancy

We are now a month into spring with much nicer conditions for walking and being outside. What causes our seasons? Two things are key, our orbiting the sun and the tilt of Earth’s axis.

As Earth orbits the sun each year, traveling 1.6 million miles each day, our polar axis nearly points to Polaris, our North Star. Our axis is not perpendicular to the plane of the Earth’s orbit but at a tilt of 23.5 degrees to the side. This tilt causes the sun to peak high in the mid day sky in late spring/early summer and low in the mid day sky in late fall/early winter.

In late spring/early summer, the sun rises in the east northeast and sets in the west northwest. The sun then moves over 210 degrees across the sky. In the late fall/early winter, the sun rises in the east southeast and sets in the west southwest. The sun then moves only 150 degrees across the sky. The relative length of the sun’s path causes our daylight hours to vary from 15 hours at the start of summer to 9 1/3 hours at the start of winter.

Besides our temperatures and weather, our seasons have a great impact on both plants and animals. Most of our trees sprout leaves in spring, which die off in autumn. Our insect populations rise in the warm weather and shrink in the cold weather periods. Birds migrate over huge distances. Some animals hibernate; others grow a thicker coat for the cold weather, which sheds when warmth returns.

In regards to axial tilt, the Earth is in the middle among the eight major planets. Three of these planets have a negligible tilt: Mercury at 0 degrees, Venus at 0.6 degrees and Jupiter at 3.1 degrees.

(Venus rotates backwards so its tilt is often stated as 177.4 degrees.) Three other planets have tilts comparable to Earth’s tilt: Mars at 25.2 degrees, Saturn at 26.7 degrees and Neptune at 28.3 degrees.

The oddball planet is Uranus with a 98 degree tilt, nearly rolling on its side as it orbits the sun.

A fellow faculty member at Frostburg State recently asked me, “How would things change if the Earth had 0 axis tilt?” The easiest way to answer this question is to imagine that conditions on the first day of spring would persist all through the year.

The sun would rise due east, peak in the south about halfway up in the sky, and set in the west. The daylight hours each day would stay about 12 hours and 8 minutes throughout the year. At the poles, the sun would skim along the horizon for 24 hours each day. There would be no darkness there so Santa couldn’t see the stars at all, just the sun and moon.

At the equator, the sunlight would be most intense as the sun would pass overhead in mid day. So the temperatures on the Earth’s surface would peak at the equator and drop the farther north or south of the equator.

We would still have the six Hadley cells, where the wind direction shifts about every 30 degrees of latitude. (In the north temperate cell that we are in, the prevailing winds are from the west.) Temperatures would not change much, except for the passage of warm and cold fronts. We could not count on snow in February or heat waves in August.

To go skiing, you would probably have to travel to Canada where snow would fall year around. Warm water temperatures would continue year around in the Caribbean but it would be rather chilly for swimming in the Great Lakes. There would be great adjustments of our plants and animals in the absence of tilt.

Farming could be done year around in low latitude but there would be no crops in high latitudes (60 degrees and above). Birds would not have to migrate across the equator, just move within the same hemisphere to find suitable conditions for mating and raising their young.

Winter and summer stars in view

In the early evening, the best winter evening stars can be briefly seen in the western sky. Orion, the brightest star group (seen since December) is just above the trees. Orion’s three star belt points left to Sirius, the night’s brightest star. To the right of the belt is Aldebaran, the Bull’s eye and the Seven Sisters star cluster. Late in the evening, the sparkling white-blue star Vega appears in the northeast.

Vega is the brightest evening star through the summer and fall.

Continuing at the Frostburg State Planetarium is “Animal Sky Stories” with free Sunday public programs at 4 p.m. and 7 p.m. in Tawes Hall. At about 5 p.m. and 8 p.m., I give tours of the Compton Science Center Exploratorium just across the street from Tawes Hall. All of our presentations are free and open to all interested. Call (301) 687-7799 for a free bookmark which has a small map of the FSU showing Tawes Hall, Compton Center and where to park.

Bob Doyle invites comments and questions from readers; his email is rdoyle@frostburg.edu .