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Curt Nason

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Despite the jokes and complaints, many people in our part of the world are grateful for the variety afforded by the changing seasons. Those less fortunate than us, who live in year-round warmth, can still mark the seasons by watching the parade of constellations across the night sky.

Did it feel cold to you last night? About an hour before midnight, earth was at its farthest from the sun for the year. Many people believe our planet's changing distance from the sun - it is about three percent farther in July than in January - is the cause of our seasons. Obviously, it is not, for summer began about two weeks ago.

The reason for the seasons is that earth does not stand up straight in its orbit. It is tilted a quarter of the way over, with the north polar axis pointing toward a distant star we call Polaris, the North Star.

To understand why seasons occur, shine a flashlight directly onto a table and notice the size of the beam. Now, tilt the light and see how the beam covers a larger area of the table and also looks dimmer. In late June the northern hemisphere is tilted toward the sun and the sunbeams are at their most intense for us. From our viewpoint, the sun rises at its farthest north on that day, reaches its highest point of the year, and sets at its farthest north. Our cousins in southern New Zealand are viewing their noonday sun at its lowest and cursing the onset of winter.

Although the sun's gravity keeps us in a stable orbit, the endless gravitational effects of the other planets try to change the earth's tilt. Fortunately, the relatively large size and nearness of the moon prevents the tilt from changing significantly, which would have a devastating effect on our climate. The tilt does change slightly over a period of 41,000 years, and also the earth's axis wobbles like a top every 26,000 years.

Another effect of the planets is a variation in the difference between earth's nearest and farthest distance from the sun. These combine to vary the intensity of sunlight we receive, and some scientists believe that plays a significant role in the occurrence of ice ages.

Most of the other planets are tilted and undergo seasonal changes. The tilt of Mars is similar to ours and we can observe the shrinking of its polar ice caps in Martian summers through telescopes.

Saturn's tilt is slightly more than ours, which gives us stunning views of the top and bottom of its rings. This year Saturn enters its spring season so we see the rings edge-on. Uranus is an oddball, being tilted almost on its side so that the sun is directly over its poles in summer. With giant telescopes, astronomers are finally able to detect subtle seasonal changes in the atmosphere of this secretive planet.

Stargazers enjoy the changing seasons because each brings a different group of constellations. As earth orbits we see only those stars that are opposite the sun. Orion is the master of the winter sky, Leo roars through spring, Cygnus the Swan soars overhead on summer nights, and the square of Pegasus marks our autumn.

Although earth's axis wobbles slowly, our calendar is adjusted so that spring will continue to occur in March, but this means the constellations will shift through the seasons. In 13,000 years I'll have to get used to seeing Orion peeking above the southern horizon in summer.

For International Year of Astronomy events, visit astronomy2009.ca, or contact me at 648-9063, nasonc@nbnet.nb.ca).

Curt Nason is a health physicist in the nuclear industry.