Eye Astronomy #5: Seasons in the Sky

by Dale E. Lehman

We organize time by the calendar: days, weeks, months, years. And, of course, the seasons. What has that to do with astronomy? Simple: calendars have deep connections to the sky.

• A day is what it is because that’s how long it takes the Earth to rotate once on its axis.

• A month is what it is because that’s how long it takes the Moon to go through one complete cycle of phases. More or less, anyway. A complete lunation lasts 29.53 days.

• A year is what it is because that’s how long it takes the Earth to complete one orbit about the Sun. Again, more or less. A complete orbit takes about 365.256 days.

As for seasons, those are caused by the tilt of the Earth’s axis. The Earth orbits the Sun in an ellipse, as shown in the graphic below. As you’ll recall from geometry, an ellipse is a two-dimensional figure that lies in a plane. The Earth is spinning like a top about an axis of rotation that is tilted at an angle of 23.5 degrees from straight up-and-down (perpendicular) to the plane of its orbit, like this:

Over the short term, that tilt is always pointing the same direction in space. In the few months centered on July, the northern hemisphere tilts toward the Sun and enjoys summer, while the southern tilts away and shivers in winter. In the few months centered on January, it’s reversed. Then, the southern hemisphere gets summer and the northern hemisphere gets winter. Between those extremes, both hemispheres equally face the Sun, resulting in spring and autumn.

Fun fact: The Earth is farthest from the Sun in the middle of the northern hemisphere’s summer and closest during the northern hemisphere’s winter. These somewhat moderates both seasons. The reverse is true in the southern hemisphere. However, the distribution of land and water is a much bigger factor in climate than proximity to the sun. The oceans moderate temperatures, and there is more ocean in the southern hemisphere. Accordingly, our friends below the equator enjoy milder seasons than we up here.

The sky shows us this tilt. Look at that graphic again. In summer, the Sun rides high in the daytime sky, while the winter sun rides low. But what about at night? Then, it’s reversed. The Zodiac—the band of stars through which the Sun, Moon and planets roam — rides high in the winter sky and low in the summer.

This is a bit awkward for planetary observers. Objects look better in a telescope when they’re high in the sky. The closer to the horizon you look, the more air you’re looking through and the more atmospheric disturbance will distort the image. In the lovely warm summertime, the planets are unfortunately low in the sky. In winter, when nights grow frigid, they’re nice and high! Sometimes life just isn’t fair.

But there are compensations. If you go outside at about 9:00PM on a clear night, you’ll see the winter constellations rising in the east. And are they ever glorious! The brilliant star Capella in the northeast, Jupiter blazing forth between the horns of Taurus, the bull, and the mighty hunter Orion following hard on his heels. We’ll look at these constellations and more next month. Until then, bundle up, step outside, and enjoy the show!