Bees drone lazily in the late afternoon sunshine. A rainbow of flowers blooms along roads, in gardens, and on Hawaiian shirts. Some of the shyer birds have quieted down, while vireos and thrushes still sing their hearts out on their breeding territories. Loons have fluffy chicks riding on their backs. Fox kits play and sun themselves outside the den. Mosquitoes buzz in your ears, and shimmering dragonflies come to your rescue.
In this season of growth and vitality, it is easy to forget how stunning the fall colors can be, or how the yard looked covered in snowdrifts. Every season has pleasant and unpleasant aspects, and they are all made possible by the tilt of the Earth on its axis.
The Earth’s axis is an imaginary line going right through the planet between the north and south poles. The axis is tilted 23.5 degrees off the plane of the Earth's orbit around the Sun. For several months of the year, the half of the Earth that’s tipped toward the Sun receives more direct rays than the other half. We are now absorbing more energy from the Sun than we will at any other time of year. Plants sense it, and they grow furiously – photosynthesizing like crazy – before cool weather and lower sun angles bring on winter dormancy again.
Without the tilt of the Earth’s axis, our day length would not change, Alaska would have perpetual twilight, and we would not have the wonderful variety of the four seasons. Instead, two slightly different seasons might emerge based on the distance of the Earth from the Sun. Our elliptical orbit takes us farthest from the Sun (to a point known as the aphelion) around July 3. The perihelion, or the closest point in our orbit, happens around January 4. The difference between the two distances is about 3,000,000 miles, a variation of only about 3%. This causes a fairly minor change in the amount of energy from the Sun that reaches Earth, and would not lead to our rainbow of seasons by itself without the tilted axis.
Although the word solstice derives from a combination of Latin words meaning "Sun" + "to stand still," the Solstice is not constant over the years. The tilt of the Earth’s axis changes by 2.4 degrees (between 22.1 and 24.5 degrees) over 41,000 years. We are comfortably in the middle of that range right now. When the Earth tilts less, the Sun is lower on the horizon in the summer and higher in winter. Thus, summers are cooler while winters are warmer. This changing tilt is one of several large-scale factors influencing the advance and retreat of glaciers.
Glaciers shaped our landscape, while the seasons decorate it. Both owe some thanks to the tilt of the Earth on our axis. While the tilt of the Earth has a big impact on our lives, that tilt may have been caused by a large impact itself. One theory suggests that a huge chunk of space dirt in the early solar system may have slammed into the still-molten Earth – ejecting material that would become the moon.
No matter what you believe, I hope you enjoy these long summer days, and sweet summer nights.