The red maple outside my bathroom window has been there a long time. It’s probably 70 feet tall now, and mutely overlooks the birches, poplars, oaks and hemlocks around it.

This maple is always first to bud in spring and first to turn in fall. It’s gorgeous during all the seasons, and has been since we moved here 11 years ago. It’s also taller – I used to look out from a sitting position and watch its topmost leaves transform from bare, to bud, to green and then to luminous red, and then subside again. Now the top is lost to sight from inside the house. Time marches on.

In Maine the elemental units of time are the seasons. Winter and summer are polar opposites, but we most anticipate the crisp, clear air and colors of fall, remembering it in muddy March as if it was a dream of another lifetime.

The trees are emblems of the changes, waking and sleeping by the seasons. Humans seem to have the task of knowing about them. Why do seasons exist at all?

Summer turns into fall and winter, not because we wander too far from the sun, but because the Earth is tilted.

To understand what happens, imagine a tennis ball spinning in front of your face. The top of the ball has an N (for North Pole), and the bottom has an S (for South Pole). If the ball spins on an up and down axis, you can just see both the N and the S. But if you tilt the ball toward you a little bit, you only see the N. If your face is the sun, then your light is shining on the N, but not on the S.

Now hold the ball steady and circle it around your head until it’s behind you. If you turn and face it, now you see the S at the bottom tilted toward you, but not the N. The light from your face now shines on the ball’s South Pole, and the North Pole is in the dark.

The Earth is tilted toward the sun by 23.5 degrees. Summer comes when sunlight is hitting us in the Northern Hemisphere more directly and longer each day. As the Earth continues to circle the sun, the Northern Hemisphere moves out of the direct rays. Winter sets in.

Since it takes about 12 months for the Earth to travel once around the sun, the change from long days to short days and back again is gradual. In Maine we’re about halfway between the Earth’s equator and the North Pole, and the resurgence of light in spring is sharp and distinct. And when the sun’s rays lower in autumn, the air grows cool and clear.

The maple by my house has no knowledge of this, but it has a deep natural understanding. When light and warmth increase, it awakens and buds. When the days diminish in September and frost stings the grass at night, it withdraws the sap from its leaves, and becomes an element of autumn. And a sign of the time.