A Tilted Earth Nears the Balance Point

The stars and constellations of summer remain in the evening sky, even though down here on Earth it feels like summer has ended. We are in an in-between time.

With Labor Day come and gone, you can now make a left turn onto Route 6, and it’s easy to park at the beach in the middle of the day, but the three bright stars of the Summer Triangle still shine. You can also still catch sight of Scorpius very low in the southwest after sunset; it’s an elusive constellation, visible only in summer.

Summer lingers on the calendar for a little while longer, too. The fall equinox, marking the end of summer and start of autumn, is Sept. 22 this year. The fall and spring equinoxes are when day and night are nearly equal in length and mark the first days of fall and spring. The summer and winter solstices are the longest and shortest days of the year, respectively, and are the start of summer and winter in the northern hemisphere.

The solstices, equinoxes, and four seasons all relate to Earth’s orientation to the Sun.

Most things in space orbit something: Earth orbits the Sun once every 365 days. And most things in space rotate, too: Earth rotates, turning toward, away from, and back to the Sun once every 24 hours.

What’s perhaps less obvious in our everyday calculations of things is that Earth’s rotation has an axis: an imaginary line through the Earth around which it rotates. One end of this axis is the North Pole. The other is the South Pole. In an imaginary universe in which all things were nicely ordered and symmetrical, you might picture Earth’s axis of rotation to be tilted a neat 90 degrees from the plane of its orbit around the sun, like a spinning top that stands perfectly upright. But we live in a universe with an interesting dash of chaos and asymmetry.

Earth’s axis is tilted from perfectly upright by 23.5 degrees. That tilt causes the patterns of a year that cycles through warmth and cold, rain and snow. What I’m saying is that the mechanism that drives the change of seasons is less the changing length of daylight throughout the year, as is commonly thought, and more the varying intensity of solar energy throughout the year (though the two are related). And the reason the intensity of solar energy varies is the Earth’s tilt.

Over these past months of summer, because of the tilt, the northern hemisphere has been tipped toward the Sun. The Sun has been higher overhead, and we have received more intense sunlight than at other times of year when the Sun is lower and the light that reaches us less intense. More sunlight equals more energy, which equals more heat.

But since the summer solstice on June 21, when the northern hemisphere was at its maximum tilt and the days were longest, we’ve been slowly tipping back the other way. The days have been getting shorter, and solar energy has become less intense. Heat waves in July or August seems at odds with this. But heat accumulates in the oceans and atmosphere, then takes time to dissipate as solar energy diminishes. (The opposite is also true — I’ll get to that.)

On Sept. 22, we will reach the balance point. The Sun will appear directly overhead at noon at the equator, and solar energy will be as evenly distributed between northern and southern hemispheres as it ever gets. Day and night will be almost the same duration. It’s an equinox, when all things are (nearly) equal.

Earth will continue its orbit, the tilt will shift past the balance point, and the northern hemisphere will tip away from the Sun. We’ll receive less solar energy, the atmosphere and ocean will cool, and the days will shorten.

On Dec. 21, the winter solstice, the tilt will reach its other extreme. We will have our shortest day of the year, and sunlight will be at its weakest. Picture winter afternoons with their distinctive low, cool sunlight and imagine being tilted away from the sun.

After the solstice, the days will lengthen during the coldest days of the year. The paradox is similar to that of the summer months after the June solstice. Solar energy increases, but the warming of the atmosphere and ocean lags behind. On March 20, we’ll reach the spring equinox, another balance point. Then the northern hemisphere will tip toward the Sun again.

Finally, on June 21, 2027, we’ll be back at the summer solstice.

There are other ways to think about the passage of a year. You can imagine how Earth will have completed an orbit of 584 million miles around the sun at over 66,000 miles per hour. But I find those facts can be hard to grasp. On the other hand, autumn leaves, the first snow of the winter, or a long summer twilight at First Encounter Beach always remind me of the fascinating phenomena at work beneath our feet. Clear skies!