Geologic Column: On the Summer Solstice

 Illumination of Earth by Sun on the day of summer solstice on northern hemisphere. Credit: ©Przemyslaw "Blueshade" Idzkiewicz, CC BY-SA 3.0. Illumination of Earth by Sun on the day of summer solstice on northern hemisphere. Credit: ©Przemyslaw "Blueshade" Idzkiewicz, CC BY-SA 3.0.

By John Copeland

There is no world organization that designates the official days on which seasons begin. In fact, there are different ideas about how to define the seasons. When I was a child, summer began, for my friends and me, at 3 p.m. on the last day of school. Meteorologists and climatologists may tell you that summer begins on June 1. Astronomers, however, use the summer solstice — which falls this year on Sunday, June 21 — to mark the beginning of summer in the Northern Hemisphere and the beginning of winter in the Southern Hemisphere.

For many of Earth’s creatures, nothing is as fundamental as the length of the day. The solstice marks the longest day and shortest night of the year in the Northern Hemisphere (and vice versa in the Southern Hemisphere). On June 21, the sun will rise to the north of due east and set to the north of due west, making its longest traverse and reaching its highest point of the year. Thus, on the solstice, the noon sun appears to hang motionless in the sky for a time. The word “solstice,” in fact, comes from the Latin “sol” (sun) and “stitium” (to stop).

Along with the winter solstice and the spring and fall equinoxes, the summer solstice has been celebrated throughout human history by cultures around the world. Our ancestors were keen observers who noted that the sun’s path across the sky, the length of daylight, and the location of both sunrise and sunset shifted throughout the year. Around the world, you can still visit many ancient structures associated with the summer solstice. Perhaps the most famous — and one of my favorite places — is Stonehenge, on the Salisbury Plain in Wiltshire, England. Stonehenge’s main axis is aligned with the solstice sunrise, an orientation that was probably for ritualistic rather than scientific purposes.

Today, we can only speculate on the significance that the summer solstice had for our ancestors. But, we know that the solstice is determined by the tilt of Earth’s axis and the sun’s alignment over the equator. In summer, the Northern Hemisphere is tilted toward the sun at an angle of 23.44 degrees, making the sun appear high in the sky; in winter, it is tilted away, making the sun appear much lower. This affects both day length and the angle at which sunlight hits Earth’s atmosphere, which varies the amount and intensity of solar insolation received at different latitudes throughout the year.

Because Earth’s orbit is an ellipse with the sun at one foci, our planet’s orbital speed varies as it travels around the sun and the seasons are not of equal length. The periods of time from the Northern Hemisphere spring equinox to the summer solstice, to the autumn equinox, to the winter solstice and back to the spring equinox are roughly 92.8, 93.6, 89.8 and 89.0 days respectively. In the Northern Hemisphere, our spring and summer seasons last longer than autumn and winter.

Although the summer solstice occurs on what we think of as the beginning of summer, for farmers it is the midpoint of the growing season, halfway between planting and harvesting — which is why it is often referred to as “Midsummer.” As the name suggests, the day is considered the height of summer and an occasion for celebration. Yet there is also an undertone of darkness. On the solstice, ancient peoples celebrated the power of the sun, but they were also noting its decline. From the summer solstice on, the hours of sunlight each day will decrease.

This leads many to wonder: If June 21 is the longest day of the year, why does the hottest summer weather occur in late July and August? This arises from the so-called lag of the seasons, which is essentially the same reason it is often hotter in the late afternoon than it is at noon. Earth takes a while to warm up after a long winter. As I write this, ice and snow still blanket the ground in some places. Meltwater and rainwater have been percolating down through snow on the tops of glaciers. But the runoff from glaciers isn’t as great now as it will be in another month, even though sunlight is striking the Northern Hemisphere most directly now. The sun has to melt the ice and warm the oceans and then, after that happens, we will experience sweltering summer heat.

So wait another month. The heat will come when the days are already beginning to shorten again, as Earth continues its annual trek around the sun, bringing us closer to yet another winter.

John Copeland

John Copeland headshot. Credit: Fran LoCascio

Copeland is a filmmaker in California who has produced television programs ranging from “Babylon 5” to “Faces of Earth” (produced with the American Geosciences Institute). Copeland also works with MIT’s Experimental Study Group to instruct undergraduate science and engineering students in the art of visual communication and storytelling. The views expressed are his own.

Sunday, June 21, 2015 - 06:00

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