The summer solstice occurred today at precisely 5:13 a.m.. The truth is, I’ve avoided writing about it for years. I remember studying these matters in college and saying to myself, “English major. Yeah, that sounds good.”
However, I’ll give it a go by popular demand, knowing full well that my fellow astronerds will bomb me with text messages and emails for not including this or that detail.
On a practical level, the summer solstice is that day when the sunlit hours are at their longest. Combined with Daylight Saving Time, the solstice means that evening twilight doesn’t end until after 11 p.m., a backyard barbecuer’s dream but a stargazer’s nightmare.
Those of us who work the night shift can’t do any serious stargazing until everybody else is safely in their beds.
To the celestial cartographer, the solstice is an imaginary point on an imaginary sphere.
Given how much we know about the universe, it’s ironic that for the practical purposes of sky mapping, we still imagine the universe as a great sphere that encloses Earth. Of course, we can see only half the celestial sphere at one time because the danged Earth blocks our view of the other half.
On the celestial sphere, we draw several imaginary circles. One is the celestial equator, the extension of Earth’s equator into space. Another line, called the ecliptic, marks the sun’s apparent yearly path across the sky.
Because Earth’s axis is tilted 23 degrees with respect to its orbit around the sun, the celestial equator and the ecliptic are tilted with respect to each other. As a result, the two circles touch each other on the ecliptic at two opposite points, which also happen to be the points along the sun’s path where day and nights are equal in length.
Those points, which also correspond to the first days of spring and autumn, are called the vernal and autumnal equinoxes, respectively.
Are you following this so far? Me neither. In any case, the summer solstice is the point on the ecliptic where the sun is at equal distances between the vernal and autumnal equinoxes, or in other words, as far as it’s going to get from the celestial equator.
In effect, the sun is at its farthest point north. Since we live in the Northern Hemisphere, the sun reached its highest point north of the celestial equator at 5:13 today.
Therefore, in Central Ohio, the sun reached its highest point in the sky at local noon, 1:32 p.m.
Why 1:32 and not, say, noon, you ask? Blame Daylight Saving Time and the fact that we are 32 minutes west of the eastern edge of our time zone. (Earthlings measure time in funny ways.)
The solstice also means that the sun rises and sets at its most northern points, north of east for the rising and north of west for the setting. That means it stays up as long as it’s going to all year.
During the spring, the sun rises higher and higher in the north each day, making the daylight a little longer. After the solstice, it will be a little lower each day, and the daylight time will get shorter and shorter, thank goodness.
Remember, all of this happens because Earth’s axis is tilted about 23 degrees with respect to its orbit around the sun. The Northern Hemisphere tilts most directly toward the sun on the summer solstice. As a result, the sun rises to its highest point in the sky.
In the Southern Hemisphere, the Earth is tilted away from the sun. As you wish everyone around you a “happy summer solstice” today, don’t forget to text your friends and family in the Southern Hemisphere a “happy winter solstice.”
Earth’s axial tilt is thus responsible for the change of seasons. During summer, it gives us more hours of sunlight. On the solstice from our 40-degree latitude, the sun is up for 15 solid hours.
Compare that number to the winter solstice, when the sun peeks over the horizon for only nine hours and 18 minutes.
Another factor affecting the seasons is how high the sun rises in the sky. The sun rises to almost 70 degrees above the horizon on the summer solstice at local noon. The sky’s zenith (straight overhead) is 90 degrees above the horizon. The sun is practically overhead.
On the winter solstice, the sun scrapes the horizon at only 24 degrees at local noon.
So why isn’t the summer solstice the hottest day of the year? Any meteorologist will tell you that it takes time for the atmosphere to heat up. That’s why we suffer through the “dog days” in August instead of June.
The same thing goes for the winter solstice. December 21 would be the coldest day if it didn’t take the atmosphere a while to cool down.
Since the summer solstice happens on the longest day, one might expect that it is also the day of the earliest sunrise and the latest sunset. Oh, that the universe were that simple, fellow seekers after truth.
The earliest sunrise occurs about a week before the solstice. The latest sunset happens about a week after. Sadly, the reasons for this strangest of celestial phenomena are too mind-meltingly complex to explain without images, charts, and graphs.
In short, it has to do with the tilt of the Earth on its axis and the fact that Earth’s orbit around the sun is an ellipse, not a circle.
At Earth’s closest point to the sun, called its perihelion, our planet is 91.4 million miles away from it. At its farthest, called its aphelion, it is 94.5 million miles distant.
That difference of 3.1 million miles between aphelion and perihelion affects the gravitational interaction of the sun and Earth.
As Earth approaches its nearest point to the sun, the sun’s gravity speeds it up a bit. As Earth recedes from the sun, the sun’s gravity slows it down.
Consequently, the earliest sunrise occurs about a week before the summer solstice, while the latest sunset occurs about a week after.
If you haven’t noticed already, astronomical time with all its changing variables doesn’t follow the uniform way humans keep time.
The way we measure time’s passage is just an average of the complex and changing motions that determine astronomical time.
If you want to know more, go to your local library (or the Internet, if you must) and look up the “equation of time.” However, don’t blame this old English major for not providing a more detailed explanation.
The summer solstice was a day of celebration in ancient times, but not so much today.
After all, we celebrate the winter solstice with Christmas, Hanukkah, and Kwanzaa in the western world. Why doesn’t the summer solstice have an associated holiday?
Typically, older “pagan” celebrations were absorbed and Christianized as the Catholic Church spread across Europe. The Roman debauch called Saturnalia became Christmas, for example.
Likewise, the ancient summer solstice celebrations became the feast of St. John the Baptist.
Today, we identify the solstice as the first day of summer, but our ancient forebears celebrated it as midsummer.
Those old midsummer solstice celebrations have faded but have not disappeared completely. The solstice festivals that still exist are rooted in pre-Christian pagan traditions.
For example, the Swedes celebrate Midsommarstang with ancient gusto. Celebrants of all genders wear flower wreaths called midsommarkrans. They eat traditional foods like pickled herring, dance the maypole, and perform other traditional folk dances.
According to the “culture trip” website, “if unmarried girls place seven flowers under their pillow on midsummer, they’ll dream of their future husband.”
On Salisbury Plain in Wiltshire, England, stands a great ring of standing stones called Stonehenge. Built sometime between 3000 and 2000 BCE, Stonehenge is one of the oldest monuments to our human obsession with the summer solstice.
In ancient times, the Druids gathered in the circle and watched the sun rise majestically over a carefully placed hunk of sandstone called the Heel Stone.
In homage to our ancient past, self-styled neo-Druids still gather at Stonehenge on the summer solstice to watch the sun rise over the Heel Stone.
I know. I know. It all sounds so “pagan.” But consider what we will do instead. Throw some burgers on the barbie? Perhaps a good, old-fashioned maypole is in order.
Tom Burns is the former director of the Perkins Observatory in Delaware.