Lawrence LeBlond for redOrbit.com
Wednesday June 20, 2012 is the longest day of the year; the one day with more sunlight hours than any other day in the calendar year; it is also the first day of summer and is known as the Summer Solstice.
The longest daylight day of the year occurs when the Sun appears at its highest point in the sky. At that point, the Sun appears to stand still just briefly before it begins its long journey toward the Winter Solstice — the shortest daylight day, which occurs around December 21.
During the Summer Solstice, most of us are treated to more than 14 hours of daylight, some as much as 15 hours and 2 minutes. But what causes this event?
Carolyn Sumners, vice president of astronomy and the physical sciences at the Houston Museum of Natural Science, told the Houston Chronicle that the Earth is always tilted on its axis roughly 23.4 degrees. It orbits around the Sun in that position, and in the Northern Hemisphere, when that tilt leans toward the Sun, we call it summer.
“We are reaching that point in the Earth´s orbit where we are tilted most toward the sun,” Eric Schlegel, professor of physics and astronomy at the University of Texas at San Antonio, told the Chronicle’s Alyson Ward. “And when we get there, that´s the summer solstice.”
During the event, and only for those who are lucky enough to be on the Tropic of Cancer — an invisible line around the Earth — a rare event will occur: for a brief moment on Wednesday, nothing will cast a shadow. But most of us are too far north to witness this event; the closest we will come is at “solar noon,” which does not occur at exactly noon on your clock, but rather when the sun is at its highest point in the sky. The actual time is different depending on where in the Northern Hemisphere you are located.
The reason solar noon does not occur at “noon” is due to Daylight Savings Time, which is in effect for us in the Northern Hemisphere this time of year. And based on where you are located in your time zone, solar noon may be at 1 p.m. for one area, but further west it could occur at 1:15 p.m. because it takes light 15 minutes longer to reach that area than it does where the time-zone meridian begins.
During the Summer Solstice, we in the Northern Hemisphere have more sunlight than those in the Southern Hemisphere. But from this point on, we will lose increments of daylight as the Earth´s northern axis turns a little bit away from the Sun each day. The Southern Hemisphere, in turn, receives those increments of sunlight that we lose, and in about 6 months, when we are receiving our shortest day — the Winter Solstice — the Southern Hemisphere will have its Summer Solstice, when the southern axis is pointing toward the Sun.
If not for the tilt of the Earth´s axis, we would not have seasons. Both day and night would be exactly the same length all year. Both northern and southern hemispheres would share the same amount of sunlight each day. But because of the Earth´s axis, that only occurs on two days through the year — The Vernal Equinox on March 20 and the Autumnal Equinox on September 22 this year.
While the Summer Solstice typically occurs on June 21, this year it is occurring one day earlier because of our calendar, said Schlegel. It takes the Earth 365.25 days to orbit the Sun, which means every four years we must add in an extra day so we do not fall behind. During a leap year, which occurred this year, the Summer Solstice gets moved up one day on the calendar. This does not mean that the actual event is taking place a full day earlier, however.
So what can we expect on the Summer Solstice?
Since the Sun is at its highest point in the sky today, it means more solar radiation is getting dumped on the Earth. “You´ll get your quickest sunburn on the summer solstice – the sun is most intense,” noted Sumners.
Interestingly enough, as we are treated to upwards of 15 hours and 2 minutes of daylight, today is also expected to be the hottest day of 2012 so far in many parts of the country. And tomorrow, the actual first full day of summer, it is supposed to be hotter.
What´s perhaps even more interesting, is that during the summer, when, in the Northern Hemisphere, we are farthest away from the Sun, we receive our warmest days. In the winter, when we are closest to the Sun, it is the coldest. Why this is so, is because during the summer, the northern axis is more directly facing the Sun, and in winter, not so much, which means less solar radiation.
What does the solstice mean?
Don Wheeler, an associate professor of natural science at Louisiana Delta Community College said the word solstice is derived from Latin origins meaning, “Sun standing still.”
“The first part of the word, ℠sol,´ means sun and the suffix means ℠to stop,´” he told TheNewsStar.com. “On the Summer Solstice, the sun appears to stop rising in the sky because it has reached its highest point.”
So what do we do during the Summer Solstice?
For many here in the US, we celebrate the Sun´s longest visit with parades and festivals. In England, one of the most famous celebrations occurs: each June, thousands gather at Stonehenge the night before the Summer Solstice, waiting all night for the Sun to rise up over the stone monument; as it lines up perfectly with the outer Heel Stone.
Why such a celebration at Stonehenge? In that part of the world, the seasons are very different than they are here, said Sumners.