It was not just a regular long Monday.
It was the longest day of the year, signaling the official beginning of summer and highlighting the astronomical forces that drive the seasons.
Tennessee Tech Professor of Physics Steve Robinson said the summer solstice occurs when the Northern Hemisphere receives its greatest amount of daylight due to Earth’s tilted axis.
“This is not to do anything to do with how far away from the sun we are, it’s to do with the fact that, the sun’s rays are hitting the Earth, in a more concentrated fashion, or hitting the Earth in our region, I should say, in a more concentrated fashion, which increases the warming effect,” Robinson said.
Robinson said the hottest temperatures of the year typically occur after the solstice because the Earth and its atmosphere retain heat. He said the warming trend continues to build even as daylight hours begin to decrease slightly following the longest day.
Robinson said the date of the solstice can shift from year to year because the Earth takes approximately 365 and a quarter days to complete a full orbit around the sun. He said the calendar uses leap years every four years to catch up with this extra time, causing the specific timing of the solstice to vary.
“I think the common misconception, let me broaden that out a little bit, is that we get summer because we’re closer to the Sun,” Robinson said. “And that’s actually not true. We’re actually a little bit further away from the Sun. The Earth as a whole is a little bit further away from the Sun now than it was in January. And so people think it’s the distance from the Sun that matters. And that’s perfectly understandable that people think that because our everyday experience tells us if we get closer to a fire, we get hotter. And so we think well if we’re getting hotter in the summer, we must be closer to the Sun. We’re actually not, it’s the almost completely due to the tilt of the Earth’s axis and how concentrated the sunlight is, on when it hits the ground in our area of the Earth.”
Robinson said the tilt of the Earth is entirely responsible for the seasons, comparing the concentration of energy to a flashlight beam hitting a sheet of paper.
“If we had no tilt, we would have no seasons, basically,” Robinson said. “It would be the same climate all year round at every part on the Earth. So the equator would always be the hottest region and the poles would always be the coldest region and it would just be a pretty simple transition in between. It’s the tilt of the axis as the Earth orbits the Sun that gives us our seasons.”
Robinson said the most fascinating aspect of the science is how sunlight concentration works on the curved surface of the Earth. He said the way light spreads out depends on the angle at which it hits the surface, much like moving a flashlight up and down in front of a basketball.