What Is The Summer Solstice?

Author: News Bureau
Posted: Sunday, June 12, 2022 12:00 AM
Categories: Pressroom | Faculty/Staff | School of Health and Natural Sciences

Macon, GA

The summer solstice, which this year occurs on June 21, marks the official beginning of summer in the Northern Hemisphere. But what, exactly, is it? Dr. Larry Camarota, assistant professor for Middle Georgia State University’s Department of Natural Sciences, explains this astronomical phenomenon and talks about other upcoming celestial events.

What is the summer solstice and why do we have one?

The word ‘solstice’ roughly translates as ‘when the sun stands still’, and is related to where exactly the sun rises. It is common knowledge that the sun rises in the east, but it usually doesn’t rise due east. In the summer, it rises slightly to the north, and in the winter it rises slightly to the south. Similarly for sunset, it is slightly north of west in the summer, slightly south of west in the winter. During most of the year, the exact position of sunrise and sunset slowly drifts day to day. In the spring, the direction of sunrise is drifting northward. As the summer solstice approaches, the drift slows down, then comes to a stop. After the summer solstice, sunrise will start drifting toward the south.

Another property of the summer solstice is that it is the longest day of the year. This is related to the location of sunrise because, since the Earth is spinning, the sun appears to travel in a circle around us, with parts of that circle being blocked by the ground. The orientation of that circle depends on where on Earth you are standing, as well as the time of year. At the summer solstice, the circle appears to be the highest it ever gets. This means that more of the sun's apparent motion is above the ground, which in turn means that there is more time spent in daylight.

Physically, the reason for this is because the axis of the Earth's orbit around the Sun is not in the same direction as the axis of the Earth's rotation, which is sometimes called the tilt of the Earth. The summer solstice happens when the Earth is in the part of its orbit where the North Pole is tilted toward the sun. This means that for people living in the northern hemisphere, the sun's apparent motion around the Earth is higher in the sky.

The solstices, and the equinoxes, have been very important culturally. To start, we define a year as the time it takes the Earth to orbit the sun, but how do we know how long that is? Since the summer solstice happens exactly once each year, we could simply define it as the period from one summer solstice to the next, and many societies did just that. 

Toward that end, many ancient civilizations built monuments that appear to aid in determining the date of the summer solstice. Probably the most well-known example is Stonehenge, although there is some debate as to whether it was set up intentionally as such. A more striking example would be the Karnak Temple in Egypt. On the summer solstice, as the sun sets, sunlight shines through a long corridor of the temple complex to illuminate the central shrine of the Precinct of Amun-Re. Other examples include some of the towers at Angkor-Wat, the Mayan Uaxactun structures, and the Temple of the Sun in Machu Picchu.

Today, the summer solstice marks the beginning of the summer season. The summer solstice is when the sun stays up the longest, and is most directly overhead. All this together means that the summer solstice is the day that the environment gets the most thermal energy from the sun. This causes the temperature to rise and become hot for the summer months.

Do other planets in our solar system have solstices?

Yes, to varying degrees. The thing that makes a solstice happen is for the planet to have a rotational axis that is different than its orbital axis, and the magnitude of the effect of the solstice depends on how large that difference is. 

The planet with solstices most similar to our own is Mars. Mars has a 25° axial tilt, compared to the Earths 23°. Mars also has a day that is just a little longer than Earths, so people living on Mars would experience the same phenomenon of the position of sunrise moving north and south over the course of the Martian year. Mars also experiences seasons similar to Earth, with summers and winters being warmer and colder respectively.

The next most similar would be Saturn, with an axial tilt of 27°. In the case of Saturn, the summer and winter solstices are in some ways more interesting to us, because they represent the time when Saturn’s rings are most visible. For people living on Saturn, summer might be considered the time when the rings are lit up by the sun, and winter is the time when they are in their own shadow.

There are regions on the Earth where the sun doesn’t set for multiple days. This is true in the Arctic Circle during the summer, and the Antarctic Circle during the winter. With an axial tilt of 82°, most of Uranus’ surface is in that planets Arctic or Antarctic Circle equivalents. For Uranus, the sun very nearly does stand still on the summer solstice.

Mercury has no axial tilt to speak of, and it spins slowly enough that a solar day lasts two years. However, its orbit is elliptical enough that once per year the sun appears to go backwards for a very brief time. Venus has a small axial tilt, and a dense enough atmosphere that one can neither see the sun nor feel any temperature differences between the seasons. Jupiter has very little tilt, and Neptune is far enough away that the sun is about as bright as a single lightbulb at the opposite end of a medium sized classroom. 

What are some other upcoming celestial events you recommend we check out?

In the near term, there are going to be a pair of overlapping meteor showers that run from mid-July to mid-August. The Delta Aquarids will peak around July 31, and will be mostly coming from the south The Perseids will peak around August 13, and will be coming from the north. The Perseids are usually one of the most active meteor showers, but their peak this year is close to a full moon, which might make them harder to see. The Delta Aquarids are not as active, but their peak happens closer to the new moon, so if you can get to a dark area you should be able to see one every couple of minutes.

The best way to view a meteor shower is to find a dark area with a lot of open sky, and lie down. Bring a blanket and a pillow, and get comfortable. During the peak of a meteor shower, meteors come at a rate of about one per minute. Bring, music, or a friend, or both, and try to avoid looking at your phone to keep your eyes night-adapted.

In the longer term, Macon is going to have a pair of really nice solar eclipses coming up. The first one will be on October 14, 2023 from 11:45-2:45, peaking at 60% at 1:15. The second one will be on April 8, 2024 from 1:45-4:20, peaking at over 80 percent at 3:04. That second one is being called the Great American Eclipse, because it will be visible from everywhere in the continental United States, and has a path of totality that stretches from the Texas panhandle to the tip of Maine.

When viewing an eclipse, the most important thing to remember is to never look directly at the sun with your bare eyes. Even when almost entirely blocked, the sun is still bright enough to damage your eyes. If you want to look at the eclipse, you will need to get eclipse viewing glasses. They have filters strong enough that you can look at the sun without risk, and are pretty cheap, but you will need to get them ahead of time. You can also indirectly observe the eclipse with a pinhole. The basic idea is that if you have the sun shine through a small opening, the light that goes through will make a dot the shape of the eclipsed sun. You can make a pinhole viewer by putting a small hole in an index card, or even by making a hashtag out of your index and middle fingers. If a tree has a medium amount of leaves, the gaps in the foliage make a bunch of pinholes, and you can see many images of the eclipse in them. A third way to view the eclipse is with optical devices designed for viewing the sun with. Most telescopes have solar filters available, and there are projector sets that will shine an enlarged image of the sun onto a surface.

The exception to the ‘no direct viewing’ rule is if you can see a total eclipse. When the sun is completely eclipsed it is safe to look at. If you are going to do that, I strongly recommend looking up exactly how long the eclipse will last where you are, and setting a timer on your phone so that you can put your protection back on before direct sunlight starts peaking past the moon.

In space exploration, the big thing these days is the James Webb Space Telescope. Will there be any significant events related to that this summer?

The James Webb Space Telescope (JWST) has already released some images that have excited the astronomical community. However, these pictures have all been calibration images; as stunning as they have been, none of them are at the quality that JWST will be capable of. Once the calibration is complete, JWST will start taking scientific pictures in earnest, probably in early July. 

Technically, most of the pictures taken by JWST are only available to the researcher(s) that requested them for a period of 12 months. However, the Space Telescope Science Institute has set aside some time during the first few months of operations for pictures that will be released to the public as early as possible. These pictures will include Jupiter and the Jovain moons, black holes, exoplanets, and forming stars. It is also not uncommon for researchers to publicly release an exceptional example of their exclusive pictures.

Later in the summer we may get to see the launch of the Artemis I mission. The purpose of the Artemis program is to get humans back to the moon, with the medium term goal of a permanent habitat on the surface of the moon, and a long term goal of getting humans to Mars. The Artemis I mission will not be crewed; it will spend a month in space to test every part of the ship that will eventually carry the Artemis astronauts to the moon.

Dr. Larry Camarota graduated with a B.S. in physics and a B.S. in aerospace engineering from the University of Florida. He earned a Ph.D. in physics with a focus on astronomy from the University of Arizona. He is a member of the American Association of Physics Teachers, the Royal Astronomical Society, and the Friends Society. His areas of interest are astrophysics, cosmology, electronics, and wavelets.