Today is the 2018 summer solstice, which marks the longest day of the year and the official start of the summer season for those of us in the northern hemisphere. Many of us in the Midwest enjoy celebrating the occasion by going outside to fire up the barbecue, soak up the sun, and enjoy some much needed vacation time in our flip flops and swim trunks. Any Pagan readers out there most likely recognize the June solstice as an event of great spiritual significance with a number of associated rituals and religious practices. But for most of us, it’s just a warm weekday toward the end of June and an opportunity to spend more time outside. Back in March, we discussed the science behind the spring equinox. But what happens (astronomically speaking) on the summer solstice? Here are the most important facts to know about the longest day of the year.
Seasons are caused by Earth’s tilt
It’s easy to assume that we experience summer when our planet is closest to the Sun. But in reality, Earth makes its closest approach in January! So even though residents of the southern hemisphere do experience summer weather at this time (more on that in a bit), the changes in Earth’s distance from the Sun across the year are too small to influence our seasons.
Over the course of the year, our planet orbits along a flat, nearly circular path around the Sun. Additionally, as we already discussed in our post back in March, Earth also spins around its own axis, which passes through the north and south poles and is tilted at an angle of 23.4° with respect to its orbit. As the planet circles the Sun, this axis always points in the same direction. Because of this, the North pole will tilt toward the Sun on the summer solstice, and then six months later, it will point away from the Sun on the winter solstice. Consequently, we have more hours of daylight during the summer when we’re facing the Sun, and fewer during the winter. Additionally, the sunlight we do receive is more intense during this time. Between the solstices, we have the equinoxes, which mark the first days of fall or spring, when we have roughly equal hours of day and night because the poles are not tilted at all with respect to the Sun.
During the June solstice, days last a full 24 hours anywhere north of the Arctic Circle, and the Sun’s path in the sky peaks directly overhead for places along the Tropic of Cancer.
For those of us who live far away from the equator, summer has longer hours of daylight that allow us to spend more time outside at barbecues and picnics. The farther north you go, the longer the summer days become: In the Arctic, the Sun never actually sets during these months! If you lived all the way at the poles, you’d only see the Sun rise and set once per year (at the spring and fall equinoxes, respectively) and you’d see permanent daylight or night for the rest of the year. Anyone who has traveled way up north during the summer knows that a good pair of opaque window shades are an absolute necessity for sleep!
Scientifically, though, why does this happen? The answer again relates to the geometry of our planet’s tilt. Earth’s equator sits at 0° N/S in latitude, and the north pole is at 90°N. Since the northern hemisphere angles toward the Sun at an angle of 23.4° on June 21, any location within 23.4° of the north pole, or above
90°N – 23.4° = 66.6°N
will be in permanent daylight during the summer solstice. This latitude defines the Arctic Circle (or the Antarctic Circle in the southern hemisphere).
For those who live in the tropics, summer can be unbearably hot. Why is this the case, though, if days are not as long closer to the equator? As was briefly alluded to in our equinox post, any location near a latitude of 23.4°N (such as Cabo San Lucas, Mexico) will be facing directly toward the Sun at solar noon on the summer solstice, meaning that the Sun will appear directly overhead in the sky at that time. This latitude is called the Tropic of Cancer (or the Tropic of Capricorn in the southern hemisphere). When the Sun is high in the sky, its rays are more intense because they strike the Earth’s surface more directly. As a result, places near the Tropic of Cancer are some of the warmest during the summer. Since the Sun is directly overhead on the summer solstice in the tropics, this means that your shadow completely disappears!
In the southern hemisphere, seasons are reversed
Given the planet’s tilt, one pole facing toward the Sun naturally means that the other is facing away. As a result, residents of the Northern and Southern hemispheres experience opposite seasons. In places like Australia, the summer solstice is in December, on the same day as our winter solstice here in the northern hemisphere. For this reason, it is common for residents down under to hit the beach on Christmas. Additionally, some species of migratory birds take advantage of the seasonal differences between hemispheres to avoid winter altogether; the Arctic tern actually migrates from the Arctic to the Antarctic and back, so it experiences two summers per year!
Those of us who live near the Great Lakes may have seen these birds during their northward journey in the spring. Now that it is summer here, most of Antarctica is currently under permanent nightfall since the south pole is tilted away from the Sun, and regions beyond 66.6°S are receiving no sunlight. For seabirds like the Arctic tern, these dark, frigid conditions are unsuitable for breeding (or for living), forcing them to make the arduous trek from pole to pole twice each year. If only it were feasible for us to travel across the globe to avoid winter!