What is the summer solstice? How is it linked to the seasons, the famous Stonehenge monument, and the Strawberry Moon?
Although the temperatures outside may make it feel otherwise, summer has only just begun, at least in the northern hemisphere. Every year, the summer solstice, on or near June 20th, marks the longest day of the year and the beginning of summer for those above the equator.
How is the summer solstice linked to the seasons, the famous Stonehenge monument, and what’s called a Strawberry Moon?
What Causes the Seasons?
Our four seasons, and the associated temperature differences experienced in many parts of the world, are all due to a 23.5 degree tilt in the Earth’s rotation axis. Contrary to a still commonly held misconception, the seasons are not due to differences in the Earth-Sun distance. In other words, we do not experience colder winter temperatures because the Earth is farther from the Sun in a noncircular orbit.
It is true that the Sun (in combination with our atmosphere) is our heat source, and so in very simplified circumstances, the Earth would be hotter overall when closer to our star. However, the Earth’s orbit ultimately does not vary in distance by enough to cause much of a noticeable difference in temperature. In fact, our planet is actually at its closest to the Sun during the winter months in the northern hemisphere. This orbital distance-related reasoning would also not be able to explain why the northern and southern hemispheres experience opposing seasons at the same time.
Instead, the seasons arise from the slight tilt in the Earth’s rotation axis. The Earth spins on its own axis while traveling in orbit around the Sun, but that spin axis is not perfectly perpendicular to the plane of its orbital path (called the ecliptic). This lean causes different portions of the Earth to receive more direct sunlight for a longer period of time each day at different times throughout the year. The northern hemisphere is tilted toward the Sun during the months of June, July, and August and so receives more direct sunlight in what we northerners call the summer.
So what caused this tilt? We are still putting together the pieces of our understanding of planet formation, but we suspect it was a very violent process. As stars form from collapsing clouds of gas and dust, initially small chunks of debris will grow larger as their gravitational pull assists the accumulation of more material. A collision with another significantly-sized proto-planet or proto-moon could knock off kilter a planet in the process of forming.
In fact, just look at Uranus—this gas planet lies almost completely on its side at a 98-degree angle with its orbital path. A year on Uranus, or the time it takes the planet to make one lap around the Sun, is roughly equal to 84 Earth years. Thus, someone residing on the pole of Uranus would experience 42 years of summer while pointed almost directly toward the Sun, followed by 42 years of winter while pointed away. The less extreme tilt on our home planet makes our earthly temperature fluctuations much more hospitable to life like us.
What Are the Summer Solstice and the Strawberry Moon?
On the summer solstice, the longest day of the year in the northern hemisphere, the northern pole is pointed as directly toward the Sun as it can get, which results in the pole receiving 30% more sunlight than the equator.