Why Is the Summer Solstice the Longest Day?

Depending on where you are (aka the Northern Hemisphere), the summer solstice typically takes place between June 20th and the 21st and is the longest day of the year. In the Southern Hemisphere, the longest day instead shifts to December 20th or the 21st. With all that in mind, you might be wondering what actually makes the summer solstice the longest day.

Earth is round and spins a lot

You’re probably aware that the Earth is both round and does quite a bit of spinning. The Earth rotates around its axis, and orbits around the Sun at the same time. At the same time, the Sun is also orbiting around the center of our galaxy, making our motion through space this kind of weird helix-spiral-thing. If you think about it, it makes teleportation as a superpower pretty impressive, since you’d have to math out where in space the Earth is going to be. 

Anyway, we can draw a line through the North and South poles of the Earth. The Earth rotates around this line, forming its axis of rotation. If you imagine the Solar System as a kind of “disc” with the Earth moving along it, Earth’s axis of rotation isn’t perpendicular. There’s roughly a 23.45 degree angle between Earth’s axis and plane of orbit around the Sun. This means that at any given time, one pole is facing the Sun more than the other. It’s also why if you track the Sun’s highest point in the sky each day, you’ll notice that it kind of forms a figure-eight in the sky. 

Except there’s more. The Earth isn’t a perfect sphere, in part thanks to the gravity of both the Sun and Moon acting on it. This also changes the Earth’s axis of rotation over time. You know how a top kind of wobbles before it falls over? Earth is doing that as it rotates but really slowly thanks to the gravitational pulls of the Sun and Moon. This means objects that appear fixed in the sky actually move about 0.014 degrees west to east each year, completing a full 360 degree cycle once every 26,000 years. 

Why doesn’t the solstice fall on the same day every year?

As a result of the Earth’s tilt relative to its rotation, one pole spends more time in front of the Sun than the other most of the time. That’s why solstices exist, and it’s why the summer solstice in the Northern Hemisphere lines up with the Southern Hemisphere’s winter solstice.

But you might be wondering why the solstice fluctuates between a window of about two days year on year.

The big reason is that years aren’t actually 365 days long. They’re about 365.24 days long—which is why we have leap years every four years.  The Earth doesn’t orbit around the Sun in a perfect circle, it orbits the Sun in an ellipse (which is kind of like an oval). This means that the Earth doesn’t orbit the Sun at a constant speed, hence the whole leap year thing.

You can push this to the extreme with Mercury, which occupies all sorts of weird extremes. It has a very elliptical orbit (much more than the Earth’s), and is super close to the Sun. Here’s a comparison, the Earth fluctuates between 91.4 and 94.5 million miles from the Sun in its orbit. This is a difference of about 3.34%. On the other hand, Mercury fluctuates between 28.5 million and 43 million miles from the Sun, a percent difference of 44.56%. 

Here’s a word ladder for those who are feeling the Summer Solstice.