The longest day of the year is the Summer Solstice. This year, the Summer Solstice falls on Sun., June 21. But what does that mean? And why does it happen?

When we say it’s the “longest day,” we really mean this is the day with the longest amount of sunlight in the Northern Hemisphere. The word “solstice” originates from the Latin term “solstitium,” which is derived from “sol”, meaning Sun, and “sistere”, meaning “to stand still.” The term denotes a point in which the Sun’s movement in the sky, as viewed from Earth, appears to pause before reversing its direction. On this day, the Sun’s declination, or the angular distance it is away from the celestial equator, reaches its highest point.

Though, of course, it is not the Sun that is moving, rather it’s the Earth that is orbiting the Sun. We are constantly in motion around the Sun, orbiting in the shape of an ellipse. Because of this, there are points along our orbit where we are closer in distance to the Sun than others.

An important contributor to the phenomenon of a solstice is the Earth’s axial tilt. Earth is tilted on its orbital axis by 23.5 degrees. As a result, Earth experiences seasonal changes. Our seasons are affected by the Sun’s influence on Earth. If we are in a position where we’re tilted closer to the Sun, we experience summer. If we are further from the Sun, we experience winter.

Every June, our planet reaches a point in its orbit where the Northern Hemisphere is tilted toward the Sun. This marks the very first day of summer in the Northern Hemisphere. The opposite is true for the Southern Hemisphere; they experience the first day of winter during the June solstice.

In the summer, you can expect to see the Sun for a longer amount of time as it arches high across the daytime sky. As we approach the winter months, we will see the Sun’s path get lower and lower toward the horizon. Each day will feel as though it is seeing less and less sunlight. It will reach its lowest point on the Winter Solstice in late December; this is known to be the shortest day of the year!

In Pittsburgh, the Sun will rise at about 5:47 a.m. and set at around 8:53 p.m., giving us over 14 hours of sunlight in a single day. Remember to enjoy the summer sunlight, because starting on Mon., June 22, the days will start to get shorter and shorter until we reach the dark skies of the Winter Solstice in December.