Original photo by ju_see/ Shutterstock
The solstice in June marks the longest day of the year and the official beginning of summer for the Northern Hemisphere. This celestial event has been marked by humanity for at least 7,000 years (it likely helped our Neolithic ancestors chart the growing season), and in fact it could be one of humanity’s earliest astronomical discoveries. Today, scientists understand much more about the complicated celestial mechanics that create the solstice every year around June 20 to 21, but these six facts explore some of the lesser-known aspects of this amazing astronomical event.
The Summer Solstice Isn’t an Entire Day, But a Precise Moment
Although we often think about the summer solstice as an entire day, the solstices (meaning “sun standing still” in Latin) are actually precise moments. The solstice in the Northern Hemisphere marks the exact minute when the sun reaches its max northerly ascent, stopping (or seeming to stand still) directly above the Tropic of Cancer. This specific latitude is located roughly 23.5 degrees above the equator. The winter solstice similarly marks the end of the sun’s southern journey above the Tropic of Capricorn (also 23.5 degrees away from the equator). In 2025, in the Northern Hemisphere, the summer solstice will occur on Friday, June 20, at 10:42 p.m. ET (or Saturday, June 21, at 2:42 a.m. UTC).
The Latest Sunset Actually Occurs the Week After the Solstice
People know that the June solstice is the longest day in the Northern Hemisphere (and the shortest day down south), but because of differences between solar time and clock time, the latest sunset actually occurs throughout the following week. During the vernal and autumnal equinoxes, the solar day is actually shorter than 24 hours, and during the solstices, the 24-hour day is slightly longer. Due to this small difference, as well as the planet’s tilt and elliptical orbit, the latest sunsets occur on different days after the summer solstice, depending on latitude. At 40 degrees north (around Philadelphia), the latest sunset occurs on June 27, while farther north near Seattle, the latest sunset is around June 25. The same is true with the winter solstice, as the earliest sunset occurs weeks before that date.
The Egyptians Were the First To Have Some Knowledge of the Solstice
Located about 550 miles south of Egypt’s Pyramid of Giza is the world’s oldest astronomical site: the Nabta Playa. Built around 7,000 years ago — around two millennia before Stonehenge — it’s the first known evidence that Neolithic humans had some understanding of the summer solstice. Believed to be constructed by a cattle-worshipping cult that predates even the pharaohs, this incredible megalith monument appears to track both monsoon season and the arrival of the summer solstice. The stones were only discovered in 1973, when a Bedouin named Eide Mariff found them and showed the discovery to American archaeologist Fred Wendorf. Wendorf first thought the stones were natural, but then realized that their location on an ancient lakebed should have worn them away, meaning they were placed there purposefully. Archaeoastronomers eventually discovered the stones would’ve aligned with certain constellations around 4800 BCE. At that time, the arrival of the solstice marked the beginning of the monsoon season, which brought rains that were vital to the survival of the Nabta Playa people.
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The Earth Is Farthest From the Sun Around the Summer Solstice
It may seem counterintuitive, but the summer solstice is around the same time the Earth is farthest from the sun. Contrary to grade school illustrations of the solar system, the Earth is actually in an elliptical orbit around the sun, and throughout the year its distance from our host star can vary by some 3 million miles. Seeing as the Earth is 93 million miles away from the sun on average, this may not seem like much, but scientists estimate that the Earth experiences a 4-degree Fahrenheit total temperature change between the two astronomical extremes. On July 6 (at least in 2023), the Earth experiences its aphelion, or its farthest distance from the sun, and on January 3 (in 2024), the planet reaches its perihelion, its closest approach.
The Hottest Days May Arrive Weeks After the Solstice
Longer days mean more sunlight, and more sunlight means higher temperatures, right? Well, not exactly. Even though the days get shorter and shorter following the solstice, the temperatures only climb in the Northern Hemisphere due to what scientists call “seasonal lag.” The hottest temperature for any given area is largely due to differences in heat capacity (how much heat it takes to warm something up) of land and water. Water takes longer to heat than land, so San Francisco, which is surrounded by water on three sides, doesn’t experience its hottest days until September, whereas cities like El Paso actually do sweat through their hottest days near the solstice. This is also why islands in the Pacific and cities along the coast feel less temperature change between seasons, while deserts experience the opposite. The hottest day in a particular region or city can also be influenced by humidity, summer storms, and even human irrigation.
People Still Celebrate the Summer Solstice at Stonehenge
There are some 35,000 megaliths spread throughout Europe. Some are as small as a single stone, while others are massive structures of mind-boggling complexity. But none of these impressive, ancient monuments is as well known and beloved as Stonehenge, the famous solstice calendar of antiquity nestled in the hills near Salisbury, England. For thousands of years, druids, pagans, and other peoples who used the site gathered to celebrate the solstice, and it’s a tradition that continues to this day. Every year, thousands gather at Stonehenge to watch the sun rise on the summer solstice. On the morning of June 21, the sun will rise behind the Heel Stone and illuminate Stonehenge in a dazzling display (unless it’s a cloudy day). Although the monument closed its doors to visitors during the pandemic, locals, pagans, and many others were welcomed back in 2022 to walk among the ancient stones once again. After watching the sun rise on the Earth’s longest day, one visitor told the BBC, “It’s good to be back.”
Darren Orf lives in Portland, has a cat, and writes about all things science and climate. You can find his previous work at Popular Mechanics, Inverse, Gizmodo, and Paste, among others.
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