Solstice: When is it, and what causes it?
The solstices occur on two particular days each year and mark either the start of winter or the start of summer. On the winter solstice, the Sun’s position at noon is the lowest of the year, and the Sun spends the least time above the horizon. On the contrary, the summer solstice is the day when the Sun climbs to its highest point and spends the longest above the horizon.
Contents
- What is a solstice?
- Winter and summer solstices
- What are the dates of each solstice?
- Special circles of latitude
- Ancient sites aligned with solstices
- Common questions related to solstices
- When is the longest day of the year?
- When is the shortest day of the year?
- What are June 21 and December 21 called?
- What is the difference between a solstice and an equinox?
- Why is the hottest time not during the summer solstice?
- The shortest day of the year 2024, how long is it?
- How long is the longest day of the year 2025?
- Bottom line
What is a solstice?
Solstices occur because the Earth’s axis of rotation is tilted at about 23.5° relative to the Earth’s orbit around the Sun. The moment when the Sun reaches its most northerly or southerly excursion from the celestial equator is called a solstice. Although a solstice is actually defined as an exact instant on a particular day, it’s still correct to talk about solstices in terms of days.
Winter and summer solstices
There are two solstices each year: winter solstice and summer solstice. From the winter solstice to the summer one, the Sun gradually climbs higher in the sky at noon, spending a little bit more time above the horizon each day. On the summer solstice, this gradual lengthening of days stops, and from then on, the Sun rises lower and lower at noon each day, and the days shorten. This continues until the winter solstice when the Sun starts to climb again.
In the Northern Hemisphere, winter solstices happen in December, and summer solstices in June.
In the Southern Hemisphere, winter solstices happen in June, and summer solstices in December.
What happens on the winter solstice?
When the winter solstice happens in the Northern Hemisphere, the North Pole is tilted about 23.5° away from the Sun. The Sun’s rays are shifted southward from the Equator by the same amount so that the vertical noon rays are directly overhead at the 23.5°S latitude.
Six months later, the South Pole becomes inclined about 23.5° away from the Sun. This day is the winter solstice in the Southern Hemisphere. The Sun’s vertical overhead rays progress to their northernmost position at the 23.5°N latitude.
The winter solstice marks the shortest day of the year and the start of winter.
What happens on the summer solstice?
When the summer solstice happens in the Northern Hemisphere, the North Pole is tilted about 23.5° toward the Sun. Because the Sun’s rays are shifted northward from the Equator by the same amount, the vertical noon rays are directly overhead at the 23.5°N latitude.
Six months later, the South Pole becomes inclined about 23.5° toward the Sun. This day is the summer solstice in the Southern Hemisphere. The Sun’s vertical overhead rays progress to their southernmost position at the 23.5°S latitude.
The summer solstice marks the longest day of the year and the start of summer.
What are the dates of each solstice?
Solstices occur twice a year:
- The first one occurs between June 19 and 22. This century, there are 47 solstices on June 20 and 53 solstices on June 21. That’s why most sources say that June solstices fall on June 20 or 21. The next time the solstice falls on June 22 is in the year 2203; the next June 19 solstice is in the year 2488.
- The second solstice of a year happens between December 20 and 23. This century, there are five solstices on December 20, 82 solstices on December 21, and 13 solstices on December 22. The next time the solstice falls on December 23 is in the year 2303; the next December 20 solstice is in the year 2080, but that doesn’t prevent many articles from claiming that December solstices fall on December 21 or 22.
When is the winter solstice?
In the Northern Hemisphere, the winter solstice will occur on December 21, 2024 at 09:20 GMT (December 21, 04:20 a.m. EST). In the Southern Hemisphere, it will occur on June 21, 2025 at 02:42 GMT (June 20, 9:42 p.m. EDT).
When is the summer solstice?
In the Northern Hemisphere, the summer solstice will occur on June 21, 2025 at 02:42 GMT (June 20, 9:42 p.m. EDT). In the Southern Hemisphere, it will occur on December 21, 2024 at 09:20 GMT (December 21, 04:20 a.m. EST).
Dates of upcoming solstices
- December 21, 2024, 09:20 GMT (4:20 a.m. EST)
- June 21, 2025, 02:42 GMT (June 20, 10:42 p.m. EDT)
- December 21, 2025, 15:03 GMT (10:03 a.m. EST)
- June 21, 2026, 08:25 GMT (4:25 a.m. EDT)
- December 21, 2026, 20:50 GMT (3:50 p.m. EST)
- June 21, 2027, 14:11 GMT (10:11 a.m. EDT)
- December 22, 2027, 02:43 GMT (December 21, 9:43 p.m. EST)
- June 20, 2028, 20:02 GMT (4:02 p.m. EDT)
- December 21, 2028, 08:20 GMT (3:20 a.m. EST)
- June 21, 2029, 01:48 GMT (June 20, 9:48 p.m. EDT)
- December 21, 2029, 14:14 GMT (9:14 a.m. EST)
- June 21, 2030, 07:31 GMT (3:31 a.m. EDT)
- December 21, 2030, 20:09 GMT (3:09 p.m. EST)
- June 21, 2031, 13:17 GMT (9:17 a.m. EDT)
- December 22, 2031, 01:56 GMT (December 21, 8:56 p.m. EST)
- June 20, 2032 19:09 GMT (02:09 p.m. EDT)
Special circles of latitude
Circles of latitude are imaginary east-to-west circles that connect all places with the same latitude. The reference plane for all circles of latitude is the equatorial circle of latitude that bisects the Earth perpendicular to its axis of rotation. There are four more special circles of latitude that have a unique relationship with solstices.
The Tropic of Cancer
The Tropic of Cancer (or the Northern Tropic) is the northernmost latitude where the Sun passes directly overhead in its most vertical position at noon. This happens on the summer solstice for the Northern Hemisphere in June. The tropic got its name thousands of years ago when the Sun was in the constellation Cancer at the solstice. Since then, due to the Earth’s axial precession, the Sun passed Gemini and came into Taurus in 1989.
Currently, the Tropic of Cancer is positioned at 23°26’14” N of the Equator. The latitude equals the current axial tilt of the Earth.
The Tropic of Capricorn
The Tropic of Capricorn (or the Southern Tropic) is the southernmost latitude where the Sun passes directly overhead in its most vertical position at noon. This happens on the summer solstice for the Southern Hemisphere in December. The tropic got its name thousands of years ago when the Sun was in the constellation Capricornus at the solstice. Since then, due to the Earth’s axial precession, the Sun came into Sagittarius in 130 BC and is expected to pass into Ophiuchus in 2269.
Currently, the Tropic of Capricorn is positioned at 23°26’14” S of the Equator. The latitude equals the current axial tilt of the Earth.
The Arctic Circle
The Arctic Circle is the southernmost latitude where the Sun doesn’t rise on the winter solstice for the Northern Hemisphere, which happens in December. It is also the southernmost latitude where the Sun doesn’t set on the summer solstice for the Northern Hemisphere in June.
Currently, the Arctic Circle is positioned at 66°33’46” N of the Equator. This latitude equals 90° minus the current axial tilt of the Earth.
The Antarctic Circle
The Antarctic Circle is the northernmost latitude where the Sun doesn’t set on the summer solstice for the Southern Hemisphere, which happens in December. It is also the northernmost latitude where the Sun doesn’t rise on the winter solstice for the Southern Hemisphere in June.
Currently, the Antarctic Circle is positioned at 66°33’46” S of the Equator. This latitude equals 90° minus the current axial tilt of the Earth.
Ancient sites aligned with solstices
Stonehenge
Scientists believe that Stonehenge was specifically designed to line up with solstices. If you stand at the center of Stonehenge on the summer solstice, you’ll see the Sun rising exactly behind the monument’s Heel Stone.
The Great Pyramids of Giza
In Egypt, the Great Pyramids at Giza appear aligned with the Sun. When viewed from the Sphinx, the Sun sets precisely between the pyramids of Khufu and Khafre during the summer solstice. What’s more, this spectacle forms an image that replicates the Egyptian hieroglyph Akhet, meaning “horizon.”
The Temple of the Sun
The Temple of the Sun was a sacred precinct built by the Incas to pay tribute and give offerings to the Sun inside Machu Picchu. During the sunrise on the summer solstice, when the very first light rises over the distant mountains, it shines through one of the two windows of the Temple of the Sun and illuminates the ceremonial stone within. The second window was for the Sun’s rays on the winter solstice.
Bonus: The well in Syene and the radius of the Earth
The summer solstice and the well in Syene were all it took for Eratosthenes, the chief librarian of the great library of Alexandria, to calculate the radius of the Earth in 240 BC.
Eratosthenes had heard from travelers about a well in Syene (now Aswan, Egypt) with an interesting property: at noon on the summer solstice, the Sun's rays shone directly down a well, casting no shadow at all. From this, he concluded that the Sun was directly overhead at Syene. Eratosthenes then measured the shadow cast by a stick at noon on the summer solstice in Alexandria and found it made an angle of about 7.2° (1/50 of a complete circle).
Astronomers from 500 BC already knew that the Earth is spherical, so Eratosthenes realized that it was the angle subtended at the center of the Earth by radial lines drawn from both locations. All he had to do was find the distance from Alexandria to Syene and multiply it by 50 to get the Earth's circumference.
Eratosthenes probably hired someone to accomplish the task and found out that the distance between Alexandria and Syene was about 5000 stadia or 805 km (500 miles). Then he used a simple ratio method to calculate the circumference of the Earth: C = 805 * 360 / 7.2 = 40,250 km (25,000 miles).
After that, he calculated the Earth’s radius: r = C / 2π ≈ 6,405 km (3,979 miles). The modern value for the radius of the Earth is 6,371 km (3,958 miles).
Common questions related to solstices
When is the longest day of the year?
The day with the most hours of sunlight in a year occurs at the summer solstice. In the Northern Hemisphere, this takes place around June 21, and around December 21 in the Southern Hemisphere.
When is the shortest day of the year?
The day with the fewest daylight hours in a year occurs at the winter solstice. In the Northern Hemisphere, this takes place around December 21, and around June 21 in the Southern Hemisphere.
What are June 21 and December 21 called?
Two solstices occur annually, around June 21 and December 21. In the Northern Hemisphere, the June solstice is called the summer solstice, and the December solstice is called the winter solstice. And vice versa in the Southern Hemisphere.
What is the difference between a solstice and an equinox?
In short, solstices are the longest and shortest days of the year, while equinoxes are dates when day and night are equally long. You can learn more about equinoxes from our article on this topic.
Why is the hottest time not during the summer solstice?
The Earth’s land and oceans take time to heat up or cool down. Thus, although the summer solstice brings the longest day, the warmest temperature comes a month or two later. In science, this effect is called seasonal lag.
The shortest day of the year 2024, how long is it?
The winter solstice marks the 24-hour period with the fewest daylight hours of the year. The exact day length depends on the location of the observer. The farther the place is from the equator, the more the day length changes.
In the Northern Hemisphere, the shortest day of the year 2024 will occur on December 21. In Ka Lae, Hawaii, USA (18°54′N), there will be 10 hours, 59 minutes, and 40 seconds of daylight, while in Anchorage, Alaska, USA (61°13′N), there will be only 5 hours, 27 minutes, and 44 seconds. In the places within the Arctic Circle, the Sun will not rise at all — people will experience a polar night there.
In the Southern Hemisphere, the shortest day of the year 2025 will occur on June 21. In Futiga, American Samoa (14°22′S), there will be 11 hours, 17 minutes, and 03 seconds of daylight, while in Melbourne, Australia (37°48′S), there will be only 9 hours, 32 minutes and 27 seconds. In the places within the Antarctic Circle, the Sun will not rise at all — people will experience a polar night there.
How long is the longest day of the year 2025?
It depends on your location. For example, on June 21, 2025, in New York, the USA, there will be 15 hours, 5 minutes, and 38 seconds of daylight, while in Oslo, Norway, there will be 18 hours, 49 minutes, and 58 seconds. On December 21, 2025 — the longest day of the year in the Southern Hemisphere — people in Sydney, Australia, will experience 14 hours, 24 minutes, and 47 seconds of sunlight. Learn about the day length for your location with the Sky Tonight app. Open the app, and tap the calendar icon on the panel at the bottom of your screen; from there, select the Sky tab.
Bottom line
The next solstice will occur on December 21, 2024, at 09:20 GMT (04:20 a.m. EDT). In the Northern Hemisphere, it is the shortest day of the year, marking the start of winter. In the Southern Hemisphere, it is the longest day of the year, marking the start of summer.
Thanks for reading this article! If you have questions about solstices, please feel free to ask them on our social media. If you want to expand your astronomical knowledge, take our quiz on equinoxes and solstices. Happy solstice to all!
