What ARE they and why are they so special?
—Milpitas, CA
The longest day of the year in the Northern Hemisphere is the summer solstice, which occurs for approximately 24 hours between June 20th and June 22nd every year. 2021’s summer solstice starts at 8:31 P.M. PST on June 20th.
Since solstices are based on the opposing Northern and Southern hemispheres, the winter solstice in the Northern Hemisphere is the same as the summer solstice in the Southern Hemisphere and vice versa.
During this time, the Earth's axis of rotation will incline 23.4 degrees relative to its orbit around the sun, resulting in solstices as sunlight reaches earth more directly. This is why it is officially known as the start of summer. It is also known as the day that receives the most sunlight during the year. This phenomenon even occurs outside of Earth. Scientists determine seasons on other planets using their solstices, too! For example, the next solstice marking Mars's summer is on August 25, 2021.
The shortest day of the year in the Northern Hemisphere is known as the winter solstice. In 2021, it is on December 21st at approximately 7:58 AM PST. In contrast to the summer solstice, this day receives the least amount of sunlight out of all the days of the year.
During the solstices, the sun travels its longest or shortest path depending on the season. As it travels, it appears to stop before changing direction, thus, the word “solstice” comes from the latin word “sol,” which means sun, and “sistere,” meaning “to stand still.”
In the Northern Hemisphere’s summer solstice, the entirety of the top of the northern hemisphere tilts towards the sun, receiving 24 hours of daylight, while the tip of the southern hemisphere tilts away, receiving no sunlight. Similarly, the Northern Hemisphere’s winter solstice where the tip will receive no sunlight marks the beginning of the Southern Hemisphere’s summer solstice, which simultaneously receives 24 hours of sunlight.
Regardless of how the solstices relate to the sun, they do not correlate with the sunset and sunrise timing. Since our planet travels around the sun, the solstices are unrelated to Earth's distance from the sun as well as the coldest and hottest days. Instead, the solstice simply marks a specific time of the year that receives the most or least sunlight and when the seasons start.