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Celebrating the Summer Solstice: June 20, 2024

Yesterday marked the summer solstice, occurring precisely at 4:51 PM EDT, signaling the official start of astronomical summer in the Northern Hemisphere. As the Earth’s axial tilt reached its maximum inclination toward the sun, regions north of the Tropic of Cancer experienced the longest day of the year, with the sun at its highest point in the sky.

Diagram illustrating the Earth's axial tilt during different seasons - courtesy of the NWS
Diagram illustrating the Earth’s axial tilt during different seasons – courtesy of the NWS

Understanding the Summer Solstice

The summer solstice is a pivotal astronomical event when the sun reaches its northernmost position relative to Earth’s equator, directly over the Tropic of Cancer at 23.5° latitude north. This geographic line intersects various countries, including Mexico, Egypt, India, and China, marking the farthest point north where the sun appears directly overhead.

Longest Daylight Hours and Cultural Traditions

On this day, locations north of the Tropic of Cancer enjoyed extended daylight hours, characterized by the sun’s highest elevation and minimal changes in its noontime position for several days surrounding the solstice. This phenomenon provides ample daylight for summer activities and has been celebrated across cultures throughout history, reflecting on the sun’s power and the abundance it brings during the peak of the growing season.

Solstice Insights and Meteorological Implications

Solstices occur twice yearly, marking the changing of seasons and featuring the year’s shortest and longest daylight hours depending on your hemisphere. These extremes in the length of day and night make solstice days more noticeable to many observers than the subtle equality of day and night experienced during equinoxes. Solstices were some of our earliest astronomical observations, celebrated throughout history via many summer and winter celebrations.

In 2024, solstices occur on June 20 at 4:51 PM EDT (20:51 UTC) and December 21 at 4:19 AM EST (9:19 UTC). The June solstice marks the moment when the Sun is at its northernmost position in relation to Earth’s equator, and observers outside of the tropics experience the longest amount of daylight for the year. Conversely, during the December solstice, the Sun is at its southernmost position, and observers outside of the tropics experience the least amount of daylight and the longest night of the year.

Zero Shadow Days and Observations

While solstice days are very noticeable to observers in mid to high latitudes, they bring a different phenomenon to observers in the tropics—zero shadow days. These occur twice yearly when the Sun is directly overhead at solar noon, causing objects to cast minimal shadows. Observers near the Tropic of Cancer experience a zero-shadow day on the June solstice, while those near the Tropic of Capricorn experience it on the December solstice. Equatorial regions experience zero-shadow days on the March and September equinoxes.

Scientific Insights and Historical Observations

The solstices not only mark seasonal changes but also provide opportunities for scientific observations. In ancient times, observations of solstice shadows contributed to significant discoveries about Earth’s shape and dimensions. For instance, around 200 BC, Eratosthenes calculated Earth’s circumference by measuring solstice shadows in Alexandria and Aswan, demonstrating early evidence of Earth as a sphere.

So celebrate the summer solstice and its astronomical significance by enjoying the extra summer sun while we have it, until December 21st, 2024.