Why the June solstice was the key to measuring Earth 2,200 years ago
June 2023 offers a celestial spectacle as Mars and Venus converge in the evening sky. This guide highlights the month's best planetary alignments, stellar constellations, and the historical significance of the solstice, which once allowed an ancient scholar to calculate the circumference of our planet with remarkable precision.
The June solstice, occurring on the 21st, marks the longest day of the year in the Northern Hemisphere and the shortest in the Southern. Beyond its seasonal role, this event holds a unique place in scientific history. Approximately 2,200 years ago, the scholar Eratosthenes used the solstice to determine Earth's size. By measuring the difference in shadow lengths cast by poles in two cities separated by 800 kilometers, he calculated Earth's circumference to be about 40,000 kilometers—a figure that matches modern measurements. He also successfully calculated the tilt of Earth's axis, which drives the seasonal cycle.
Skywatchers this month can observe Mars passing through the Beehive Cluster (M44) early in June, while Venus and Mars draw closer together in the west throughout the month. In the morning, Saturn rises around midnight, followed by Jupiter. Overhead, the bright stars Arcturus and Spica dominate the evening, joined by the rising Summer Triangle of Vega, Deneb, and Altair.
