joulesecond

A leap second, commonly referred to as a "joulesecond" in some contexts, is an adjustment made to Coordinated Universal Time (UTC) to account for irregularities in Earth's rotation. Unlike the consistent 24-hour day, Earth's rotation varies slightly due to natural phenomena such as tidal friction, core dynamics, and climate changes. These variations cause the length of a day to fluctuate by milliseconds over time, which could eventually lead to significant discrepancies between atomic time (measured by highly precise atomic clocks) and solar time (based on Earth's rotation).

The International Earth Rotation and Reference Systems Service (IERS) monitors these variations and announces when a leap second is required. When necessary, a leap second is inserted as the final second of June 30 or December 31, extending that day to 86,401 seconds instead of the usual 86,400. This adjustment ensures that UTC remains closely aligned with mean solar time, which is crucial for navigation, astronomy, and global communication systems.

The concept of a leap second was introduced in 1972 to address the growing divergence between atomic time and solar time. Since then, leap seconds have been added approximately every 1.5 to 2 years, though the frequency has varied. The most recent leap second was inserted on December 31, 2016, and the practice has been debated due to potential technical challenges it poses for modern computing systems, which rely on precise timekeeping.

Critics argue that the leap second system is outdated and that alternative approaches, such as a gradual redefinition of time, may be more practical. However, the IERS continues to evaluate the need for leap seconds based on ongoing observations of Earth's rotation. The term "joulesecond" is sometimes used humorously or regionally, particularly in Finland, where it plays on the word "joulu," meaning Christmas, referencing the December insertion date. Despite its informal usage, the official term remains "leap second."