tähtäinvuonna

Tähtäinvuonna, also known as the "star year" or "astronomical year," refers to the time it takes for the Earth to complete one full orbit around the Sun relative to the fixed stars. This period is distinct from the tropical year, which measures the time between successive vernal equinoxes, and is slightly longer due to the gradual precession of the equinoxes.

The tähtäinvuonna is approximately 365.25636 days long, or about 365 days, 6 hours, 9 minutes, and 9.5 seconds. This duration is determined by observing the positions of distant stars against the backdrop of the celestial sphere, as their apparent positions remain nearly fixed over human timescales. The concept is rooted in ancient astronomy, where civilizations such as the Babylonians and Egyptians tracked celestial cycles for calendrical and agricultural purposes.

Unlike the tropical year, which forms the basis of the Gregorian calendar, the tähtäinvuonna is not directly used in modern timekeeping. However, it remains a fundamental reference in astronomy for understanding Earth's orbital mechanics and long-term celestial alignments. The slight discrepancy between the tähtäinvuonna and the tropical year accumulates over centuries, contributing to the gradual shift in the timing of seasonal events.

In Finnish astronomy, the term *tähtäinvuonna* is occasionally used to emphasize the distinction between the orbital period relative to the stars and the shorter tropical year. While not commonly discussed in everyday contexts, it plays a role in advanced astronomical studies and historical calendrical research.