Today marks Tallinn's longest day for the next 20,000 years

21 June 2026 is not only this year's longest day but also, scientifically speaking, the longest day for the next 20,000 years. The reason lies in the slow decrease of Earth's axial tilt, which is part of the Milankovitch cycles. In Tallinn, the longest day shortens by an average of only about 0.15 seconds per year.

Today, 21 June 2026, is in Tallinn and across the northern hemisphere not only this year's longest day but, from a scientific perspective, also the longest day over the next 20,000 years, provided the observer's geographical location remains unchanged.

Earth's axial tilt decreases slowly

Behind this surprising fact lies one of the Milankovitch cycles, an approximately 40,000-year cycle during which Earth's axial tilt oscillates between roughly 22.1° and 24.5°. Currently, the axial tilt is in a declining trend, meaning that as time progresses, Earth's tilt relative to its orbit decreases, and the extra hour of daylight that illuminates the summer nights grows shorter.

The greater the axial tilt, the brighter the summer night on the northern hemisphere's higher latitudes. If there were no axial tilt at all, everywhere on Earth would have exactly 12 hours of night and 12 hours of day year-round, with no white nights or long winter darkness.

The change is imperceptible to humans

In practical terms, this change is completely unnoticeable on the human timescale. In Tallinn, the year's longest day shortens by an average of only about 0.15 seconds per year. Even over a century, the accumulated difference would be only several dozen seconds. On Earth's surface, such an axial tilt change corresponds to roughly a 13-metre shift per year, which sounds impressive as a number but is negligible on a planetary scale.

Within the long-term trend, there is, however, a smaller oscillation: graphs reveal an approximately 19-year fluctuation, caused by a peculiarity of the Moon's orbit. The Moon's gravitational pull affects the position of Earth's rotation axis and creates long-term small variations in the duration of the longest day.

When will the turning point come?

In approximately 9,800 years, Earth's axial tilt will begin to increase again. Returning to the current state will take almost the same amount of time, so about 20,000 years in total. Only then will some future summer day surpass today's record.