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This December will offer a unique astronomical spectacle that only occurs every 18.6 years and will reach its fullest on Sunday 15 December. This phenomenon is known as Lunasticism, a term that comes from Latin and means "still Moon" because, for several days, the Earth's satellite seems to stop moving, although this is not the case.
The Lunastice began at the last autumn equinox (in September) and will end at the spring equinox in 2025 (in March). But, as its visibility depends on the lunar phase in which it is observed, this Sunday will be the ideal time to enjoy this phenomenon as it coincides with the last full moon of the year, the so-called cold moon. It so called because the native North American peoples related it to the longest nights and lowest temperatures in the northern hemisphere. Its particularity is that, in this last full phase of the year, is when the Moon reaches its highest elevation in the night sky.
But to return to the Lunasticism, to understand why it happens and why every 18.6 years, we must take into account the movements and degrees of inclination of both the Earth with respect to the Sun and the Moon with respect to the Earth. The inclination of the respective rotation axes of the planets orbiting the Sun varies. The Earth's is tilted by 23.5 degrees. In turn, the rotation axis of the Moon is also tilted with respect to the ecliptic on which the planets orbit the Sun, namely by 5.1 degrees. As a result, the boundaries of the Moon's rising and setting change over a period known as nodal precession, which takes 18.6 years to complete.
Just at that moment, which is known as the Lunastix, the Moon's rising positions are as far north as possible and its setting positions are as far south as possible, i.e. their separation is greatest, it occurs at their most extreme points. This is the reason why the Moon appears to stop its motion, the so-called "lunar standstill". If this effect is combined with the spectacular nature of the cold moon, the phenomenon takes on greater magnitude.
The best time to see it is expected to be during the night of Sunday 15 December. It is recommended to look for an elevated location with good visibility and clear skies, away from sources of light pollution.
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