![]() The time it takes for the Earth to orbit the sun is approximately 365.25 days, meaning we have an extra day every four years. This also affects the onset of the seasons over the length of a year as part of this 26,000 year cycle, and therefore has important implications for anyone attempting to attribute any cultural significance to a particular point in a given season. Given that we can predict this motion, we know that 13,000 years from now our north star will be Vega, in the constellation Lyrae. In 3,000 BC, the celestial north was the star Alpha Draconis (Thuban), in the constellation Draco. A visible example of this on a smaller scale is shown several times during the film Inception, where the precession of a spinning top was used to determine whether the main character was in reality, or still dreaming.įor the Earth, this precession traces out a circle on the sky once every 26,000 years (see image below). The result on Earth is called the precession of the equinoxes – a phenomenon which affects our observations of the stars. Given that the Earth is already spinning, any force applied to it, such as gravity from the moon or other bodies in the solar system, will modify this rotation (known as torque). The rotation of a spinning object, like the Earth, can be affected by external forces. Currently, our celestial north is marked by the position of the star Polaris. Interestingly, the stars appeared to have moved position by the same amount, and he realised that the location of north in the sky must have moved in the intervening centuries. After compiling his catalogue, he noticed that the position of the stars had changed from those in much earlier records, such as the Babylonian. Hipparchus, one of the founders of modern astronomical techniques, wrote one of the first comprehensive star catalogues in 129 BC. ![]() These variations have been known about since the time of the ancient Greeks. It also causes the sun to rise later and set earlier, giving us fewer hours of sunlight and colder temperatures.Īs it happens, the direction of the Earth’s tilt changes over time. In winter, the Earth’s tilt away from the sun causes sunlight to be spread out over a larger surface area than in summer. ![]() The winter and summer solstices, and the vernal and autumnal equinoxes, are the extreme points in each of these seasons (see image). Tilting planetīut what causes the winter solstice? Our planet has an axial tilt (of 23.4°) with respect to its orbital plane around the sun, which results in the seasons. The pagan Anglo-Saxon event known as Yule was in full swing during the winter solstice a few centuries after that, eventually evolving into the festival we now know as Christmas. The last day of Saturnalia was referred to as the “dies natalis solis invicti” (birthday of the unconquered sun) by the Romans, who celebrated it by giving gifts to each other on December 25. Millennia later, the Romans celebrated Saturnalia (until the fourth century AD) – a festival over the week of the winter solstice dedicated to the god Saturn, involving games and merriment. This focus on the winter solstice was an important time marked by feasting and possibly animal sacrifice. Stonehenge and the nearby Neolithic site of Durrington Walls (circa 2,500 BC) were each built to be orientated to face the midwinter sunset and sunrise respectively. This is the longest night of the year – once celebrated as “ Yule” by the pagan people of Northern Europe before it became Christmas. We are fast approaching the winter solstice for the Northern hemisphere, which takes place on December 21. From the Neolithic to present times, the amount of sunlight we see in a day has had a profound impact on human culture.
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