To an observer in an inertial frame of reference, planet Earth is seen to rotate about an axis and revolve around the Sun in an elliptical path with the Sun at one focus. From a fixed position on the ground, the Sun appears to orbit around Earth. Relative velocity is the motion of an object from the point of view of an observer in a frame of reference. The Romans used "standing" to refer to a component of the relative velocity of the Sun as it is observed in the sky. It contains two Latin-language morphemes, sol, "sun", and -stitium, "stoppage". Pliny uses it a number of times in his Natural History with a similar meaning that it has today. This modern scientific word descends from a Latin scientific word in use in the late Roman Republic of the 1st century BC: solstitium. In that sense, solstice means "sun-standing". However, the Sun's motion in declination comes to a stop at the moment of solstice. The Sun's westerly motion never ceases as Earth is continually in rotation. When it is the summer solstice at one Pole, it is the winter solstice on the other. Similarly, for an observer on the South Pole, the Sun reaches the highest position on the December solstice day. The day this occurs is called the June solstice day. The word solstice is derived from the Latin sol ("sun") and sistere ("to stand still"), because at the solstices, the Sun's declination appears to "stand still" that is, the seasonal movement of the Sun's daily path (as seen from Earth) pauses at a northern or southern limit before reversing direction.ĭefinitions and frames of reference įor an observer at the North Pole, the Sun reaches the highest position in the sky once a year in June. Alternative terms, with no ambiguity as to which hemisphere is the context, are " June solstice" and " December solstice", referring to the months in which they take place every year. The day of a solstice in either hemisphere has either the most sunlight of the year ( summer solstice) or the least sunlight of the year ( winter solstice) for any place other than the Equator. The term solstice can also be used in a broader sense, as the day when this occurs. In many countries, the seasons of the year are determined by the solstices and the equinoxes. Two solstices occur annually, around June 21 and December 21. Equinoxes and solstices on Earth eventĪ solstice is an event that occurs when the Sun appears to reach its most northerly or southerly excursion relative to the celestial equator on the celestial sphere.
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