The days are longer around the summer solstice and shorter around the winter solstice. The path of the Sun, or ecliptic, sweeps north and south between the northern and southern hemispheres. Outside of the tropics, the maximum elevation occurs at the summer solstice and the minimum at the winter solstice. At maximum or minimum elevation, the relative yearly motion of the Sun perpendicular to the horizon stops and reverses direction. The component of the Sun's motion seen by an earthbound observer caused by the revolution of the tilted axis – which, keeping the same angle in space, is oriented toward or away from the Sun – is an observed daily increment (and lateral offset) of the elevation of the Sun at noon for approximately six months and observed daily decrement for the remaining six months. The Sun's path each day can be seen from right to left in this image across the sky the path of the following day runs slightly lower until the day of the winter solstice, whose path is the lowest one in the image. This is a long-exposure photograph, with the image exposed for six months in a direction facing east of north, from mid-December 2009 until the southern winter solstice in June 2010. An observer on Earth therefore sees a solar path that is the result of both rotation and revolution.Ī solargraph taken from the Atacama Pathfinder Experiment at the Llano de Chajnantor Observatory in the southern hemisphere. Earth's axis is tilted with respect to the plane of Earth's orbit and this axis maintains a position that changes little with respect to the background of stars. 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.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |