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Celestial dating standards

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.

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.

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.

The word solstice is derived from the Latin sol ("sun") and sistere ("to stand still"), because at the solstices, the Sun's declination "stands still"; that is, the seasonal movement of the Sun's daily path (as seen from Earth) stops at a northern or southern limit before reversing direction.

For an observer on the North Pole, the Sun reaches the highest position in the sky once a year in June.

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However, the Sun's motion in declination comes to a stop at the moment of solstice. This modern scientific word descends from a Latin scientific word in use in the late Roman Republic of the 1st century BC: solstitium.

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A solargraph taken from the Atacama Pathfinder Experiment at the Llano de Chajnantor Observatory in the southern hemisphere.Two solstices occur annually, on about 21 June and 21 December.The seasons of the year are directly connected to both the solstices and the equinoxes.Relative velocity is the motion of an object from the point of view of an observer in a frame of reference.From a fixed position on the ground, the Sun appears to orbit around Earth.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.