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Sun, Moon and Earth

7 6 Each day, the Sun appears to rise from an easterly direction, traces a clockwise arc across the heavens and then sets towards the west, disappearing for the dark time we call night. This cycle repeats perpetually it is the diurnal rhythm, called, more simply, the day, 24 hours of our clock time. What we see is caused by the daily rotation of a spherical Earth on its own axis. To complicate matters the Sun each day appears to move about a degree anticlockwise eastwards with respect to the fixed stars, along a path known as the ecliptic. This may be observed as the stars rising by just under four minutes earlier each evening and is caused by the Earth orbiting the Sun. Thus the solar day, to which we set our clocks, exceeds the sidereal star day by 3 minutes and 56 seconds. The axial tilt of the Earth page 8 causes the Sun to rise and set each day at different positions on the horizon. Only at the summer and winter solstices is this daily change in the Suns rise and set positions reduced to zero, at their extreme standstill positions. Subsequent sunrises and sunsets gradually reverse back along the horizon, the observed range being dependent on the latitude of the observer opposite below. This is the fourfold rhythm of the year. The Earths orbital period is 365.242199 days. Our solar calendar of 365 days keeps pace by adding a leapyear day every four years except once every four centuries, and by occasionally inserting the odd leap second or two. the Sun the day and the year
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