If we could see the stars during the day (the glare of the Sun blots them out) then as Earth moves along its anticlockwise path around the Sun, our line-of-sight with the Sun would move westwards among the stars in a great arc encircling Earth. This arc is known as the ecliptic, so-named because lunar (Moon) and solar (Sun) eclipses can only occur when the Moon crosses it. Its plane is coplanar with Earth’s orbit.
At night as the hours pass, the stars retain their relative positions to each other in patterns we call constellations. The Sun can take weeks to ‘pass through’ a constellation so each revolution of Earth about its axis can provide a snapshot of the Sun’s apparent journey. On a clear night just after sunset (the horizon fixes the Sun’s position and blocks out the Sun’s direct light) a constellation visible just above the western horizon will a few weeks later be replaced by the next constellation when our line-of-sight with the Sun moves on to it.For each complete revolution about its own axis Earth travels 360/365≈1° of its orbit around the Sun. From day-to-day the Sun therefore appears to move approximately two angular diameters (2×0.5) westwards against the background of the fixed and normally invisible stars.
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