APEX, SOLAR, the point of the heavens towards which the sun's motion relative to the mean of the stars is directed. Its position was indicated approximately by Sir William Herschel in 1783. It is near right ascension 18h declination -f- 3o°, not far from Vega, but the position varies considerably according to the class of stars chosen for reference. The speed of the sun's motion is approximately 20 kilometres per second.