Finally, the movement of the solar system as a whole through space is defined in terms of the celestial sphere, the imaginary sphere of the heavens that has Earth at its center. The solar system appears to be moving toward a point on the sphere at a velocity, relative to nearby stars, of about 20 km/sec (12 mi/sec). This point, the solar apex, lies in the constellation Hercules near the star Vega, at a right ascension of about 18 hours and a declination of about 30° north.