Astronomy Picture of the Day . ... The Plane of the Ecliptic is illustrated in this Clementine star tracker camera image which reveals (from right to left) the Moon lit by Earthshine, the Sun's corona rising over the Moon's dark limb, and the planets Saturn, Mars, and Mercury. ... Its distance has been accurately measured by astronomers using one of the fundamental yardsticks of the extragalactic distance scale - pulsating variable stars known as Cepheids. ... NGC 6872: A Stretched Spiral . ...