Morning Stars, Evening Stars: Venus and Mercury

Abstract

Track the Moon long enough and it will guide you to new celestial sights. Some morning just before new Moon, or some evening just after it, the slender crescent will share the twilight with a star that outshines all others. If the timing is just right, a second speck of light – fainter, a bit redder – will hover in the unsteady air close to the horizon. With that observation your grasp of the universe swells over 200-fold, reaching far beyond the orbit of the Moon to encompass the planets Venus and Mercury. These two “stars” never appear more than a few hours ahead of or behind the Sun. They emerge into the morning or evening twilight only to reverse course and slip back into the Sun’s glow. The terms “morning star” and “evening star” usually refer to any planet bright enough to stand out in the twilight glow of dusk or dawn, a condition that all of the classical planets satisfy sooner or later. But only two planets spend all or most of their visibility periods in twilight, so these terms best apply to Mercury and Venus, the planets whose progress through the sky are most closely tied to the Sun’s.