Venus

Venus is the second planet from the Sun and Earth's closest planetary neighbor, orbiting at 0.723 AU. Often called Earth's 'twin' due to its similar size (12,104 km diameter, 95% of Earth's), Venus is in reality a hellish world with a crushing atmosphere 90 times denser than Earth's, composed of 96.5% carbon dioxide. A runaway greenhouse effect heats the surface to about 465°C — hotter than Mercury despite being nearly twice as far from the Sun. The planet is permanently shrouded in thick sulfuric acid clouds that completely obscure the surface in visible light.

Venus is the third-brightest object in the sky after the Sun and Moon, reaching magnitude -4.6 at peak brilliance — bright enough to cast shadows from a dark site. As an inferior planet, it shows dramatic phases visible in even a small telescope at 50-100x: a large thin crescent when closest to Earth, shrinking to a tiny full disk when on the far side of the Sun. The crescent phase is the most spectacular, with Venus appearing surprisingly large (up to 60 arcseconds). A violet or blue filter (Wratten #47) can sometimes reveal subtle cloud banding. Venus is best observed in twilight — its brilliance overwhelms in a dark sky. Greatest elongation occurs roughly every 9.5 months.

Known since prehistory and often called the Morning Star or Evening Star. The ancient Greeks, like with Mercury, initially believed these were two separate objects — Phosphorus (morning) and Hesperus (evening). Galileo observed its full range of phases in 1610, providing crucial evidence that planets orbit the Sun. The thick atmosphere was noted by Mikhail Lomonosov during Venus's 1761 transit of the Sun. The Soviet Venera program landed multiple probes on the surface between 1970 and 1985, returning the only photographs ever taken from Venus's surface.