Venus Puts on Its Brightest Show of the Year This September

September brings a packed skywatching calendar, anchored by Venus at its most dazzling. NASA says Venus will reach peak brightness during its current evening appearance on September 18, appearing low above the western horizon shortly after sunset as an intensely bright point of light. The same evening also marks the planet's "greatest brilliancy," according to Space.com, when it shines at magnitude -4.8 while just 26% lit.

Through a small telescope, Venus looks very different from the blazing dot visible to the naked eye. Its disk is dimming from 36% to 32% lit as September progresses, shrinking toward a thinner crescent even as its apparent size grows because the planet is moving closer to Earth. Venus reached its greatest eastern elongation, about 46 degrees from the sun, on August 15. It is now swinging toward inferior conjunction on October 24, when it will pass roughly between Earth and the sun and vanish into the sun's glare. On September 14, a 16%-lit waxing crescent moon will pass extremely close to a 29%-lit Venus.

Venus is not the only sight this month. Between September 14 and 20, the Moon can guide observers to the bright star Antares and the "Teapot" star pattern in Sagittarius, near the center of the Milky Way. September 19 is International Observe the Moon Night, a global event encouraging skywatchers to learn about lunar science and exploration. The September equinox falls on September 22, marking the start of fall in the Northern Hemisphere. On September 26, the Harvest Moon rises in the east near Saturn, with faint Neptune completing a triangle with the two.

Venus's crescent phase, and its underlying cause, also connects to history: when Galileo observed the full range of Venus's phases through a telescope in the early 17th century, the sight helped demolish the old Ptolemaic model in which Venus was assumed to orbit Earth. Venus will also vanish behind the Moon twice more this year in events called lunar occultations, on September 14 over parts of Asia, Africa, Europe and western Russia, and on November 7 over southern South America. Those follow a first such occultation that daytime skywatchers across the U.S. and parts of Canada saw on June 17.