He explained “Manhattanhenge,” the term he coined to describe the phenomenon that happens when the setting sun aligns perfectly with the Manhattan grid. In 2002, Tyson first published the precise times to view the sun framed by the city’s concrete and steel canyons on the website of the American Museum of Natural History, where he is director of the Hayden Planetarium. Since then, in ever increasing numbers Manhattanhenge fans race out into traffic during the twice-annual occurrence to snap photos and watch the sun dip below the horizon. “It has gotten out of control in the most wonderful way,” he said. He described other, lesser known cosmic phenomenon involving architecture, including how the sun set on the observation deck of the old World Trade Center’s Twin Towers two minutes later than on the ground floor, due to the curvature of the earth and the height of the buildings.

Tyson also talked about images that changed the way much of humanity viewed the planet, including the so-called “Earthrise” photo shot from Apollo 8 in 1968. Considered by many as the most influential environmental photograph ever taken, it shows the earth as a small blue marble and highlighted both its beauty and fragility. He presented another, more recent image—one taken by the unmanned Cassini spacecraft which just ended its 20-year mission exploring Saturn in September. In the same frame, the photo captures Saturn’s rings and shows earth hundreds of millions of miles away, as a tiny spec of light. Before leaving the stage, Tyson entreated his audience “to preserve and cherish that pale blue dot.”