Civil twilight to nautical twilight: The science behind sunrises and sunsets

The sky above Boston's skyline is bright orange, golden and pink with clouds. In the foreground boats on the river. — Beautiful sunrises and sunsets happen during civil twilight and nautical twilight.

Dave Epstein is a meterologist who partners with GBH News to provide daily weather updates and weekly higher-level conversations on Morning Edition.

Senior Producer, writing, editing and producing local news. Feedback? Questions? Story ideas? Reach out to Meghan at meghan_smith@wgbh.org.

April 25, 2024

Sunrise and sunset are two of the most beautiful times of the day, as the sky is washed with pinks and oranges. But what happens before and after?

GBH meteorologist Dave Epstein joined Morning Edition co-hosts Jeremy Siegel and Paris Alston to explain these wondrous moments and the science behind these stellar shows of nature.

Sunrise is when the center disk of the sun moves above the horizon, and sunset is the opposite. Civil twilight is what happens directly before sunrise and after sunset, when the sun is zero to six degrees below the horizon.

“I love civil twilight. It's probably my favorite time of the day, to be honest,” Epstein said. “It's still light. The sun's not up above the horizon. Your sunrises and sunsets can be absolutely stunning at that point. And that's the 30 minutes before sunrise or the 30 minutes after sunset, roughly.”

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When the sun is six to 12 degrees below the horizon, that’s nautical twilight, which comes before civil twilight in the morning and after civil twilight in the evening.

“Nautical twilight is when we begin to see the brightest planets and the stars start to come out,” Epstein said.

It’s also a cautious time for animals.

“I call this coyote time,” Epstein said. “You have a lot of the crepuscular animals which love this time of the day, kind of emerging and moving around. Deer are very active during this time. It's also when there can be deer strikes with cars because that's the time that they're moving around.”

Then there is astronomical twilight, when the sun is between 12 and 18 degrees below the horizon.

“It's when we can observe faint objects, even the Milky Way. But it's still not technically night. Night comes after that,” Epstein said.

In summer months like June, astronomical twilight comes very early and late, making night very short.

“In some parts of far northern New England, night doesn't start occurring until like, 11:00 in the evening,” he said. “And we get into astronomical twilight around 2:30, 3:00 in the morning. So true night is very short on those longest days of the year.”