From Sunrise to Astronomical Twilight: Understanding the Science Behind Each Stage of Twilight

As a journalist, I had the opportunity to interview GBH meteorologist Dave Epstein to learn more about the science behind sunrise and sunset. He shared with me that these are not just two beautiful moments, but also different phases of twilight.

Before sunrise and after sunset is civil twilight, which occurs when the sun is situated zero to six degrees below the horizon. During this time, while there is no direct sunlight, it still provides light without the sun being visible. Sunrises and sunsets during civil twilight can be breathtakingly beautiful and occur approximately 30 minutes before or after each event.

Following civil twilight is nautical twilight when the sun is between six and 12 degrees below the horizon. This is a great time for stargazing as bright planets like Jupiter and Saturn become visible, along with stars like Sirius. It’s also an active time for crepuscular animals such as deer, which increases their chances of collisions with cars.

Finally, astronomical twilight occurs when the sun is between 12 and 18 degrees below the horizon. This marks the transition from day to night where faint objects like constellations become visible but it’s not yet considered nighttime. In summer months like June, astronomical twilight can occur very early in morning or late at night resulting in short nights especially in northern New England where true nighttime may not begin until late evening and astronomical twilight occurs in early morning hours.