Tonight – June 24, 2016 – if you’re located around 40o north latitude, it’s your latest evening twilight for the year. The longest evening twilights always happen around the summer solstice. Although the Northern Hemisphere’s summer solstice, and longest day, happened a few days ago, on June 21, the latest twilight at 40o north latitude always occurs several days afterwards, on or near June 24.
The parallel 40o north passes through the cities of Philadelphia, Pennsylvania and Columbus, Ohio; it sweeps through the northern suburbs of Indianapolis, Indiana and Denver, Colorado.
Want to know for your latitude? Click here and check the “astronomical twilight” box.
The year’s latest sunsets don’t come exactly on the solstice either. For 40o north latitude, the latest sunset happens about a week after the summer solstice, on or near June 27.
Let us introduce you to the three different kinds of twilight:
Civil twilight starts at sundown and ends when the sun is 6o below the horizon.
Nautical twilight occurs when the sun is 6o to 12o below the horizon.
Astronomical twilight happens when the sun is 12o to 18o below the horizon.
North of 50o north latitude, there’s no true night in the month of June. In June, that far north, the sun never gets far enough below the horizon for true night to occur.
It’s the land of the midnight twilight from 50o north latitude to the Arctic Circle (66.5o north latitude).
It’s the land of the midnight sun from the Arctic Circle to the North Pole (90o north latitude).
At the temperate zones and the tropics, the longest period of twilight after sunset or before sunrise happens around the summer solstice, and the shortest period around the equinoxes. At 40o latitude, astronomical twilight ends about 2 hours after sunset on the summer solstice; and on the equinoxes, astronomical twilight ends about one and one-half hours after sunset. Believe it or not, the duration of astronomical twilight reaches a secondary peak around the winter solstice, lasting about one and two-thirds hours after the sun goes down at 40o latitude.
Bottom line: Although the latest sunset won’t happen at 40o north latitude for another few days, the latest twilight happens on June 24.
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.