The duration of the longest day in the Northern Hemisphere is a relative duration. In San Francisco, the length of the day on the Solstice is 14 hours 47 minutes, but that duration depends upon the latitude north of the Equator. The further north you are located, the longer the longest-day at Solstice. For example, in Seattle the Solstice day lasts 16 hours, and in Anchorage Alaska, the Solstice day lasts 19 hours 21 minutes, and in addition when you are that far north twilight never fully fades to dark and the sky remains in twilight glow the entire time the Sun is below the horizon. Of course, extreme situations emerge in locations such as the far northern reaches of Alaska, Canada, Greenland, Iceland, the Nordic Countries and Russia that lie above the Arctic Circle where the Sun is above the horizon for 24 hours on the day of the summer Solstice.
|Solstice & Equinox|
This year's Solstice also features a Full Moon, so enjoy the shortest night with a bright and shiny Moon to accompany you.
This excellent article in Sky & Telescope provides further illumination on the subject.
Image courtesy Sky & Telescope.