At 7:09 p.m. (EDT) today, the sun will shine directly overhead at the Tropic of Cancer while the North Pole reaches its maximum tilt towards the sun. For the Northern Hemisphere, this marks the official start of summer and the longest day of the year.
Relationship between latitude, sun angle, and day length
Since day length depends on latitude, northern cities like New York and Chicago see the sun above the horizon for more than 15 hours. At lower latitudes the amount of daylight is shorter but the sun climbs higher in the sky. So while cities like Atlanta, Miami, and Los Angeles have somewhat shorter days during summer, the midday sun there is also more intense.
As one moves north from the tropics towards the North Pole, the sun stays above the horizon longer but is also noticeably lower in the sky. At nearly 65ºN latitude, residents of Fairbanks, Alaska see the sun circle the sky for nearly 23 hours in mid-June. Yet even on the longest day of the year the sun shines at a very low 48.6º declination during midday. At this time of year areas along and north of the Arctic Circle experience 24-hour daylight.
The summer solstice in other cities around the world
Much of the continental U.S. enjoys 14 to 16 hours of daylight on the summer solstice, but how do other major world cities compare? Many cities in East Asia, such as Tokyo and Beijing, are at about the same latitude and have similar periods of sunlight. Other places – notably in Europe – get considerably more daylight than even the northernmost metropolitan areas of the mainland United States.
Across the Atlantic in London, for example, the sun stays above the horizon for well over 16 hours – from before 5 a.m. until after 9 p.m. Factor in the amount of light from dawn until dusk and the period of visible light extends beyond 18 hours. The graph below compares the day length and twilight duration of a handful of cities in the Northern Hemisphere. Note that locations in higher latitudes have not only longer days but also much longer periods of twilight (see graphical comparison).
Why the earliest sunrise is before the June solstice (EarthSky.org)
Day and night world map (timeanddate.com)
More from Capital Weather Gang