Tuesday, June 26, 2012

Summer solstice 2012: Longest day of year - how does daylight compare around the world?

Summer solstice 2012: Longest day of year - how does daylight compare around the world?

Sunrise and sunset times in major world cities on the summer solstice. While locations in the Northern Hemisphere have their longest daylight period, cities in the Southern Hemisphere such as Sydney, Australia have their shortest. (Data source: timeanddate.com)
If you’ve enjoyed these bright, sun-filled evenings for an outdoor barbecue or a neighborhood stroll, the summer solstice tonight marks the culmination of our long-lasting daylight.

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.

On the summer solstice, Earth’s northern hemisphere reaches its maximum tilt towards the sun. Locations north of the Arctic Circle experience 24 hours of daylight. See larger image. (NOAA)
On the summer solstice in Washington, D.C., the sun rises in the northeastern sky at 5:43 a.m. and remains above the horizon for nearly 15 hours before setting to the northwest at 8:37 p.m. With morningcivil twilight beginning at 5:11 a.m. and evening twilight stretching until 9:09 p.m., we get nearly 16 hours of daylight to enjoy. On the longest day of the year the sun also reaches its highest point in the sky at solar noon (1:10 p.m. in D.C.) so remember to use plenty of sunscreen when outdoors.

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.

Related: The summer solstice - and other interesting sun facts - explained

Sunrise and sunset times on the summer solstice. Day length refers to the elapsed time from sunrise to sunset, not including twilight. (Data source: timeanddate.com)
Consider Miami, where the sun is up for less than 14 hours but climbs to a height of 87.7º above the horizon by midday. Just to the south and very close to the Tropic of Cancer, Key West, Fl., sees the midday sun shine almost directly overhead at local noon.

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).

The amount of visible light on the longest day of the year in the Northern Hemisphere. Day length refers to the period from sunrise to sunset. Civil twilight refers to the time before sunrise and after sunset when the sun is 6º or less below the horizon. (Data source: timeanddate.com)
Despite these regional differences in day length and sun intensity, sky observers around the world can admire the June solstice for signaling a turning point in the seasons. Going forward, the days become shorter in the Northern Hemisphere while our Southern Hemisphere neighbors can look forward to more daylight. While northern locations are seeing a lot of sunlight now, it will be quite a different story six months from now. As the saying goes, what goes up must come down…

Additional reading

Why the earliest sunrise is before the June solstice (EarthSky.org)
Day and night world map (timeanddate.com)

More from Capital Weather Gang

Spring Equinox 2012
Winter Solstice 2011
Autumn Equinox 2011
Summer Solstice 2011

By | 09:20 AM ET, 06/20/2012