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Posted at 10:50 AM ET, 06/21/2011

The summer solstice – and other interesting sun facts – explained

The solstice sun setting over Old Town Alexandria, seen from the Woodrow Wilson Bridge (6/21/09) (By author)
Most of us can agree that summertime heat and humidity have been well underway in our area since meteorological summer began on June 1. Yet for those who prefer the astronomical definition of the seasons, today marks the official start of summer in the Northern Hemisphere.

This year the summer solstice occurs at 1:16 p.m. EDT. It marks the time at which the sun’s rays are at zenith (directly overhead) at 23.5 degrees north latitude, better known as the Tropic of Cancer.

If you are an avid sun observer like me, the summer solstice has some special significance. For areas north of the equator, June 21 has the longest daylight period and shortest night of the year, and the noontime sun angle reaches its maximum height.

For the Washington, D.C. area, this means:

* The sun is above the horizon for 14 hours, 54 minutes
* Sunrise occurs at 5:43 a.m. and sunset at 8:37 p.m.
* The noontime (1:10 p.m.) sun angle reaches its maximum height at 74.6º above the horizon (compared to only 27.7º above the horizon on Dec. 21)
* The sun rises at its northeasternmost point and sets at its northwesternmost point all year (58º and 302º from due north, respectively)
* Twilight lasts longer than at any other time of year.

Sunrise and sunset on the summer solstice relative to other days (,, U.S. Naval Observatory)

A look at the sun’s path across the sky at the solstice

Visually speaking, the sun appears to move in a circular path about our horizon year round. When the sun is above us, we see its arc move across the sky. After sunset, it continues its circular path “below” our horizon, and it is nighttime.

The summer sun is above the horizon much longer than during winter. (Daniel V. Schroeder, Weber State University)
On the summer solstice, the sun appears above the horizon for the longest amount of time and also reaches its highest point in the sky. On the equinoxes, exactly half of the sun’s apparent circular path is above us and half of it is below, leading to 12 hours of daylight followed by 12 hours of night. In the winter, days are short because most of the sun’s path is below the horizon.

The image to the left also illustrates how the summer sun rises in the northeast and sets in the northwest, whereas the winter sun rises in the southeast and sets in the southwest. Contrary to popular belief, only on the equinoxes does the sun rise and set due east and due west of us respectively.

Why does the day length also depend on latitude?

At 38.9º north latitude, Washingtonians experience moderately long days in summer and short days in winter. Travel north of here and summer days become even longer (the downside being even shorter winter days). Move south toward the equator, and the seasonal difference in day length becomes less noticeable.

Why the difference? Closer to the Poles, locations have very long “day arcs” in summer and very short ones in winter. This also means that the sun angle is relatively low year-round, which leads to colder climates. On the equator, however, the noontime sun is almost directly overhead all year. As a result, equatorial regions experience warm climates and about 12 hours of daylight all year long. Interestingly, the low sun angle in northern latitudes also means that twilight lasts significantly longer than at the equator.

Days getting shorter, but things still heating up

Due to seasonal lag, average temperatures in our area will continue to increase until late July – even as the daylight period gradually diminishes. In fact, D.C.’s earliest sunrise already occurred on June 17. But for those of you wishing to enjoy dusk extend past 9 p.m., rest assured: we still have another fifteen days before the sun starts disappearing earlier from the sky.

Sources and further exploration:
The sun and the seasons
How an observer’s latitude affects view of the sky above
How the location of sunrise and sunset changes throughout the year
Why the earliest sunset isn’t on the shortest day of the year
Length of twilight by latitude
The duration of twilight
Why day length changes little around the solstices

By  |  10:50 AM ET, 06/21/2011

Categories:  Astronomy, Science, Latest

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