Sunrise at 4 a.m. seen from Sólfar the Sun Voyager in Reykjavik, Iceland, on Jun. 5. (Justin Grieser)

Summer heat has been in full swing across much of the country the last few weeks, but not until Monday’s summer solstice can we officially — or at least, astronomically — say goodbye to spring.

The 2016 solstice occurs at 6:34 p.m. Eastern Time on June 20, marking the first day of astronomical summer and the longest day of the year in Earth’s northern hemisphere.


(NASA)

Every year on the June solstice the sun’s direct rays reach their northernmost position on the Earth. At solar noon on Monday, the sun will appear directly overhead at 23.5 degrees north latitude, along the Tropic of Cancer. The North Pole also reaches its maximum annual tilt toward the sun.

Since the sun’s direct rays reach their northernmost point from Earth’s equator, the June solstice brings us the northernmost sunrise and sunset of the year. In the Northern Hemisphere, the sun rises in the northeast and follows its longest and highest path across the southern sky before setting again in the northwest sky. Along and north of the Arctic Circle, however, the sun never sets. Instead, the sun circles through the sky for 24 hours as the Earth rotates on its axis.

Latitude and day length

We hear it’s the longest day (and shortest night) of the year, but how long is the sun up exactly? The amount of daylight you’ll see on the solstice depends on your latitude, or distance from the equator.

In D.C., the sun is up just under 15 hours on the solstice, rising at 5:43 a.m. and setting at 8:37 p.m. Locations to our north see the sun up even longer, while to the south there is a bit more nighttime.

[Sunrise and sunset times on the summer solstice]

The map below — created by Alaska-based climatologist Brian Brettschneider — shows how widely daylight hours vary on the summer solstice across the United States:


(Brian Brettschneider)

In the Lower 48, day length ranges from less than 14 hours in Florida and southern Texas to more than 16 hours across the northern tier. Then there’s Alaska, where the sun is up for 18 hours or more across large portions of the Last Frontier.

Of course, even though the sun shines for nearly 22 hours in Fairbanks, its rays are relatively weak due to the low angle at which the northern sun circles the sky. Places closer to the equator like Phoenix or Miami get less daylight on the solstice, but the midday sun climbs higher in the sky, making it easier to get a nasty sunburn at this time of year.

For an additional map showing how daylight hours vary across the entire Northern Hemisphere, click here.

Longest day and longest twilight

Ever notice how dusk lingers a bit longer around the summer solstice? Interestingly, the solstice brings not only the longest day of the year, but also the longest period of morning and evening twilight in the Northern Hemisphere.

(Daniel Schroeder, Weber State University) (Daniel Schroeder, Weber State University)

Because the summer solstice is the shortest night of the year, the sun doesn’t drop as far below the horizon. As the sun sets, its path on the celestial sphere slowly curves below the horizon instead of dropping quickly. The shallow angle at which the sun crosses the horizon allows light to linger longer both before sunrise and after sunset.

How much daylight is there on the solstice when factoring in twilight?

There are three ways to define twilight. The brightest phase — civil twilight — occurs just before sunrise and after sunset, when the sun is less than 6 degrees below the horizon. When we consider the time from sunrise to sunset plus civil twilight, the summer solstice provides 15 to 17 hours of usable daylight across much of the Lower 48, as seen in the map below:


Hours of visible light on the summer solstice, including time from sunrise to sunset and civil twilight, when the sun is within 6 degrees of the horizon. (Brian Brettschneider)

In northern latitudes, twilight is even further prolonged because the sun not only stays up longer but also crosses the horizon at a shallower angle. Once you reach the 49th parallel (the U.S.-Canadian border), there are over 17.5 hours of visible light on the summer solstice. And north of 60°N latitude (around southern Alaska), twilight lasts all night.

Earliest sunrise and latest sunset not on the solstice

No discussion of the solstice would be complete without mentioning that the earliest sunrise and latest sunset don’t neatly coincide with the longest day of the year. Most places in the middle latitudes see their earliest sunrise about a week before the summer solstice, while the latest sunset comes nearly a week later. The misalignment is the unique result of the Earth’s 23.5-degree tilt and our elliptical orbit around the sun (read more).

This third (and final!) map shows how the date of the earliest sunrise and latest sunset varies with distance from Earth’s equator.


(Brian Brettschneider)

At high latitudes, the earliest sunrise and latest sunset occur closer to the solstice while closer to the equator they can be nearly a month apart. D.C. — located at about 39 degrees latitude — typically sees its earliest sunrise around June 13 or 14, whereas the latest sunset occurs around June 27 or 28.

Note that the gap between earliest sunrise and latest sunset is less pronounced than the gap between the earliest sunset and latest sunrise that happens around the winter solstice in December. Starting next week, though, the nights will slowly get longer again, even as the warmest days of summer are still upon us.

But before we think about diminishing daylight, or the dog days of summer, let’s enjoy the longest day of the year in all of its sunny glory!