The June solstice is here once again, marking the longest daylight period of the year and the start of astronomical summer in Earth’s northern hemisphere.
At 5:04 UTC (1:04 a.m. EDT) on June 21, the sun can be seen straight overhead along the Tropic of Cancer, while the North Pole reaches its maximum annual tilt toward the sun. As the planet rotates on its axis, areas within the Arctic Circle see the sun circle through the sky for 24 hours.
Link: Day and night world map
Of course, in the mid-latitudes of the Northern Hemisphere we don’t see the sun up for 24 hours, but there’s still plenty of daylight to enjoy. Most places in the continental U.S. see the sun above the horizon for 14 to 16 hours on the summer solstice – the exact amount depends on your latitude.
Historically, the summer solstice has been celebrated as an astronomical turning point. But what makes this day unique to humans observing the annual movement of our nearest star? Let’s sum it up in four main points (and delve into some detail along the way):
1. North of the Tropic of Cancer, the sun takes its longest and highest path through the sky.
On the summer solstice, areas of the Northern Hemisphere outside the tropics see the longest daylight period of the year and the midday sun is at its highest point in the sky.
In Washington, D.C., the sun is above the horizon for 14 hours and 54 minutes, climbing 74.5º above the horizon at solar noon (1:10 p.m.). The higher your latitude, the longer you’ll see the sun above the horizon – however, this also means the sun will appear lower in the sky.
In the table of world cities below, we see that in more northern locations, the sun is up longer on the summer solstice, but shines from a lower angle at midday. Compare Washington and London, for example: The sun in London (if not masked behind clouds), is up nearly two hours longer than in Washington. However, at midday it’s only as high in the sky as Washingtonians see it in mid-April.
Why can’t we say that the sun is at its highest point in the sky everywhere north of the equator? The reason is that in the tropics (locations within 23.5º latitude of the equator), the midday sun can appear toward either the northern or southern horizon depending on the time of year (see graphic). Along the equator, the noontime sun is at its highest point – zenith – on the equinoxes, while on the solstices, the sun actually takes its lowest path in the sky.
Interestingly, the June solstice sun appears higher in the sky at solar noon in cities like Washington, New York, or Minneapolis than it does on the equator. As we see in the first table above, equatorial Singapore only sees the sun rise 67.9º above the horizon on the summer solstice, while the sun in Minneapolis appears at a slightly higher 68.5º above the horizon at midday (see second chart).
2. The sun rises and sets at its northernmost points on the horizon.
Everywhere on earth (outside the polar regions), the sun rises and sets at its northernmost position relative to due east and due west, respectively. This is true even in the Southern Hemisphere, where the June solstice marks the shortest day of the year.
In Washington, D.C., the sun rises and sets within 58º of due north along the horizon (that’s compared with 90º from due north on the equinoxes when the sun rises at due east and sets at due west on the horizon).
The farther one travels from the equator, the closer sunrise and sunset appear to due north on a compass. For example, on the summer solstice in Seattle, sunrise and sunset occur within 53º of due north (see graphic), and a mere 15º from due north in Fairbanks, Alaska (see graphic). As you keep moving toward the Arctic Circle, sunrise and sunset practically overlap in the northern sky, until eventually the sun never sets at all.
3. The earliest sunrise of the year comes before the summer solstice.
You know the days are long when the sun is up an hour or two before you wake up in the morning. Indeed, sunrise on the solstice is close to its earliest of the year, though depending on latitude, many places already saw their earliest sunrise nearly a week ago.
Even though the June solstice marks the longest day of the year in the Northern Hemisphere, the earliest sunrise usually precedes the solstice by several days (this happens up to about 45ºN latitude). The cause of this misalignment is a discrepancy between our clocks and the apparent motion of the sun with respect to the horizon.
EarthSky nicely summarized this topic last year:
“In June, the day (as measured by successive returns of the midday sun) is nearly 1/4 minute longer than 24 hours. Hence, the midday sun (solar noon) comes later by the clock on the June solstice than it does one week before. Therefore, the sunrise and sunset times also come later by the clock [pushing both sunrise and sunset later simultaneously].”
In D.C. the earliest sunrise was at 5:42 a.m. around June 14. Yet on the solstice, sunrise is a minute later, mainly because the time of solar noon is later than it was a week ago. By late June, solar noon moves another two minutes later, delaying sunrise by another three minutes, while the time of sunset holds steady.
A similar but more pronounced version of this phenomenon occurs around the winter solstice in December.
4. Twilight in the Northern Hemisphere is longer on the solstice than at any other time of year.
The fact that the summer solstice features the shortest night of the year also means the sun doesn’t drop as far below the horizon at night. At sunset, the sun’s apparent path on the celestial sphere tends to curve below the horizon instead of dropping quickly, which results in longer periods of twilight both before sunrise and after sunset.
While the longer period of twilight is less discernible at lower latitudes, northern cities experience noticeably longer dawn and dusk on the summer solstice than at other times of year.
Put simply, the sun is not only up longer in northern latitudes, but it also crosses the horizon at such a low angle that the sky remains illuminated for an exponentially longer period of time. Along the 49th parallel (the U.S.-Canadian border), there are over 17 hours of visible light on the summer solstice. North of 60ºN latitude, civil twilight lasts all night.
You can see twilight start and end times in the two tables of global and U.S. cities above, or take a look at this graphical comparison of twilight by latitude.
Long hours of daylight only slowly fading into darkness make the summer solstice a day worth savoring. If you live in the Northern Hemisphere, take a moment to admire this seasonal and astronomical turning point before the nights grow longer again. In the meantime, the warmest days of summer are just arriving – even as the sun begins to spend ever less time in the sky.