When it’s still dusk at 9 or 10 p.m. and gets light again around 5 o’clock the next morning, we know the longest day of the year is upon us. This Saturday is the June solstice, marking the longest day of the year and the astronomical start to summer in Earth’s northern hemisphere.
At 6:51 a.m. EDT on June 21, the sun can be seen directly overhead along the Tropic of Cancer at 23.5º north latitude. With the Earth’s North Pole at its maximum tilt toward the sun, locations north of the equator see the sun follow its longest and highest path across the southern sky. This means the shadow you cast at local solar noon will be the shortest of the year.
The exact amount of daylight we see on the summer solstice is highly dependent on latitude (see image). North of the Arctic Circle, the sun circles the sky for 24 hours. But while the high latitudes see continuous daylight, the sun’s low angle in the sky provides very ineffective heating. As a result, temperatures in the Arctic aren’t exactly summerlike even when the sun is up for weeks or months at a time.
Seasonal turning point
The summer solstice has been celebrated throughout human history as an astronomical turning point. Daylight is at a maximum and the location of sunrise and sunset appear to “stand still” (from the Latin sol sistere) with respect to the horizon before reversing direction. For the next few days, the sun rises and sets at its northernmost point on the horizon before slowly migrating southward again for the next six months.
Tools for visualizing the sun’s path
A number of websites offer avid sun observers interactive tools to learn the time and position of sunrise and sunset anywhere in the world. An app called SunCalc allows users to zoom in on any location to see where on the horizon the sun rises and sets on any particular date (photographers take note!).
This map of downtown Washington, D.C. illustrates how the sun sets well to the northwest on the summer solstice. Take a far northern location such as central Alaska, and the SunCalc app shows sunrise and sunset nearly converging at due north, as we would expect when nearing the Arctic Circle.
Timeanddate.com – an early pioneer in all things timekeeping – is another great resource for learning positional sun and moon astronomy. Recent website enhancements allow users to track how far above or below the horizon the sun is at any given time of day. The diagram below shows the height of the sun at solar noon in Washington, D.C. and the slow curve below the horizon at sunset.
One thing we see here is that because the summer solstice has the shortest night of the year, the sun also doesn’t drop as far below the horizon at night. If you pay close enough attention throughout the year, you’ll notice that around the solstice twilight lingers longer than usual. This is especially true as one moves farther north, where even though the sun sets, it stays close enough to the horizon for twilight to last most of the night.
Much like people did in ancient times, the longest day of the year is worth celebrating. Even as the warmest days of the year still lie ahead, the nights will slowly get longer as summer heats up. As EarthSky poetically describes the solstice: “Even in summer’s beginning, we find the seeds of summer’s end.”
Still have questions about the solstice? Find your answer in one of our previous posts below: