June 21 is the longest day of the year in 2017, at least when it comes to northern daylight hours.
That's a good thing for people living in the US — we all enjoy a little more light during our morning and evening commutes, after-work playtime, and summer vacations.
But there's a popular misconception about summer days: That sunsets get later after summer begins.
In fact, the opposite happens. After the summer solstice, daylight starts fading from the morning and evening, beginning the north's slog toward the cold depths of winter darkness.
To understand where this misconception comes from, we need to define what summer actually is.
How the summer solstice works
Tauʻolunga/Wikipedia (CC public domain)
The Earth orbits the sun once every 365 days and 6 hours. Our planet also rotates once per day around a tilted axis.
That tilt, which is currently 23.5 degrees, bathes different parts of the world with different intensities of light over the course of a year. The planet's rotation, meanwhile, keeps the heating even, like a 7,917-mile-wide rotisserie chicken made of rock and a little water.
The summer solstice occurs when sunlight reaches its maximum extent, either in the northern or southern hemisphere. This marks the beginning of summer. In the north, it happens between June 20 and 22; in the south, summer hits six months later — between December 20 and 23.
NASA; cmglee/Wikipedia (public domain)The spots on the Earth that get the maximum amounts of sunlight form what we call the Tropic of Cancer (or Northern Tropic), and the Tropic of Capricorn (or Southern Tropic).
Between each summer solstice is an equinox, when the sun's maximum intensity lines up directly with the equator. Summer ends with the fall equinox, and winter ends with the spring equinox. (And no, it's not any easier to balance an egg on its end during these times.)
This animation by NASA's Goddard Space Flight Center clearly shows the progression:
Why sunsets sometimes seem later in the summer
So if sunrises happen later and sunsets happen sooner as summer wears on, why can it seem like the days are still growing longer? A bunch of small factors may come together to convince you of this.
On either side of a summer solstice, the sun sets and rises at nearly the same time, right down to the minute, for a week or two. This seeming pause can fool us into thinking the days are getting longer if we're not paying attention to exact sunset and sunrise times.
Charts called sun graphs illustrate this point well, though they differ depending how far north or south you live. The closer you live to a tropic around the solstice, the more consistent those endless summer (and, technically, spring) days seem to last.
We'll note a technicality, however: For a few days after a summer solstice, the sunset can happen at a slightly later time depending on where you live. This is because a day on Earth is actually less than 24 hours. (TimeAndDate.com has a great explainer on this.) But even when that happens, the total amount of daylight still shrinks. And what we're talking about here is the broader perception that the sun sets later for weeks after the start of summer.
Another reason for this mistaken perception is that because of Earth's tilt, people who live in northern latitudes get more incident sunlight and thus more twilight hours during the summer than people farther south do. That might make summer days seem longer than they technically are — especially for those who live farther from the equator.
Additionally, a now-pointless tradition called daylight-saving time may contribute to this issue. Advancing clocks an hour earlier in March makes more people wake up in darkness and experience a later sunset. By the time the solstice rolls around, though, the sunrise usually happens before many people wake up, and they're back indoors before the sunset occurs. This might give the impression that the days are lasting longer than they do.
Furthermore, we often think of summer in terms of our schedules — not when the solstice actually occurs. Many people find themselves outside more often during the evenings, and also take vacations for a week or two post-solstice. That also makes the days seem longer.
How long June 21 — the "longest" day of the year — lasts depends on where in the northern hemisphere you're located. But here in New York City, the sun rose at 5:25 a.m. EDT and sets at 8:30 p.m. EDT — a total of 15 hours and 5 minutes of glorious summer daylight.
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