The summer solstice is upon us: Wednesday, June 21, is the longest day of 2023, and the start of the summer season, for anyone living north of the equator.
Technically speaking, the summer solstice occurs when the sun is directly over the Tropic of Cancer, or 23.5 degrees north latitude. This will occur at exactly 10:57 am Eastern Wednesday. If you’re a fan of sunlight, wearing shorts, eating ice cream, and enjoying all summer has to offer, this is likely a big day for you.
Below is a short scientific guide to the longest day of the year.
1) Why do we have a summer solstice, anyway?
The summer and winter solstices, the seasons, and the changing length of daylight hours throughout the year are all due to one fact: Earth spins on a tilted axis.
The tilt — possibly caused by a massive object hitting Earth billions of years ago — means that for half the year, the North Pole is pointed toward the sun (as in the picture below). For the other half of the year, the South Pole gets more light.
Here’s a time-lapse demonstration of the phenomenon shot over the course of a whole year from space. In the video, you can see how the line separating day from night (called the terminator) swings back and forth from the poles during the year.
A fun fact about the terminator: It’s not the case that it splits the Earth into even halves of light and dark. That’s because our atmosphere bends sunlight a bit, essentially stretching it over a slightly greater area of land. This “results in the land covered by sunlight having greater area than the land covered by darkness,” the National Oceanic and Atmospheric Administration (NOAA) explains.
2) How many hours of sunlight will I get on the 21st?
That depends on where you live. The farther north you are, the more sunlight you’ll see during the solstice. Alaska-based climatologist Brian Brettschneider created this terrific guide (click to see some charts): The northernmost latitudes will see an entire 24 hours of sunlight, while most of the US will see anywhere between 14 and 16 hours.
Hours of daylight on the summer solstice. pic.twitter.com/O7F2v4ngu4— Brian Brettschneider (@Climatologist49) June 19, 2022
On the chance you live near the Arctic Circle, the sun never really sets during the solstice.
Here’s another cool way to visualize the extreme of the summer solstice. In 2013, a resident of Alberta, Canada — several hundred miles south of Fairbanks but still in a high latitude — took this pinhole camera photograph of the sun’s path throughout the year and shared it with the astronomy website EarthSky. You can see the dramatic change in the arc of the sun from December to June. (You can easily make a similar image at home. All you need is a can, photo paper, some tape, and a pin. Instructions here.)
3) Is the solstice the latest sunset of the year?
Not necessarily. Just because June 21 is the longest day of the year for the Northern Hemisphere doesn’t mean every location has its earliest sunrise or latest sunset on that day.
If you live in Washington, DC, the latest sunsets will start the day after the solstice, the 22nd. If you like sleeping in, that’s arguably the most exciting day of the summer. TimeAndDate.com can tell you when the latest sunset will occur in your area.
4) Is the solstice really the first day of summer?
Well, depends: Are you asking a meteorologist or an astronomer?
Meteorologically speaking, summer is defined as the hottest three months of the year, winter is the coldest three months, and the in-between months are spring and fall.
Here’s how NOAA breaks it down:
Meteorological spring includes March, April, and May; meteorological summer includes June, July, and August; meteorological fall includes September, October, and November; and meteorological winter includes December, January, and February.
So by this meteorological definition, summer has already started. But astronomically speaking, yes, summer begins when the sun is directly over the Tropic of Cancer, which is on the solstice.
5) Do I need to wear sunscreen?
Yes, you should wear sunscreen in the intense summer sun. Though, as Vox’s Julia Belluz has reported, the research on whether sunscreen actually helps prevent the more aggressive form of skin cancer is lacking. As she writes:
The US Preventive Services Task Force summed up the evidence on the health effects of sunscreen use ... [and] found that sunscreen reduced the incidence of squamous cell cancer, but that it had no effect on basal cell cancer. What’s more, “There are no direct data about the effect of sunscreen on melanoma incidence.”
Still, research is always evolving and newer studies are emerging that show sunscreen can curb melanoma risk, such as this long-term trial from Australia.
That said, it does definitely help prevent sunburn, which is unpleasant. For more on the science of sunscreen, read Belluz’s explainer.
6) Are there solstices and equinoxes on other planets?
Yes! All the planets in our solar system rotate on a tilted axis and therefore have seasons. Some of these tilts are minor (like Mercury, which is tilted at 2.11 degrees). But others are more like Earth (23.5 degrees) or are even more extreme (Uranus is tilted 98 degrees!).
Below, see a beautiful composite image of Saturn on its equinox captured by the Cassini spacecraft (RIP) in 2009. The gas giant is tilted 27 degrees relative to the sun, and equinoxes on the planet are less frequent than on Earth. Saturn only sees an equinox about once every 15 years (because it takes Saturn 29 years to complete one orbit around the sun).
During Saturn’s equinox, its rings become unusually dark. That’s because these rings are only around 30 feet thick, and when light hits them head-on, there’s not much surface area to reflect.
7) I clicked this article accidentally and really just want a cool picture of the sun
We got you:
The image above was taken by NASA’s Solar Dynamics Observatory, a spacecraft launched in 2010 to better understand the sun.
In 2018, NASA launched the Parker Solar Probe, a spacecraft designed to come within 4 million miles of the surface of the sun (much closer than any spacecraft has been before). The goal is to study the sun’s atmosphere, weather, and magnetism and figure out the mystery of why the sun’s corona (its atmosphere) is much hotter than its surface. Still, even several million miles away, the probe will have to withstand temperatures of 2,500 degrees Fahrenheit.