The summer solstice is upon us: Wednesday June 21st is the longest day of 2023 and the start of the summer season for those living north of the equator.
Technically, the summer solstice occurs when the sun is directly over the Tropic of Cancer, or 23.5 degrees north latitude. This will be at exactly 10:57 am Eastern Wednesday. If you’re a fan of sunshine, wearing shorts, eating ice cream and enjoying all that summer has to offer, this is probably a great day for you.
Below is a brief scientific guide to the longest day of the year.
1) Why do we even have a summer solstice?
The summer and winter solstices, the seasons, and the changing length of daylight hours throughout the year are all due to one fact: the earth rotates on an inclined axis.
The tilt — possibly caused by a massive object that struck Earth billions of years ago — means the North Pole faces the Sun for half a year (as in the image below). In the other half of the year it gets brighter at the South Pole.
Here’s a time-lapse demonstration of the phenomenon, captured over a full year from space. In the video you can see how the line separating day and night (called the terminator) oscillates back and forth from the poles throughout the year.
A fun fact about the Terminator: It’s not like it splits the earth into even light and dark halves. That’s because our atmosphere bends sunlight a bit, essentially stretching it out over a slightly larger area of land. This “results in the land covered by sunlight having a larger area than the land covered by darkness,” explains the National Oceanic and Atmospheric Administration (NOAA).
2) How many hours of sunshine will I have on the 21st?
That depends on where you live. The further north you are, the more sunlight you will see during the solstice. Alaskan climatologist Brian Brettschneider I made this great guide (click here to see some charts): At the northernmost latitudes, the sun shines for a full 24 hours, but for most of the US, it’s between 14 and 16 hours.
If you live near the Arctic Circle, the sun never really sets at 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 at a high latitude — took this pinhole photo of the sun’s year-round path and shared it with astronomy website EarthSky. From December to June you can see the dramatic change in the solar arc. (You can easily create a similar image at home. All you need is a can, photo paper, some tape, and a pin. Instructions are here.)
This is a 6 month pinhole photo taken from solstice to solstice in Medicine Hat, Alberta, Canada. We’re one of Canada’s sunniest cities, and it shows.
Posted by Ian Hennes on Saturday December 21st, 2013
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 it means that each location has its earliest sunrise or latest sunset that day.
If you live in Washington, DC, the latest sunsets begin the day after the solstice, the 22nd. If you like to sleep in, this is arguably the busiest day of summer. TimeAndDate.com can tell you when the latest sunset will be in your area.
4) Is the solstice really the first day of summer?
Well, it depends: are you asking a meteorologist or an astronomer?
meteorologically, Summer is defined as the hottest three months of the year, winter as the coldest three months and the–Between the months are spring and autumn.
Here’s how NOAA breaks it down:
Meteorological spring includes March, April and May; The meteorological summer includes June, July and August. The meteorological autumn includes September, October and November. and meteorological winter includes December, January, and February.
According to this meteorological definition, summer has already begun. But astronomically Yes, summer begins when the sun is directly over the Tropic of Cancer, which falls on the solstice.
5) Do I have to wear sunscreen?
Yes, in the intense summer sun you should wear sunscreen. However, as reported by Vox’s Julia Belluz, there is a lack of research into whether sunscreen actually helps prevent the more aggressive form of skin cancer. As she writes:
The US Preventive Services Task Force summarized the evidence on the health effects of sunscreen use… [and] found that sunscreen reduced the incidence of squamous cell carcinoma but had no effect on basal cell carcinoma. In addition, “there is no direct data on the effect of sunscreen on melanoma incidence.”
Still, research is evolving and newer studies are emerging showing that sunscreen can reduce the risk of melanoma, like this long-term study from Australia.
However, it definitely prevents an unpleasant sunburn. For more on the science of sunscreen, check out Belluz’s explainer.
6) Are there solstices and equinoxes on other planets?
Yes! All planets in our solar system rotate around an inclined axis and therefore have seasons. Some of these tilts are minor (like Mercury, which is tilted 2.11 degrees). But others are more similar to Earth (23.5 degrees) or even more extreme (Uranus is tilted 98 degrees!).
Below is a beautiful composite image of Saturn at the equinox taken by the Cassini (RIP) spacecraft in 2009. The gas giant is tilted 27 degrees relative to the Sun, and equinoxes are rarer on the planet than on Earth. Saturn only experiences an equinox about once every 15 years (since 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 about 30 feet thick, and when light hits them head-on, there isn’t much surface area to reflect.
7) I accidentally clicked on this article and all I really want is a cool picture of the sun
We have 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 get within 4 million miles of the Sun’s surface (much closer than any spacecraft before it). The aim is to study the Sun’s atmosphere, weather and magnetism, and to unravel the mystery of why the Sun’s corona (its atmosphere) is much hotter than its surface. But even at a distance of several million kilometers, the probe has to withstand temperatures of 2,500 degrees Fahrenheit.