Saturday, June 22nd 2019, 12:00 pm - It's summer, and we've just seen the longest day of 2019, but what's the science behind it all?
Friday, June 21 is the summer solstice for 2019 - the official first day of summer in the northern hemisphere, and the longest day of the year for that half of the planet.
Why is this day so special, though?
It should be noted that, if you are in the southern hemisphere, June 21 is the winter solstice, the first day of winter, and the shortest day of the year.
This is due to the northern and southern hemispheres experiencing opposite seasons, no matter what time of year it is.
A SOLAR 'PAUSE'
The word 'solstice' comes from the Latin word solstitium, which basically means "Sun (sol) stoppage (stitium)". It refers to how the Sun appears to 'stop' or 'pause' in the sky on this day of the year (as well as on winter solstice).
On each successive day between the winter solstice and the summer solstice, the Sun appears to have climbed a little bit higher in the sky, and on the day of the summer solstice, the Sun 'pauses', before it sinks a bit lower in the sky on each successive day between the summer solstice and the winter solstice. At which point, it 'pauses' again, before it begins climbing higher in the sky again.
The Sun traces its way across the sky, higher and higher, between February 28 and June 20, 2016. Credit: Bret Culp - Used with permission.
The image above is an example of solargraphy, wherein photographer Bret Culp placed a small pinhole camera in a secluded area, with a view of downtown Toronto, and left it there for nearly 4 months in winter and spring 2016.
As the Sun tracked from horizon to horizon, getting higher and higher in the sky each day, the pinhole projection of the Sun onto the film at the back of the camera traced the same path. Thus, with one image, we can see the motion of the Sun in the sky from the end of meteorological winter until the solstice from that year.
According to Culp, the breaks in the curves are caused by cloudy periods during that particular day, and the colours are a result of chemical reactions in the film paper, due to extreme overexposure, weather (humidity, rain), dirt, fungi, etc.
The official "pause" of the Sun, in 2019, was on June 21, at 11:54 a.m. EDT.
WHAT'S BEHIND THIS PATTERN?
Throughout human history, those that have tracked the motions of the objects in our sky - both night and day - noticed that year-by-year, those objects would trace very specific, repeating paths.
Ancient monuments like Stonehenge, the temple of Karnak in Egypt, and Chichen Itza in Mexico are just a few that have been built to form specific alignments with this pattern in the sky. These locations still draw significant crowds as we transition between seasons - at the vernal and autumnal equinoxes, and at the winter and summer solstices.
The best way to see the reason for this pattern is to look at how our planet is oriented with respect to the solar system and the Sun. You don't need to fly far out into space for this, fortunately. Just look at a globe.
Credit: princessdlaf/Getty Images
Globes are nearly always tilted to one side, and it is for more than aesthetic reasons. It reflects the tilt of the Earth itself - by roughly 23.5 degrees with respect to the path the planet traces around the Sun (the 'ecliptic plane').
It's this tilt that is responsible for our seasons.
Rather than "wobbling" back and forth by 23.5 degrees, however, the Earth's tilt is relatively constant. While it has varied by about 1°-1.5° over tens of thousands of years, the influence of the Moon's gravity keeps the tilt fairly stable.
It's the angular difference between the ground and the Sun, based on hemisphere and latitude, that drives the seasons.
Since the north and south poles point roughly in the same direction year-round, as Earth goes through one full orbit of the Sun (as seen in the video above), the planet's tilt causes the northern hemisphere to be pointed more towards the Sun during one half of the year and the southern hemisphere to be pointed more towards the Sun for the other half of the year.
Thus, sunlight shines down on the ground at a steeper angle during the summer months, and each beam of sunlight delivers its energy and thus heat to a relatively small area. In the winter months, the Sun's rays strike Earth's surface at more of an angle, spreading that same energy out over a wider area, and thus delivering less overall heat to the ground. It's this overall difference in heating that makes the summer months hotter and the winter months colder.
The equinoxes - spring and autumn - mark the transitions between those two halves of the year, when the Sun is directly above the Earth's tilted equator. The solstices mark the points when the hemispheres reach their maximum angle - one towards the Sun and one away from the Sun - and the Earth's axis lines up exactly with the axis of the Sun.
FARTHEST FROM THE SUN NOW
Contrary to what some may think, during the June solstice, the Earth is not at its closest distance to the Sun.
Earth's distance from the Sun does change throughout the year because the planet's orbit is an ellipse, rather than a circle, but right now, the planet is actually nearing its farthest point from the Sun in its orbit for the year.
In 2019, 'aphelion' as this is called, occurs on July 4, at 6:10 p.m. EDT.
Perihelion, the closest point Earth is to the Sun in its orbit, occurred on January 3, at 12:19 a.m. EST, and it will occur next on January 5, 2020, at 2:47 a.m. EST.
Earth's elliptical orbit and the planet's tilt, and how they combine to form our seasons. Earth's closest point to the Sun (perihelion) and farthest from the Sun (aphelion) are noted as well. Credit: NASA/Scott Sutherland
Although the scales of the above image are exaggerated - Earth would actually be smaller than one pixel at the proper scale and its orbit is nearly (but not exactly) circular - it still demonstrates how the planet's tilt, combined with its position along that elliptical orbit, produce our seasons.
Also, even though Earth's distance from the Sun changes, day-by-day, this only causes a small difference in the amount of energy the planet receives from the Sun (known as 'solar insolation'), and thus this difference has very little effect on our seasons.
In fact, due to perihelion (the closest approach to the Sun) happening during northern winter, and aphelion (the farthest distance from the Sun) occurring in northern summer, our seasons in the northern hemisphere are slightly nicer than they would be on a hypothetical Earth with a perfectly circular orbit. Our summers are ever-so-slightly cooler, and our winters are ever-so-slightly warmer.
THE LONGEST DAY OF THE YEAR
June 21 is the longest 'day' of 2019 here in Canada, if by 'day' you mean the exact hours of sunlight you see.
Just how many hours of daylight you get depends on exactly where you live though.
As the map shows, the farther north you go, the longer the day gets! Why is that?
With the 23.4 degree tilt of Earth, the "top" of the world, with respect to the Sun, is at around 66 degrees latitude, better known as the Arctic Circle. Anyone north of that imaginary line on the planet, in summer, is facing nearly directly at the Sun at midday, and still has a direct line-of-sight to the Sun at midnight, with it hovering above the horizon.
The Midnight Sun from Norway. Credit: Pedal Power Photos/Getty Images
In winter, it's the opposite. At midday, they would only see the barest hint of sunlight, with the Sun itself just beyond the horizon, while the rest of the day they would be in darkness.
Want to check your exact location? Try TimeandDate.com.
The view from space reveals the 24 hours of daylight at the Arctic