Summer solstice 2017 splits countries down the middle
Tuesday, June 20, 2017, 19:52 GMT - The timing of the 2017 Summer Solstice often splits countries down the middle, but how do we know when we've crossed from one season to the next? Here's the science behind the June Solstice.
Tuesday, June 20 and Wednesday, June 21, 2016, at the 1st days of northern summer for 2017! It may seem strange to have two 1st days of summer, but it all depends on where you live.
Tune in to talk Solstice and the Total Solar Eclipse
Check out the embedded livestream below, from 5 p.m. to 6 p.m. ET on Wednesday, as the Slooh Community Observatory welcomes Planetary Society CEO Bill Nye, and Syfy's Bad Astronomy science writer Phil Plait, as they discuss the solstice, and give a preview talk about the upcoming Total Solar Eclipse in August!
You can go to Slooh.com to join and watch this live broadcast, snap and share your own photos during the event, chat with audience members and interact with the hosts, and personally control Slooh's telescopes.
A Solar "Pause"
The word solstice comes from the Latin word solstitium which means sun-standing - denoting how the sun climbs higher and higher in the sky at local noon since winter, appears to "stand still" in that progression on this day of the year, and then being a progression towards lower and lower noon positions in the sky, until it once again "stands still" at the winter solstice.
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 as this pattern in the sky repeats. 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 the world for how it's 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.
The Hollow globe of the world at the Captain James Cook Memorial, at Regatta Point of Lake Burley Griffin in Canberra, Australia. Credit: Leon Brooks
The reason that globes are nearly always tilted to one side is more than decorative. It reflects the tilt of the Earth itself - by roughly 23.4o - with respect to the path it traces out as it orbits the Sun.
It's this tilt that's responsible for our seasons.
Rather than "wobbling" back and forth by 23.4 degrees, though, the Earth's tilt is relatively constant (it has varied by about 1o-1.5o over tens of thousands of years, but the Moon keeps the tilt fairly stable).
Although Earth's distance from the Sun does change throughout a year, this only causes a small difference in the amount of energy the planet receives from the Sun, and thus this difference has very little effect on our seasons. In fact, the Earth is actually closest to the Sun during northern winter (in early January) and farthest from the Sun in northern summer (in early July).
It's the angular difference, based on hemisphere and latitude, that makes the major difference and 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.
The Longest Day(s) of the Year
Although June 21 is the official 1st day of summer for 2017, since the precise moment of the solstice occurs just after midnight, the longest day of the year for 2017 varies depending on your location. In Canada, it's June 20 for the western and northern parts of the country, and it's actually a tie between June 20 & 21 for the southeastern part of the country.
As usual, the farther north you go, the longer the day get
Since Earth's rotation doesn't line up exactly with the length of the planet's orbit, the time and day when we reach solstice changes each time we go around. It even jumps back and forth between days through the years.
Going back to the beginning of the 20th century, the solstice wandered back and forth between June 21-22 from 1900-1955, but then it was only on June 21 from 1956 through until 1987. Since then, it has varied between June 20-21 and will continue to do so until 2075, when it will occur on June 20 for at least a few decades before it swings back towards June 22.
Watch Below: Why what you wear in the summer could make all the difference.