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Many gathered to watch the sun rise on the longest day of 2014.

Science Behind the Solstice: The Longest Day of the Year

Scott Sutherland
Meteorologist/Science Writer

Monday, June 22, 2015, 11:17 - Spring is coming to a close here in the northern hemisphere, but how do we know when we've crossed from one season to the next? Here's the science behind the June Solstice.

Sunday, June 21, at exactly 12:39 p.m. EDT, marks the 2015 June Solstice - the point when the Sun reaches its highest point of the year in the northern sky and its lowest point in the southern sky, ushering in the northern summer and the southern winter seasons.

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. The equinoxes - spring and autumn - mark the transitions between those two halves of the year. The solstices mark the points when a particular hemisphere reaches its maximum angle, and the Earth's axis lines up with the axis of the Sun - summer solstice for the northern hemisphere, and winter solstice for the southern hemisphere.

The Longest Day of the Year

With the Sun tracing its longest arc through the northern daytime sky on June 21, it becomes the longest day of the year for this half of the planet, and the farther north you go, the longer the day gets!

Wandering Solstice

Since Earth's rotation doesn't exactly line up with how long it takes to orbit the Sun, the time of day when Earth reaches solstice changes from year to year, and it even jumps back and forth between days through the years. Going back to the beginning of the 20th century, the solstice fell on June 21-22 from 1900-1955 and was consistently on June 21 from 1956 through to 1987. Since then, it has varied between June 20-21 and will continue to do so until around 2075, when it will occur on June 20 for at least a few decades before it swings back towards June 22.

High Noon Solstice

This year is the first time in 25 years that the exact moment of the solstice will occur close to local noon in the Eastern Time Zone. The last time was on June 21, 1990, at 11:34 a.m. EDT.

The "latest" the solstice has occurred since then, for Eastern Time, was at 11:15 p.m. EDT on June 20, 1992. Fortunately for anyone celebrating the exact moment of the event, that was a Saturday night, so staying up late tied in nicely with the weekend.

More Time to Celebrate with Dad 

This particular June Solstice also happens to coincide with Father's Day in the United States, Canada and the United Kingdom - the first time since 2009.

So, join the Sun for a slight pause at the appropriate time, and celebrate the day by spending more time with Dad.

Sources: | NASA Earth ObservatoryMerriam-Webster

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