Science behind the solstice: The longest day of the year
This year is the first time in decades that we'll see a rare event: a Full Moon on the same night of the June Solstice. But how do we know when we've crossed from one season to the next? Here's the science behind the June Solstice.
Today, Monday, June 20, 2016, at 6:34 p.m. EDT, is the 2016 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.
Full Moon Solstice
Along with this transition between the seasons, this year we also have a rare event — a Solstice Strawberry Moon.
A "Strawberry Moon" doesn't mean the moon will turn a certain shade of red tonight. That only occurs during a lunar eclipse, and unlike the past two years, we don't have any total lunar eclipses during 2016. The name "Strawberry Moon" is a traditional name in North America, given to the June full moon, and was used to indicate when the berries would be ripe for picking.
This year, the Strawberry Moon just happens to fall on the same day as the June Solstice, something that hasn't happened here in nearly 70 years!
The last time this happened was on June 22, 1967, for some parts of the world — namely Greenland and points eastward to the International Date Line. For those of us in North or South America, however, the dates of the Solstice and the Full Moon missed each other by roughly an hour on that night! We'd have to go back 68 years, to June 21, 1948 for the last occurrence!
The next time it happens after this, again for Greenland and points eastward, is 46 years from now, on June 21, 2062. The Lower 48 actually won't see another June Solstice Full Moon for the rest of this century! We will see a rare December Solstice Full Moon on Dec 21, 2094, though.
The exact moment of the Full Moon on June 20, 2016 actually happens after sunrise for the eastern third of of the country, but it will still be full enough when it rises again roughly 12 hours later that it will take a keen eye to notice the difference.
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.
