Glimpsing Summer

It is Memorial Day weekend again. It was originally called Decoration Day though both names have been in use since the 19^th century. By the mid-20^th century Memorial Day became the more common usage. President Garfield spoke in Arlington National Cemetery at the first one in 1881. It was intended as a day to honor the fallen in the Civil War by decorating their graves. May 30 was chosen as the date because it was not the anniversary of a major Civil War battle and therefore applied more broadly. In World War I the day was expanded to include the fallen in all wars. Some of the decorative traditions still in use date to then, such as red poppies inspired by the 1915 poem In Flanders Fields by Canadian soldier-poet John McCrae. In 1968 the Monday holiday bill let the date float to fall on the last Monday in May in order to make a three-day weekend.
 
The holiday makes a convenient “unofficial start of summer” in much the same way that Labor Day marks the “unofficial start of autumn.” Neither makes much astronomical sense, but both do match a shift in the weather pretty well, at least in the northern states and similar latitudes elsewhere. May rarely makes up its mind to what season it wants to belong but it is generally immediately followed by warm weather. Daytime temperatures in NJ this May, for example, have waffled between 40 and 90 F (4 and 32 C) with no obvious trend. Memorial Day itself (tomorrow) is anticipated to be 45 degrees (7 C) but the first week of June should be balmier.
 
Summer is my favorite season. As I’ve gotten older I’ve favored warmth, so I’m happy to pretend that summer starts on Memorial Day. I save my seasonal celebration until the weekend of the actual solstice (June 20 this year) however. I don’t burn a wicker man with sacrifices inside it as Caesar describes the Gauls doing in his Commentaries. I just grill burgers (arguably much the same thing) and I try not to burn them.
 
Fortunately, the shape of earth’s orbit makes summer particularly long in the northern hemisphere. The seasons are not of equal length because the planet’s orbit is an ellipse. Earth’s distance to the sun ranges from 147 500 000 km (perihelion) to 152 500 000 km (aphelion). Aphelion occurs this year on July 5. Since orbiting bodies sweep out equal areas in equal times, this makes the time between the spring and autumnal equinox about 5 days longer than the time between the autumnal equinox and the subsequent spring equinox. Summer days in the northern hemisphere are marginally cooler than if the distances were reversed, but the extra days of summer insolation more than make up for the difference so the net effect is extra warming.

nasa.gov

It will not always be so. Earth’s orbit precesses about a day every 58 years so the distances will eventually reverse. Precession combined with regular changes in the eccentricity of the earth’s orbit and polar tilt (between 21.5 and 24.5 degrees – it is presently 23.5) create the Milankovitch cycles that are believed to drive the onset and retreat of ice ages. (The northern hemisphere is key because it contains most of the world’s land mass, which responds more radically to changes in insolation than the mostly oceanic southern hemisphere.) Whatever the short term (meaning the next couple hundred years) prospects for warming might be, the longer term points to massive cooling. 12,000 years ago there was a mile of ice on top of where I’m now sitting. Short of geoengineering on a gargantuan scale it will be there again. The eccentric but award-winning astronomer/mathematician Fred Hoyle in his book Ice: The Ultimate Human Catastrophe proposed deliberately heating the oceans to prevent a new ice age. This is probably not a good idea at the current time.
 
At my age I don’t need to worry much about the date of the next glacial maximum – nor do you. Tomorrow matters though, and tomorrow is the unofficial start of summer. Even though 45 is a bit chilly, I just might put a lawn chain in the sunlight and catch some rays.

 

Blue Cheer - Summertime Blues