June 21 is typically the official first day of summer; however, this year, in the United States, it comes a day early on Wednesday, June 20 at 7:09pm. The variation in the exact date and time is primarily due to our system of measuring and recording time (the calendar). A year – one complete revolution of the earth around the sun – is 365.242199….days – a day being effectively one complete rotation of the earth. Because the number of days in a year is not an integer, or even a rational number, the solstices and equinoxes that mark the start of the seasons shift gradually within our calendar. Acting on the recommendations of astronomers including Nicholas Copernicus, Pope Gregory XIII (pictured below) in 1582 introduced reforms to the western calendar system (the Julian calendar) because the vernal equinox (first day of spring) had moved from March 21 to March 10 over the course of 1500 years. The problem with the older Julian calendar was that having a leap year every four years made the average calendar year 365.25 days – too long compared to the actual revolution of the earth by 0.007801 days or 11 minutes each year. This error accumulated from the time of the Romans causing not only the equinoxes but holidays like Easter to shift noticeable. By 1582 the situation had become embarrassing. The Gregorian calendar restored the dates of the start of the seasons and shortened the average year by introducing new rules for when a leap year is skipped. The Gregorian calendar works to keep the dates of the start of the seasons from drifting too far from the typical dates. However, the date and time for the summer solstice in a particular year can still fall on June 20th, 21st or even rarely on the 22nd.
In “A Private Universe”, the infamous documentary identifying the importance of directly addressing student’s misconceptions, Harvard graduates in their cap and gown incorrectly explain the cause of the seasons. See Harvard Graduates Explain Seasons. The common misconception that these highly educated men and women repeat is that the seasons are due to our planet being closer to the Sun in the summer and farther away in the winter. This simply cannot be the case, since the earth's orbit is very nearly a perfect circle, and our friends in the southern hemisphere are entering winter now as we begin summer. We share the same planet, so both hemispheres are the same distance from the sun. However, misconceptions learned early in life are very difficult to dispel.
Earth’s rotational axis tilt of 23.5 degrees is the ultimate cause of our seasons. The total amount of energy our planet receives from the sun is relatively (although not exactly) constant. A law of the physical universe called conservation of angular momentum causes Earth’s axis of rotation to remain pointed in the same direction as our planet revolves around the Sun - like a well-thrown Roethlisberger spiral or Meadowlark Lemon’s (if you don’t know who he is – ask your parents) basketball spinning on his finger. As illustrated below, this causes the distribution of sunlight on the globe to vary resulting in the yearly cycle of the seasons. On June 20 /21 the earth’s tilt results in the northern hemisphere receiving the most sunlight compared to any other day of the year – the longest day of the year.
Conversely, the southern hemisphere will receive the least and have the shortest day of the year. As our planet continues in its orbit every day after June 21, the northern hemisphere will receive a smaller share of the sun’s energy than the day before and the southern hemisphere will receive more until December 21. On this day, the winter solstice, the southern hemisphere will receive the maximum share of sunlight and the northern hemisphere will have the least. The closer to the poles the more extreme the difference, while closer to the equator, for instance between the tropics, the seasonal change is much less.
The longest day and shortest night of the year is not an event that makes observational astronomers happy. In fact, this time of year many observatories close for maintenance, especially in the American Southwest where monsoonal flow brings clouds and rain to a typically arid region. I imagine that vampires would not get too excited by the short nights of summer either. Perhaps that is why the release of Abraham Lincoln Vampire Hunter was slotted for Friday, June 22. While the title provokes an absurd interest, I cannot help but think that historians in the far future might think that their 21st century ancestors actually believed that our greatest President, who preserved the Union and wrote and delivered the Gettysburg Address and his Second Inauguration Address, hunted the undead in his spare time.
Perhaps it is a basic human tendency to insist on remaking our true heroes into superheroes. How else can Abraham Lincoln compete with Captain America, Iron Man, the Hulk and the rest of the Avengers? Maybe this genre will continue so that young George Washington fights French and Native American zombies in the western Pennsylvanian forests. Or, perhaps the young Teddy Roosevelt and his Rough Riders will take on an army of werewolves in the “real” storming of San Juan Hill. Soon every President will need a plague of the undead to liven up their story (take note Mitt Romney). While these flights into fantasy make an action-packed couple of hours, I wonder how much damage it does to our already fragile history education. Will Harvard graduates of the future have serious misconceptions not only in astronomy but also about our greatest President? Or like the influence Star Wars and Star Trek had on science, will this fictional rift on historic fact spur interest in the study of actual men and women who led us through difficult times? What do you think? Please comment below.