May 27, 2020

Astro-Event of the Week, June 17th-24th 2008

   The Summer Solstice is upon us: time to set those sun dials…  This week marks the beginning of summer in the northern hemisphere, a yearly event in which the sun pauses at its farthest passage north as seen from the Earth and begins its long trek south. This is not a spectacular observational event, and the summer solstice  itself only marks a specific moment: June 20th, 2008 at 07:59PM, EDT. What makes this event special is that in 2008, it occurs extremely early, in fact, the earliest since 1896.

First, to clear up some misconceptions. The solstice is not necessarily the day with the earliest sunrise; that falls this year on Saturday, June 14th. The two are interrelated, but things such as our friend, the Equation of Time, complicate affairs. In any event, days on either side are nearly as long; an appreciable notice in the length of night won’t even occur until around mid-August. The northern latitude nights, however, are extremely short. From high northern latitudes such as here in northern Maine, astronomical twilight does not even end until mid-July! Part of the reason the solstice tends to oscillate between June 20th & the 22nd has to do with the Earth’s own orbital motion. Our calendar year is based on what astronomers know as a Gregorian year. We add a leap year on every year divisible by four, such as 2008. Here is where it gets complicated; have no fear, Astroguyz is with you. We skip a leap year in every century year (1700, 1800, 1900…) except those that are divisible by 400. Such a century leap year occurred only on the years 1600 & 2000, and won’t happen again until 2400. This offsets the Tropical year of 365 days, 5 hours, 48 minutes and 46 seconds. Why bother, you might ask? Because even this infinitesimal amount adds up over centuries. Soon, you’ll have Christmas falling in Northern Hemisphere summer, and so on. By the 16th century, Easter was falling a full ten days out of sync, due to the church’s usage of the Julian calendar, which assumed a 365.25 day year. Our present day Gregorian calendar fixed this discrepancy.  This was established by Pope Gregory the XIII, who declared in 1582 that October 4th that year would be followed by October 15th, to get the calendar immediately back in sync. (Hooray, early payday!) This present day system also means that the summer solstice in the northern hemisphere will always fall between June 20th & the 22nd, although it always “creeps” between the two until leap years (or lack of century leap years non-divisible by 400) resets the cycle. From 2002 to 2014, some of the earliest and latest summer solstices are;

Latest: 19:10, June 21st, 2003

Earliest: 23:09 June 20th, 2012

So, next Friday (assuming it’s sunny,) Take a noon stroll and note how high the sun (but don’t stare at it!) sits in the sky. Notice how short the midday shadows are. Of course, if you’re down under, the reverse will be true! Enjoy it while it lasts…Northern winter is now only six months away! 

The Astro-related word of the week is ecliptic. As the earth travels around the sun, the sun seems to trace out an imaginary path on the celestial sphere. This plane is actually a product of our orbit, and is currently tilted about 23 1/3 degrees to our equator. This is known as the ecliptic, and during its yearly “journey,” the sun passes through the twelve traditional constellations of the zodiac, plus the constellation Ophiuchus. The summer solstice is merely the most northern point above the celestial equator on the ecliptic; its opposite six months later is the winter solstice. At the intersections of the celestial equator and the ecliptic are the Autumnal and Vernal (or spring) equinoxes.

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