Wednesday, June 21, 2017

Illum oportet crescere... the Summer Solstice

St. John the Baptist, from the Isenheim Altarpiece, by Matthias Grunewald.

"Illum oportet crescere, me autem minui."

Today we reach the Summer Solstice -- which takes place a precisely at 04:24 UT, 21 June 2017.  At my own home, that translated to 12:24AM this morning (Eastern Daylight Time).  This day, then, is the day in which daylight lasts the longest and summer begins in the Northern Hemisphere, and is gone the fastest and winter begins in the Southern.

For some excellent charts and information on length of days, sunset, sunrise, and the like, this site is splendid:

This site, too, allows you to customize your location and get a host of details on astronomical objects: Heavens Above

It is well to note the sort of pious religious connection to these events that was observed by the Medievals: the Summer Solstice falls near the Feast of St. John the Baptist (24 June), while the Winter Solstice is close to Christmas (25 December).  We know from Sacred Scripture that St. John the Baptist was, indeed, born six months before Our Lord.  Thus, the amount of daylight decreases from the Feast of St. John to Christmas, and increases from Christmas to the Feast of St. John.  This brings to mind the quotation of St. John the Baptist in Scripture, John 3:30: "Illum oportet crescere, me autem minui."  In our vulgar tongue: "He must increase: but I must decrease."

File:Analemma Earth.png
This chart show the analemma for Earth, showing the relative locations of the Sun at noon at the Greenwich Observatory in England.  Notice the change in both altitude and azimuth at the different points of the year.

What, exactly, is the solstice?

On this day, the Sun is directly overhead at real noon on the Tropic of Cancer.  In the days of old, the Sun would appear in front of the constellation Cancer on this day, hence the name of the line on the globe.  Thanks to the procession of the equinoxes, however, the sun is now in Gemini at the Summer Solstice.

As a side note, it is during this time of year that a Full Moon stays the lowest in the sky in the Northern Hemisphere, and reaches its highest point, in the Southern, this in terms of degrees from the horizon.

Why do we have a Summer Solstice, you ask?  Well, I am glad you did!  If the Earth sat directly upright on its axis, the Sun would always be directly overhead at noon on the equator, it would appear to move through the stars along the celestial equator, days would always remain the same length, and every day would be like the two equinox days in Spring and Fall.  As it happens, the Earth is tilted at about 23.5 degrees on its axis.  Thus, the sun appears to diverge as much as 23.5 degrees from the celestial equator in its apparent path through the stars (the ecliptic), and ends up being directly overhead up to 23.5 degrees north or south of the equator (the Tropics of Cancer and Capricorn) on the two solstice days.  This is the reason, of course, for our seasons.

File:North season.jpg
This diagram shows the positions of the Earth in relation to the sun at those four points -- the Summer solstice on the left, with the Northern Hemisphere tilted toward the Sun, and the Winter solstice on the right, with the Southern Hemisphere facing the Sun more directly, with the two equinoxes between, with the Hemispheres equally oriented to the Sun.

So, if we start at the vernal equinox in March, the sun is directly overhead at the equator at noon, and night and day are the same length.  From then until today, the summer solstice, the Sun appears to move slowly to the north both in the sky, setting a bit further north of west each day, and in its apparent path through the constellations of the zodiac.  This continues until the Sun reaches the solstice, where it stops, being directly overhead at 23.5 degrees north latitude (the Tropic of Cancer) at noon, and sitting about 23.5 degrees north of the celestial equator.  From that point, the sun drifts back south until reaching the equator once more at the autumnal equinox in September, going all the way to 23.5 degrees south at the Winter Solstice.

The reason for all of this is that as the Earth orbits the Sun the two hemispheres of the Earth take turns being tilted toward the Sun.  The following diagrams might help to illustrate what I am trying to articulate:

File:Ecliptic path.jpg
In this diagram, the sun appears to move against the background of the stars along the red line, the ecliptic, while the white line marks the celestial equator -- the imaginary line through space that is merely the extension of the Earth's equator.  The two points where the red and white lines are at greatest divergence are the solstices, while the two points where the red and white lines cross are the two equinoxes.  The yellow line shows what the sun would appear to be in front of from the Earth, the Constellation Pisces, at the vernal equinox.

I leave you, then, with the words of St. John the Baptist:  "Illum oportet crescere, me autem minui."   "He must increase: but I must decrease."

Live well!

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