This is the Classical Astronomy Update, an email newsletter especially
for Christian homeschool families (though everyone is welcome!) Please feel free to share this with any interested friends.
To every thing there is a season, and a time to every purpose under
the heaven: A time to be born, and a time to die; a time to plant, and
a time to pluck up that which is planted.... - Ecclesiastes 3:1-2
IN THIS UPDATE
Bonestell Film - DC Screening
Book Mini-Review - "American Eclipse"
The Times of the Seasons
Hello Friends,
I'm pleased to report that this film is coming to the Washington DC area on Tuesday, April 2, 7:30 PM, at the IMAX theater at the National Air & Space Museum. Click this link for more info. If you live in the DC area, beat the rush and order tickets early. If you do attend, stick around afterwards for the panel
discussion with the film's producer and some other eminent authorities.
Since DC is my "home away from home," I'm planning to attend this screening. If you're in the area, please drop me an email and maybe we can meet up.
This film continues to win prestigious awards around the country. Please visit the film's website for more info about Bonestell and his work, and to find other screenings around the USA.
For more information about topics from Classical Astronomy discussed in this newsletter, please check out a homeschool astronomy curriculum (but popular with adult readers too!)
Visit our archive of previous editions of the Classical Astronomy Update newsletters, going back to 2007.
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Book Mini-Review - "American Eclipse"
We recently had the pleasure of hearing a talk by author David Baron discussing his recent book, American Eclipse. This is a wonderful story about the American total solar eclipse of 1878 and three notable individuals who
observed it -- James Craig Watson, Maria Mitchell, and Thomas Edison.
Unless you're an avid student of astronomy history, you may not have heard of Watson and Mitchell. Their stories are fascinating. But the story of Edison's involvement in the eclipse is truly remarkable. I especially appreciate that "Young Edison" is the subject, the youthful, upcoming inventor whose fertile imagination was then still changing the world.
This is a refreshing angle since we hear so much lately about "Old Edison," portrayed as the fat, grey villain in the story of Nikola Tesla. But this is young Tom of decades previous, the slender, dark-haired inventor of the phonograph, who was a kind of "rock star" of the post-Civil War era, the light bulb and many of his other attainments still in his future.
More than just an account of the eclipse itself, this story provides much insight into the culture of the Gilded Age, when America was being transformed from the agrarian international backwater of its founding into the
industrial powerhouse of the 20th century, and when American science was first garnering respectability from the European scientific establishment.
Moreover, this book is just a fun read, plain and simple... a very enjoyable and informative look at a lesser-known slice of Americana. I encourage you all to check out American Eclipse by David Baron.
The Times of the Seasons
You might hear a weather report this week announcing that "spring begins today at 5:58 PM, Eastern Daylight Time." If you're like me, you're probably wondering why that is, how they know the time to such accuracy and precision, or what that's even supposed to mean.
Ever since the time of the Ancient Greeks, spring is traditionally reckoned to begin at the precise moment when the Sun is centered on the celestial equator, a circle of the celestial sphere that defines the midpoint between the north and south celestial poles. This is also the precise moment when the Sun is perfectly overhead as seen from the Equator on the Earth.
As seen in the sky, the apparent annual path of the Sun through the constellations is the ecliptic, which corresponds to the plane of the Earth's orbit. The vernal equinox is the point in the sky in the constellation Pisces where the ecliptic intersects the celestial equator as it ascends from the southern half of the sky to the northern half.
On the day of the vernal equinox, March 20, the Sun crosses this point of intersection. The precise time of spring's beginning occurs when the Sun is centered along the celestial equator. (For a complete explanation of this and other seasonal signposts, see our Signs & Seasons curriculum.)
As seen from the perspective of the the solar system, a seasonal year is completed when the Earth returns to the same point in its orbit where it was the year before, when the Sun was last seen to line up with the vernal equinox point. This would represent the Earth moving a complete 360 degrees between alignments with the vernal equinoctial point. However, the precise interval between two successive alignments is
called the tropical year, and is found to be 365.24219 days. This works out to be 365 days, 5 hours, 48 minutes, and 42 seconds.
This does not work out to be an even number of days, and there's no reason why it should. The cycle of the day has traditionally been referenced to the rotation of the Earth, which is completely separate from the time required for the Earth to circle the Sun. It would be a miracle if the year did work out to be an even number of days!
So after a typical standard year of 365 days, there is a shortfall of 0.24219 days, so the Earth does not revolve a complete 360 degrees around the Sun between two successive equinoxes. This annual shortfall represents the difference between a typical calendar year and the precise value of the tropical year.
Of course we all know that there is a leap year every four years to account for this annual shortfall. In a leap year, an intercalary day is added to the calendar on February 29. This helps keep the calendar roughly on track, on average, over the span of time. But if you look at the times for the beginning of spring over a period of successive years, you can see the shortfall piling up from
year to year, as the starting times occur later and later in the day for the date of March 20. But after four years have elapsed, the annual shortfall adds up to roughly a full day, and spring begins at a time close to that of the previous leap year, but not precisely the same time.
The amount of the annual shortfall is close to 6 hours, but not exactly so. The actual amount is about 11 minutes, 18 seconds less than 6 hours. For this reason, the full 24 hours of February 29 causes the time of equinox alignment to occur about 40 minutes earlier on the clock than the previous leap year. So spring began in in the leap year of 2016 at 4:30 UT (where UT is universal time), which was
12:30 AM EDT. But in the next leap year pf 2020, spring will begin at 3:50 UT, which is 11:50 PM EDT on March 19.
That extra 40 minute discrepancy every four years itself adds up over a long span of time. This is the reason why the Gregorian Calendar was adopted in 1582, to "clean up" for all those tiny amounts. According to the Gregorian Calendar, the discrepancy adds up to three extra days every 400 years! So to keep the equinoxes lining up properly on the correct date, Pope Gregory XIII decreed that three days
be dropped from the calendar between 1600 and 2000. So the centennial years 1700, 1800, and 1900 were not leap years, and did not include February 29. Assuming our descendants stick with this system, the centennial years 2100, 2200, and 2300 will also not include February 29.
Don't worry if all these numerical gyrations get confusing! They still confuse me and I've been trying to untangle these concepts for 30 years! But it's important to be very careful when approaching this subject and not get too turned around!
For your convenience, here's a list of seasonal starting times for 2019, adapted from The Old Farmer's Almanac:
SPRING Wednesday, March 20, 5:58 P.M. EDT
SUMMER Friday, June 21, 11:54 A.M. EDT
FALL Monday, September 23, 3:50 A.M. EDT
WINTER Saturday, December 21, 11:19 P.M. EST
Til next time, God bless and clear skies, -
jay The Ryan Family Cleveland, Ohio, USA When I consider thy heavens, the work of thy fingers, the moon and the stars, which thou hast ordained, what is man that thou
art mindful of him? and the son of man, that thou visitest him? - Psalm 8:3-4, a Psalm of David
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