On 21 August 2017, a rather historic event
occurred – a total solar eclipse occurred that was visible across the entire
contiguous 48 states of the United States of America, across a narrow corridor
about 110 km wide. This is the first time since June 8, 1918 that such an event
has occurred, and only the second time in the history of the United States. However,
the previous time a total solar eclipse exclusively made landfall on those land
was back in 13 June, 1257 – several hundred years before the US was even
founded.
Here in Namibia, we’ve also not seen any
total eclipse since the founding or our nation – but to be fair, it’s been a
scant 27 years. Not to worry however – shortly after our Nation’s 40th
birthday, we will be able to witness one. On 25 November 2030, at 6h24 in the
morning, just after sunrise, the sun will seem to start going out again. By
07h18, the sun will be completely obscured by the moon, and Windhoek will be in
the moon’s shadow. At 07h20, we’ll breathe a big sigh of relief as the sun becomes
visible again, and full visibility will be restored by 08h21, when the sun will
appear normal again.
How can I possibly know this? Why, science!
It works! Of course, it helps that humanity has been observing and recording
eclipses for over 2000 years. It became much easier with the invention of
writing, which is why it was the Mesopotamian civilization that first
determined the length of a Saros cycle – or 18 years, 11 days and 8 hours. This
is the amount of time it takes for the sun, moon and Earth to return to the
approximate same relative geometry – quite simply, it’s the time between one
eclipse and the next when the three celestial bodies line up in the same
straight line.
As a result, they were able to determine
with relative accuracy when an eclipse would occur – but not why or how it
does. First the Greeks entered the scene – they felt you cannot understand
something unless you can explain it. Their observations established that that
planets and the moon where spherical, and thus, that the eclipse is that shadow
of the moon cast by the sun onto Earth. Of course, the Greeks had one
fundamental flaw in their approach – the assumed the Earth was the centre of
the universe. A perfectly understandable assumption, for it is one we all tend
to make when considering our lives.
The same kind of work was being done in
China, and it is even speculated that the Mayans measured eclipses, though
their data was lost due to the Spanish Conquistadors. But in 1543, that changed
due to the work of one Nicolaus Copernicus, in his “De revolutionibus orbium coelestium,” or “On the revolution of
heavenly spheres.” Here he postulated that unlike what was assumed by
Aristotle and the other Greeks, the Earth actually orbited the sun. But
Copernicus’ model still had significant shortcomings. But soon, they were
addressed…
For in 1687 a young man by the name of
Isaac Newton published a work known as the “Philosophia
Naturalis Principia Mathematica” or “The Mathematical Principles of Natural
Philosophy.” Not only did this introduce to the world Newton’s Laws of
Motion, and his Theory of Gravity, but the this volume of his treatise, “De Mundi Systemate” or “On the system of the world,” he applied
the propositions he made earlier to the motions of the planets and moon as
observed in the solar system, and ushered in modern astronomy.
By the time telescopes were invented, these
calculations could be done with much greater accuracy, and by the 1700’s
astronomer Edmond Halley even published a path of a coming eclipse in the hopes
that people would not panic when it happens. It ushered in the era of the
modern eclipse watching. In 1824, Friedrich Bessel invented the method used for
calculating the occultations of eclipses – or the movement of the shadow and
its shape over an irregular object such as the earth, allowing for calculations
to show the precise location of the shadow accounting for latitude, longitude
and elevation above sea level.
Of course, these methods all assumed that
the moon was a perfect sphere – and today we know it’s not. Charles Burleigh
Watts spent most of the time during the 1940’s to 11963 to mapping the
variations in the moon’s surface, allowing predictions to get even more
previse, as now the shadow wasn’t a perfect oval. NASA’s Lunar Reconnaisance
Orbiter was later used to refine Watt’s work, as it was able to capture the
moon’s topography in much greater detail than would have been possible from the
surface of the Earth.
So how precise is the predictions now of
eclipses – specifically the one I mentioned for Namibia, due in 2030? Well, not
100% - after all, we still don’t know all the variables quite that precisely.
Neither the moon nor the Earth is perfectly round, and that ‘squashiness’ is
only known to a certain degree of precision. Furthermore, the movement of the
Earth and the moon are not constant. While the Earth revolves around the sun
once every 365.24219 days and the moon around the Earth every 29.530575 days, a
number of forces could speed or slow it by minute fractions, throwing
calculations off.
But even so, this means the calculations
are at most a few seconds off. Back in 1932, the New York Times warned
astronomers that the eclipse due on 31 August 1932 was the last one that they
could reliably see from US until August 21, 2017. I propose this – save this
paper, and we’ll have a discussion on Monday, 25 November 2030 at 07h21 about
the predictive power of science.
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