Thursday, August 10, 2017

Amidst the hype, an Eclipse book of value

kw: book reviews, nonfiction, science, history, eclipses

OK, let's get the ooh-and-aah stuff out of the way first. This image shows the eclipsed Sun in an intermediate state: a medium amount of corona and several prominences are visible. The solar prominences are the red bits around the rim of the Moon. The image was enhanced by unsharp masking to show more of the corona, which has a sharp drop-off of intensity with distance from the solar photosphere (the "surface").

Viewing an eclipse without magnification, you are unlikely to see the prominences, so it helps to have a telescope set up ahead of time, its clock drive running, ready for action the instant that second contact occurs. A magnification of 30-to-60x is sufficient. This is about how the Sun would look at 30x.

Perhaps you know that the Sun has an 11-year cycle of activity. During periods of low activity, it is more likely to look like this (and this photo was enhanced also). This is an older, black-and-white photo, but I suspect few prominences would have been visible in a color image.

Interestingly, even in quiet years the corona may be quite extended, though it tends to be smoother. 2009 was a very quite year, according to records at, the Sun's face was free of sunspots on 260 days, 71% of the year.

At the peak of a sunspot cycle, sunspots are typically visible every single day, or very nearly. Sunspots are evidence of the "wound-up" condition of the magnetic fields inside the Sun. Prominences and flares are triggered by magnetic re-combination events.

A large, active corona is seen here. Looking carefully (click on the image for a larger version), you can see prominences. The rather bright blob at right might be a coronal mass ejection. When one of these occurs in the center of the Sun's face, we can expect a magnetic storm on Earth in 2-3 days' time.

To see what the outer corona looks like any time, look at the LASCO images at the Solar and Heliospheric Observatory (SOHO) satellite's image and video gallery here. One cannot see the close-in corona because the satellite's coronagraph is about two solar diameters across. Sometimes I've looked at a video of the past week or so and been able to watch a comet "auger in".

Now, to the book. John Dvorak is an exceptionally good writer, with much of value to say, and in a time of extraordinary hype about the solar eclipse that will occur across the entire U.S. in just 11 days, he has produced a valuable book of lore, history, and scientific explanations: Mask of the Sun: The Science, History, and Forgotten Lore of Eclipses.

While most people through history have viewed eclipses of both Sun and Moon as dramatic omens of misfortune, there have always been a few wiser folk who realized that though they are so infrequent, they are subject to natural laws. While a total solar eclipse is visible over a small area, a swath no more than 112 km across, partial eclipses can be seen as far as about the diameter of the Moon (3,473 km) on either side of the central path…or a bit farther because of the curvature of the Earth's surface. Thus, if there is a solar eclipse going on, the majority or people on the sunlit side of Earth at the time will be able to witness at least a partial eclipse.

Since the sky doesn't darken much during a partial solar eclipse, how were they noticed in antiquity? Think pinholes. The crescents seen here were in shadows cast by leaves of a tree. If you are used to seeing the round dots on the ground or a wall in a tree's shadow, then you'll likely be drawn to the view when they change shape. Pinhole viewing of partial solar eclipses has been recorded over at least the past 2,400 years.

So, although an average location on Earth experiences one total solar eclipse about every 330 years, a partial eclipse is likely to be seen about every 2-3 years from almost anywhere. With a bit greater frequency, almost anywhere you live you'll be able to see an eclipse of the Moon almost every year, because they are visible from an entire hemisphere at once.

In classical times, one of the seven required subjects of  a classical education was Astronomy, which actually meant learning to gather naked-eye observations and make the calculations to determine the motion of the Moon and the naked-eye visible planets (Mercury, Venus, Mars, Jupiter and Saturn), primarily for astrological purposes and to (very roughly) predict eclipses. Much of Mask of the Sun discusses the ebb and flow of lore and superstition about eclipses, both lunar and solar. Kings and emperors employed skilled mathematicians to predict eclipses, because unfriendly (or hype-engrossed) persons were making the same predictions, and then predicting the likely demise of whomever was in power at the time. A leader with better advance knowledge could then take advantage of public magical ceremonies intended to stave off the disaster and survive the eclipse, which really meant to stave of the likelihood of a revolt.

Eclipses earned great practical value during the "age of sail": they can be used to determine longitude. It isn't easy, but it was too valuable an aid to navigation to not perform. First, one must have a good (relatively speaking) time-measurement device. The water clocks and other mechanical timekeeping devices in use before the pendulum clock was invented in 1656 (by Huygens) were better than counting heartbeats, but not by much. You, the seafaring captain intent on determining the location of some distant port, would contract with an astronomer at home to determine the time at which certain critical events occurred,and their location in the sky, usually during a lunar eclipse. This requires a bit of explanation.

The shadow of a planet or satellite has two parts, the Umbra and the Penumbra. When you see a total solar eclipse, during the time of totality you are standing inside the Umbra. Before and after totality, and in any place where a partial eclipse is witnessed, that is in the Penumbra. There are thus four contacts that delimit a total solar eclipse:

  1. The Moon first impinges on the edge of the Sun.
  2. The Moon fully covers the whole Sun.
  3. The Sun first begins to exit from behind the Moon.
  4. The last bit of the Moon exits the edge of the Sun.

The same four contacts pertain to a total lunar eclipse, except they refer to the impingement of first the penumbra of Earth's shadow, then the umbra, shading the Moon, and then the Moon's exit from first the umbra and then the penumbra.

By taking readings with a sextant or octant of the Moon's position in the sky when each contact occurs, and noting the time of each as exactly as possible, both you and the astronomer back at home gather data that can be used to calculate the longitude difference between the place you were and your home port. Of course, latitude is much easier to measure in the Northern hemisphere by sighting the north star. Seeing the orientation of the Big Dipper lets you correct for the star's offset from the actual pole, which is presently about one degree (Because of Earth's precession, Thuban in the constellation Draco was the star nearest the pole 5,000 years ago, when the pyramids were a-building in Egypt). Prior to the late 1700's, when very accurate marine chronometers were invented, it took months to learn "where" you had been! And then you might still be off by a few degrees (each degree is 60 nautical miles, that is 69 mi or 111 km).

During a total solar eclipse stars become visible. In 1919, this photo was taken and the two stars marked with little dashes were among those used to verify Einstein's general theory of relativity.

Spectroscopy of the solar corona was first done in the 1860's, and led to a paradox that has not yet been resolved. The spectroscope had revealed that the Sun's photosphere is at a temperature of about 5800K (about 10,000°F), and later that the middle part of the chromosphere, a thin pinkish layer just above it, is at about 3800K (about 6,400°F). But the corona had a puzzling spectrum that wasn't figured out until the 1930's and 1940's: its temperature ranges from one to three million kelvins! That's two to more than five million °F.

Before I close I must mention the two central solar eclipses I have seen. The first was July 20, 1963, when I was not quite 16. The Moon's shadow crossed from northwestern Canada to Maine. My family took a vacation starting nearly two weeks earlier, to Montreal and Quebec, and then on the 20th we crossed into Maine at a spot where the highway would be right at the center of the umbra. I had fitted a telescope with a projection screen, with which we watched from just prior to first contact until second contact. Then we looked at the sky to see the Sun and its corona. The hillside had a view to the northwest, and we saw the umbra racing toward us just before second contact. Seeing something, even a shadow, approach at 2,000 mph is amazing! Seeing the "hole in the sky" surrounded by a large corona was amazing! In just over a minute, it ended and third contact occurred. We saw the "diamond ring", the first bright ray of sunlight peeking through a mountain pass on the Moon.

The second was the annular eclipse that passed through Ponca City, Oklahoma, May 10, 1994, when I worked for Conoco. This picture shows the projection screen attached to my telescope, and the eyepiece is visible at the right edge. This is the same telescope I used in 1963, and I still use it. Annular eclipses occur when the Moon is in a further part of its orbit, near apogee, and doesn't cover the entire Sun.

Conoco management gave everyone half the day off. School groups and others were invited on-site. A filtered video camera was used to broadcast the eclipse inside the buildings on TV monitors usually used for executive communications. At least twelve telescopes were brought onsite by Conocoans and a few others, and used, usually by projection, to show the Sun to groups of people. One friend of mine brought a large telescope fitted with a full-aperture solar filter, so you could look through his wide-angle eyepiece at a 100x view of the whole Sun. Now, that was an amazing view!

While the publication of Mask of the Sun was timed to take advantage of public interest in the solar eclipse that will be seen all across North America on August 21, 2017, it is not hyping the eclipse, but instead giving us a primer into the past and continuing importance of eclipses. For example, eclipses on earth and elsewhere (notably, shadows of Jupiter's moons on that planet's cloud tops) are still one of the key ingredients to measuring planetary distances in the solar system. I have deliberately touched on only a few of the many delightful matters covered in the book. It is well worth reading by anyone with any level of scientific education.

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