August 15, 2017 > Eclipse Journey©: Introduction excerpt
Eclipse Journey©: Introduction excerpt
By Dr. Eric Wegryn Ohlone College Astronomy Department
DarknessÑ in the middle of the day.
The Sun is blocked out, and night falls ... much too early
The Moon orbits around the Earth every month, and twice a year it passes in front of the Sun. Twice a year its path aligns with Earth's orbit about the Sun as the Moon becomes new, and it passes directly between the Earth and Sun, blocking the Sun's light and casting its shadow on us.
A total solar eclipse is one of the most spectacular natural events. The minutes of total darkness are preceded by an hour of gradually decreasing sunlight. Air temperature falls. Winds and weather change quickly. Birds and other animals may become confused as night comes early. During totality, the landscape is darkened; the sky goes black, and stars become visible. The sun is hidden, but its position in the sky remains marked by the crown of light from its atmosphere. After a just a few minutes, a ray of light returns, and gradually daytime revives. A total solar eclipse is an amazing experience.
Or so I told my students. Every semester in astronomy class I would recount this celestial ballet. I knew the facts well enough to make the awesome spectacle interesting to those who had never seen an eclipse.
But the eclipse lecture always left me feeling a bit empty. The reasonÑ I was just passing on book knowledge, an understanding of concepts that I had read about ... but never experienced myself.
And so, I resolved to correct this deficiencyÑ I would go and see a total solar eclipse. Solar eclipses occur twice every year. But most are not total. And even when the Moon blocks 70 or 80 percent of the Sun, human eyes adjust to the decreased illumination, just as on an overcast day; people subjected to a partial solar eclipse may remain totally unaware of the celestial occultation above their heads.
Total solar eclipses are not only less frequent, they are very localized. The shadow of the Moon (umbra) is a tapering cone, only a few hundred kilometers wide at most when it hits the ground. As the Moon moves in its orbit, the shadow moves across the Earth, tracing out a path of totalityÑ only within this narrow region will sunlight be completely cut off and observers experience a total eclipse. The rapid motion of the shadow also means that totality only lasts a few minutes at most.
So, to experience nature's greatest astronomical show of light and shadow, one must be in the right place, at just the right time.
As a teacher of science, I continually highlight to my students the Scientific MethodÑ observation of patterns in the world around us, a hypothesis or theory to explain the regularities and a prediction or experiment to test the validity of the theory.
I also point out that astronomy may be considered the oldest form of science, because people have been practicing this method to understand and predict lunar and solar eclipses for thousands of years.
Basic predictions are easyÑ an eclipse requires a syzygy, an alignment of Earth, Moon and Sun. Lunar eclipses can only happen at Full Moon, solar eclipses at New Moon. The synodic period of the Moon's phases is 29? days.
Yet eclipses do not occur every month. This is because the Moon does not orbit Earth exactly in the same plane that Earth orbits the Sun; its orbit is tilted about 5¡ from the ecliptic plane. So, usually the Moon passes north or south of the Sun, and above or below Earth's shadow, in its monthly path around the sky. The plane of the Moon's orbit intersects the ecliptic plane along a straight line, called the line of nodes. The nodes are the two places, on opposite sides of its orbit, where the Moon passes through the ecliptic plane. Only when the line of nodes is pointed toward the Sun, so the Moon crosses the ecliptic at the same time it is New or Full, is there precise alignmentÑ an ecliptic syzygy.
As the Earth and Moon travel around the Sun, this happens twice a year, for each type. Thus, solar and lunar eclipses come in pairs, about every six months.
Astronomy predates science itself.
Modern astronomers have become very good at precisely predicting eclipses, when and where. I personally am not one of those doing eclipse calculations (my research has focused on exploration of the planets and their satellites through spacecraft missions), but over the years I have personally tested many of the predictions of other astronomers, and have come to trust them with a high level of confidence.
It was time for the big one.
The predictions told me that there would be a total solar eclipse on the 29th day of March, 2006. The path of totality would trace northwest across the Sahara Desert of North Africa, the eastern Mediterranean Sea, the Anatolian peninsula, and into western Asia. My science educator colleagues of The Exploratorium of San Francisco would be covering it live, from Turkey.
The name and image of this place resonated in my mind. Istanbul was once ConstantinopleÑ capital of the Roman Empire for a thousand years! At the crossroads of Europe and Asia, combining Greek, Roman and Turkish culture and heritage, this would be an unparalleled opportunity for not just a scientific expedition, but a historical and cultural odyssey.
And so my mind was setÑ one way or another, I would be in Turkey on 29 March 2006. And I would put myself in the right place, at just the right time, to have the Moon pass exactly between me and the Sun, and cast its shadow on me.
For four minutes.
Dr. Erõc Wegryn is a professional scientist, author and educator. After circumnavigating the planet ÑRound the World in 32 DaysÑ he finished his graduate research and doctoral dissertation detailing findings from the Mars Pathfinder mission. He then relocated to the Bay Area, where he has worked with NASA and the SETI Institute on the Cassini Mission to Saturn, discovering polycyclic aromatic hydrocarbons on Iapetus. When not exploring or teaching, he is also a science journalist, covering space missions and solar eclipses.
To order Dr. WegrynÕs book, Eclipse Journey, or view other excerpts from his books, visit: www.blurb.com/user/DrWegryn