Issue link: https://wardsworld.wardsci.com/i/1379459
Eclipse (continued) Saros An important coincidence relates lunar months and eclipse years. A total of 223 lunar months (technically, synodic months, the period of the phases) takes 6585.32 days. A total of 19 eclipse years—passages of the Sun through the same node of the Moon's orbit—takes 6585.78 days, and 242 nodical months—passages of the Moon through the node—take 6585.36 days. (Nodical months are also called draconic months, after the ancient Chinese dragon once thought to have been devouring the Sun at a solar eclipse.) Thus eclipses appear with this period of 18 years 111/3 days (plus or minus a day, depend- ing on leap years), a period known as the saros. Further, 239 periods of the variation of distance of the Moon from the Earth, the anomalistic month, is 6585.54 days, so the relative angular sizes of the Sun and Moon are about the same at this interval. (The anomalistic month differs from the nodical and sidereal months because the orientation of the Moon's elliptical orbit drifts around in its orbital plane.) As a result of the saros, almost identical eclipses recur every 18 years 111/3 days. The significance of the 1/3 day is that the Earth rotates one-third of the way around, and the eclipse path is shifted on the Earth's surface. Thus the June 30, 1973, 7-min eclipse in Africa was succeeded in a saros series by the July 11, 1991, eclipse in Hawaii, Mexico, and Central and South America, which reached maximum duration of 6 min 54 s in Mexico, and the 6 min 40 s eclipse of July 22, 2009, observed from China and the Pacific Ocean. After a saros, the Sun is slightly farther west than its original position, and the Moon is slightly north or south, depending on whether it is near an ascending or a descending node, so the eclipses in a saros drift from north to south or from south to north, starting near one pole and departing from the other. A complete series takes 1244 – 1514 years. The next solar eclipse with over 7 min of totality will be in a different saros, and will occur on June 25, 2150. Motion over Earth's surface The Moon's shadow travels at approximately 2100 mi/h (3400 km/h) through space. The Earth rotates in the same direction that the shadow is traveling; at the equator, the resulting motion of the Earth's surface makes up about 1040 mi/h (1670 km/h), making the speed of the eclipse across the Earth's surface 1060 mi/h (1730 km/h). When eclipses cross higher latitudes, the speed of motion associated with the Earth's rotation is not as high, so the eclipse speed is even higher. A supersonic Concorde (a test model, with windows cut into its top since the eclipse was high overhead) took advan- tage of an equatorial eclipse in 1973 to keep up with totality for 74 min. Ordinary jet aircraft cannot keep up with eclipses, so the term "eclipse chasing" is almost never accurate. Scientific value Even with advances in space technology, total solar eclipses are the best way of seeing the lower corona. Coronagraphs, telescopes for which special shielding and internal occulting al- low observation of the corona from a few sites in the world on many of the days of the year, are limited to the in- nermost corona or to use of special filters or polarization. Coronagraphs have been sent into orbit, notably aboard Skylab, the Solar Maximum Mission, and SOHO, but limitations in spacecraft control lead to the necessity of overocculting the photosphere, cutting out the inner corona. For example, the co- ronagraph on Solar Maximum Mission occulted 1.75 times the solar diameter. The innermost corona- graph, no longer in operation, on SOHO's Large Angle Spectro- graphic Coronagraph (LASCO) over-occulted to 1.1 solar radii, and LASCO's two remaining coronagraphs image the Sun from 1.5 to over 30 solar radii. The coronagraphs on NASA's Solar Ter- restrial Relations Observatory (STEREO) also over-occult. Other types of observations from space also apply to the corona, such as imaging x-ray observations. Further, any space observation is extremely expensive, over 100 times more for SOHO than for a given extensive eclipse expedition. So it is useful and necessary to carry out ground-based eclipse observations, ground-based coronagraph observations, and space-based observations to get the most complete picture of the Sun. Further, independent + ward ' s science Fig. 5 Solar eclipse of April 8, 2005, observed during totality from a ship in the mid-Pacific Ocean, west of the Galápagos Islands. (Credit: Jay M. Pasachoff and Dava Sobel/Science Faction)