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'Decen�ib�rl�7Q / Vol 9 Ne 1Z / APPLI�D"QPTI�S 2617

the difficult questions concerning the details of thesun's magnetic field. Throughout an increasingnumber of solar observations, the sheet polarizingmaterial introduced by Laiid well over 30 years ago hasbecome widely accepted. This material has been anextremely valuable aid for optical designers in theirdevelopment of large aperture instruments.

Use of large jet aircraft as platforms for observingsolar eclipses provides these five advantages: sup-plying the transportation, eliminating the possiblecloud cover, giving a viewing position at high altitude,optimizing the observer's position with respect to theeclipse path, and stretching out the time of totality byflying along the umbral track. Although airplaneshave been used in one way or another for observingeclipses for about half a century, the first concertedeffort for using sophisticated instrumentation came atthe 20 July 1963 eclipse over the North Americancontinent. Three large jet planes were used, and theresults were sufficiently successful that this method hascontinued to be applied for every major eclipse sincethen.

The general field of radio astronomy has contributedmuch to our understanding of the sun over the lastquarter of a century. Studies of coronal, chromo-spheric, and disk emissions in the radio frequenciesyield information which complements the opticalobservations and provides new data for our storehouseof knowledge. In particular, the study of the relation-ship between radio emissions associated with solarflares and the optical observations of the flares hasproved to be extremely fruitful. Radio observationsalso have the obvious advantage of not requiring theclear skies which are so important to the opticalstudies.

The eclipse of 7 MWarch 1970 attracted literallythousands of amateur and professional astronomersfrom all over the world. As a witness to this interest,the small airport at Oaxaca, Mexico, was a seethingmass of international humanity on S \farch, all awaitingchartered or scheduled flights to return homeward. Butthese numbers of people were relatively small comparedto the vehicles which pounded the Pan Americanhighway on the night of 6 M\/arch in order for theiroccupants to find a favorable spot for the morningeclipse. They were indeed fortunate and well rewardedbecause the cloudless skies in Southern Mexico on7 March 1970 showed a rich blue color without thecirrus clouds which had prevailed the previous week.Not so fortunate were observers in Southeastern UnitedStates who found only a thick overcast. However,the favorable weather over Northeastern United Statesprovided this heavily populated area with a once-in-a-lifetime treat that yielded much significant, information.

Figure 1 shows the weather conditions existing overmuch of the northern hemisphere on eclipse day asrecorded by the NASA ATS-3 satellite. Note thesuperposition of the approximate eclipse path, the clearskies over Southern Mexico, the cloud bank over theGulf of \'Iexico extending into Southeastern United

States, and the actual eclipse shadow over NortheasternUnited States.

Because of the high probability of clear weather inSouthern \exico, most professional astronomersselected eclipse sites from 75 km to 150 km southeastof the city of Oaxaca. Albert E. Belon of the NationalScience Foundation served as the United States coordi-nator for the 1970 eclipse, and he also helped with manyof the foreign expeditions. In his final bulletin helisted 57 optical observations, of which 52 were plannedin the Southern Mlexico location. Typical of thecampsites in this region was the French expeditionshown in Fig. 2. It was located near Nejapa in thebeautiful Tehuantepec River Valley close to the PanAmerican highway. Arcadio oveda, Director of theAstronomical Institute of the National University ofMlexico, served as the coordinator in \iexico and estab-lished a headquarters in Oaxaca. All the eclipse ob-servers owe a sincere debt of gratitude to Dr. Povedaand the iMexican Government for the excellent cooper-ation and thorough arrangements which permeatedthroughout the whole operation.

Solar astronomers always look forward to the nexteclipse. Since the earth is treated to as many as fivesolar eclipses in a year and we must always have atleast two, it may appear that plenty of occasions existfor observations. However, most solar eclipses are ofthe annular variety in which the angular diameter of themoon is smaller than that of the sun. Total eclipsesoccur only about one each year and a half. Statisti-cally we would expect the optimum observing position(the noon point) to occur three times as often over wateras it does over land, and this is the case. In addition,this position seems to have an affinity for rugged terrainand remote places when it does occur over land. (Actu-ally, this statement shows that only a small percentageof the earth furnishes ideal observing sites.) As a casein point, the next ten years will make available seventotal eclipses from which we may reasonably expect toobtain useful data.

Date10 July 197230 June 197320 June 197423 October 197612 October 197726 February 197916 February 1980

Location

Northern CanadaCentral AfricaSouth Pacific OceanSouth Pacific OceanNorth Pacific OceanNorthern CanadaCentral Africa and Indian Ocean

Although the maximum observing time of totalityduring this period for ground-based observers is about25 min, the actual time will be much less because of theremote locations and potential cloud cover.

One of the methods of improving the situation comesfrom the use of space platforms. NASA plans tolaunch a number of large orbiting platforms during thecoming decade with the first one being Skylab A in1972. Present plans call for the installation of acoronagraph with external occulting disks which Gordon

2618 APPLIED OPTICS / Vol. 9, No. 12 / December 1970

Fig. 1. Weather conditions over much of the northern hemisphere on 7 March 1970 with the approximate eclipse path.

Newkirk of the High Altitude Observatory has alreadysuccessfully flown on balloons. The big step forwardwith this system will be an opportunity to study thedynamical properties of the solar corona on a day-to-daybasis, which is impossible under earth-bound observingconditions. As instrumentation and techniques aredeveloped for solar observations from space platforms,much of the eclipse chasing expeditions to the remotecorners of the earth may well be rendered obsolete.

The papers selected for this issue of Applied Opticscover a wide range of eclipse instrumentation andrepresent the efforts of eclipse observers from fivecountries. Techniques used include those from thepurely optical-photographic to sophisticated electronicand well automated devices. Although some of theauthors have presented preliminary results from the7 March 1970 eclipse, we have asked them to stress theinstrumentation side of their eclipse work and will lookforward to the detailed results of their observations inthe various astronomical publications.

Fig. 2. Site of the French expedition near the town of Nejapain Southern Mexico.

December 1970 / Vol. 9, No. 12 / APPLIED OPTICS 2619