Methods for predicting the path edges and reducing observations of total solar eclipses for determining variations of the solar radius are described. Analyzed observations of the 1925 January eclipse show a 0.7 (arc second) decrease in the solar radius during the past fifty years

Dunham, D. W.

Dunham, J. B.

Fiala, A. D.

Sofia, S.

English

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ECLIPSERADIUSMEASUREMENTS*
DavidW. Dunhamand Joan BixbyDunham
ComputerSciencesCorporation
Alan D. Fiala
U. S. Naval Observatory
SabatinoSofia
GoddardSpaceFlightCenter
ABSTRACT
We have improvedmethodsfor predictingthe path edgesand reducing
observationsof totalsolar eclipsesfor determiningvariationsof the
solarradius. Recently-analyzedobservationsof the 1925Januaryeclipse
showa 0?7 (arcsecond)decreasein the solarradiusduringthe pastfifty
years.
INTRODUCTION
Methodsfor predictingthe locationof the edgesof pathsof total
solareclipses,methodsof reducingtimed observationsmade just insidethe
edgesof the path,and accuraciesof the solarradiusvariationsdetermined
from thesedata,are describedelsewhere(ref.I). Resultsfor the
eclipsesof 1715, 1976,and 1979have been reported(ref.2).
IMPROVEDRESULTS
Our previousprocedurefor computingthe non-spherlcalcorrectionsfor
eclipsepathedges (ref.1) has been replacedby a non-iteratlveschemeand
has been larBelyautomated. Similarly,we havewrittencomputerprograms
to efficientlyreducereportedBailybead and contacttimings,permitting
more comprehensiveanalysesof the data. The methodof reductionhas been
describedelsewhere(ref.3). For a reportedtiming,the computerprograms
printa plotshowingthe lunarlimbderivedfroma U. S. NavalObservatory
magneticdisk file of limbcorrectiondata (ref.4), the solar llmb,and a
pointerindicatingthe lunarfeaturewhich probablycausedthe event.
*Thisworkwas supportedby N.A.S.A.Grant NAS-5-24350,N.S.F.Grant
ATM-7743757,and the U. S. NavalObservatory.
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The selectedlunarfeaturecan be replacedwith anothercholce,If this
seemsappropriateafterexaminingthe plots. See Fig. I.
Besidesthe aboveimprovements,someof our other recently-published
resultshavebeen supercededbecausethe observedeventsused in the
analysiswere not restrictedto positionangleswithin30° of the lunar
axisof rotation(ref.5 and 6), in the polarregionswhere our arguments
aboutminimallibration-dependente_rorsare valid,or becausesome of the
computedresultswere misinterpreteu(ref.7). i
THE 1925 ECLIPSE
As a resultof E. W. Brown'sefforts,the total solareclipseof 1925
January24 was well-observedfromsites nearboth the northernand southern
limits(ref.8). Brown notedthat the observationsdeterminedthe location
of the actualpathedgesvery accurately,but that the resultcouldnot be
useddue to lackof knowledgeof the locationof the lunarvalleybottoms,
which causedthe obserwd contacts,with respectto the moon'scenterof
mass. Althoughthe southern-limitobservationsmade fromNew York City
were published(ref.9), the reportsfromthe northernlimitcollectedby
Brownhave apparentlybeen lost. Fortunately,we found a reportmade at a
professionalobservatorysituatedvery closeto the northernlimit (ref.
10). We havemeasuredthe positionsof the observersfrom l:24,000-scale
topographicmaps of the U. S. GeologicalSurvey, These positionsare
accurateto aboutl" in geographiclatitude,which contributesan errorof
onlyOVOI to our determinationof the solar radiusfrom the observations.
The correctionto the standardsolarradius(_59_63at a distanceof
l astronomicalunit)derivedfromthe 1925 eclipse,alongwith improved
valuesfor the correctionsfor previously-studiedeclipses,and the stand-
ard errorsof the corrections,are given in Table I. The lunarvalleys
which producedthe observedcontactsat the limitsof the 1925eciipsealso _
producedeventswhichwere timed in 1979 (twocontactsand one Bellybead
event)and in IgBO (threeBaily beadevents). The solarradiuscorrections
derivedfromsolutionsusing onlyth_seobservationsare also listed.
CONCLUDINGREMARKS
While no significantchangein the solarradiuscan be deducedfrom
observationsof recentecllpsc_,the data show that the radiusdecreasedby
at leastOV5 (0.05%or 350 km) between1925 and Ig7g. These two eclipses
havespecialsignificancesince theyare threeSaros cyclesapart,produc-
ing similargeometries,especiallysimilarlunar llbratlons.The decrease
is 0_70 ±0_13if all polartimingsare used. This resultis confirmed
(OV51_+0V24),to lessprecision,If only thoselunar featuresproOuclng
observedeventsduringboth eclipsesare used. Unlikethe resultfor the
1715eclipse,thereis no significantuncertaintyin the coordinatesof the
observersof this century'seclipses. The physicalsignificanceof varia-
tionsof the solar radiusis discussedin other papersin theseproceedlng_
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1982009140-118
REFERENCES
1. Softa, S.; Dunham,D. W.; and Ftala, A. D.: Determination of Varia-
tions of the Solar Radius from Solar Eclipse Observations. Proc. Conf.
Ancient Sun, 1980, pp. 147-]57.
2. Dunham,D. g.; Sofia, S.; Fiala, A. D.; Herald, D.; and Muller, P. M.:
Observations of a Probable Changetn the Solar Radius Between 1715 and
1979. Science, vo]. 210, no. 4475, 12 Dec. 1980, pp. 1243-1245.
3. Hera]d, D.: Observations of Batly's Beadsfrom Near the Northern Limit
of the Total Solar Ecltpse of June 20, 1974. The Hoon, vol. 16, 1976,
pp. 91-100.
4. Watts, C.B.: The Hargtnal Zone of the Noon. Astron. Papers Amer.
Ephem., vol. 17, 1963.
5. Ftala, A. D.; Dunham,D. W.; and Dunham,J. B.: Preliminary Results
from Observations madenear the Edgesof the Path of the 1980 February
16 Total So]at Eclipse. Bull. Amer. Astron. Soc., vol. 12, no. 2,
]980, p. 474.
6. Flala, A. D.; Dunham,D. W.; and Dunham,J. B.: Observations of Batly's
Beads from the Edgesof the Path of the Total So]at Eclipse of 1980
February 16. Bull. Int_an Astron. Soc., vol. 8, no. 2&3, 1980,
pp. 8]-82.
7. Dunham,J. B.; Dunham,D. W.; and F_a]a, A. D.: Solar Radius Varia-
tions from Observations of Four Eclipses. Bull. Amer. Astron. Soc.,
vo]. 12, no. 4, ]980, p. 832.
8. Brown, E. W.: Discussion of Observations of the Noonat or neap the
Ecllpse of 1925 Januaw 24. Astron. j., vol. 37, 1926, pp. 9-19.
9. Hagalhaes, F. V.; Law, C. L.; and Lteb, J. W.: Observations of the
Total Solar Ec]tpse of January 24, 1925, madeby Electric Companies
Affiliated wJ_. the Consolidated Gas Companyof NewYork. Trans.
I11umJn. Eng. $_c., vol. 20, no. 6, 1925, pp. 565-628.
10. Seagrave, F. E.: Short Eclipse Notes: At North Scituate, RhodeIsland.
Pop. Astron., vol. 33, ]925, p. 279.
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1982009140-119
YTABLEI. SOLARRADIUSCORRECTIONSDETEI_INED
FROMOBSERVMIONSNEARECLIPSEPMH EDGES
Date Ttmtngs wtthtn 30° Ttmtngs ustn9 features Lunar
of lunar axts determining 1925 events 11brattons
No. Are No. A_ long. 1at.
1715 Nay 3 4 +0':52 ±0':2 0 -- +1.°8 -0.02
1925 Jan. 24 4 +0.62 ±0.08 4 +0"62 +0'.'08 +2.5 -0.2
1976 Oct. 23 15 -0.23 ±0.14 0 -- -1.4 +0.1
1979 Feb. 26 33 -0.08 ±0.09 3 +0.11 +0.23 +1.7 -0.3
1980 Feb. 16 55 -0.03 ±0.04 3 -0.05 ¢0.35 -3.0 -0.1
120
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1982009140-120
1925 I 24
2 nd contact
l
.°°o
o.•
.°.
• °
o °°°
• • .
FigureI. Thls prlnterplotshows the predlctedposltlonsof the lunar
and solar 11_s computedfor the tlmeof secondcontactfor the observer
In New York Cityclosestto the southern11mltof the Ig2SJanuary;_4th
_t_1 ecllpsepath. The horlzontal11neof dots turks the Moon's_an
11_. S1dew_ysI's at the top and bottomIndlcatedlstances4" aboveand
l)elowthe man lunar 11_, respectlvely._terlsks connectedw|th a hand-
drawncurve show the actuallunarprof_leaccordlngto Watts'charts(_f.
4). Plusesmark the solar 11_. The vertlcalscale Is exaggerated27
tlmes relatlveto the _r4zontal scale. PosltlonanglesmeasuredIn
degreesfrom the Moon'snorthernaxlsas deflnedby Watts are glvenat the
bottom. The dot In the mean 11_ llne_Ich Is replacedby a s1_s
numberI Indlcatesthe computer-selectedcholceof the va11_ _|ch pro-
ducedsecondcon_ct, wherethe predlctedsel_mretlonof the actuallunar
11_ and the solar 11ni)was mlnlmlzed.
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1982009140-121


Eclipse radius measurements

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