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Transcript of ^&ckniccil v£ote fS4 - GPOU.K.Bldg OCT291963 \^&ckniccilv£ote 92©.fS4...
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OCT 2 9 1963
\^&ckniccil v£ote 92©. fS4
LUNAR OCCULTATIONS OF TWO DISCRETE
RADIO SOURCES IN 1963-1964
JOHN A. EDDY
*^Vv:
U. S. DEPARTMENT OF COMMERCENATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS
Functions and Activities
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for testing materials, devices, and structures; advisory services to government agencies on
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of the Government; and the development of standard practices, codes, and specifications. Thework includes basic and applied research, development, engineering, instrumentation, testing,
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ments the basic program of the Bureau or when the Bureau's unique competence is required.
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NATIONAL BUREAU OF STANDARDS
technical ^Yiote fS4Issued October 11, 1963
LUNAR OCCULTATIONS OF TWO DISCRETE
RADIO SOURCES IN 1963-1964
John A. Eddy
Central Radio Propagation Laboratory
National Bureau of Standards
Boulder, Colorado
NBS Technical Notes are designed to supplement the Bu-reau's regular publications program. They provide a
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mission is obtained in writing from the Office of the Director, National Bureau of Standards, Washington25, D. C. Such permission is not needed, however, by the Government agency for which the Report hasbeen specifically prepared if that agency wishes to reproduce additional copies for its own use.
TABLE OF CONTENTS
I. Introduction 1
II. Importance of Lunar Occultations 1
III. Occultations of Taurus A and IC^3 in I963-I96J+ 5
IV. Summary 12
V. References 15
LUNAR OCCULTATI ONS OF TWO DISCRETE RADIO SOURCES IN I963 - I96U
John A. Eddy
The bright, extended radio sources Taurus A. andICUU3 lie close to the ecliptic and are occulted by themoon about every &§• years, providing rare opportunities to
study their angular brightness distribution with very highresolution. The nature of the next series of occultationsof these sources, which will occur for observers at Boulderduring the period from November 1963 through August 196k,is discussed.
I. INTRODUCTION
Eight years ago Bakulin and Shklovski [1955] published a reminder
to radio astronomers that lunar occultations of two major radio sources
were due to occur at different times over the earth in the year to come.
The sources were Taurus A (Crab Nebula) and IC^3j a galactic nebula in
Gemini; they are the only extended radio sources of significant flux
density that lie near the ecliptic. The period of recurrence of these
events, fixed by the ecliptic latitude of the source, is eight to nine
years. The last series were observed in Europe and Australia in October
and November 1955 an|I in January 195&- They will recur in North America
throughout the period from winter 1963 through summer I96J+.
II. IMPORTANCE OF LUNAR OCCULTATIONS
There are three principal reasons for observing the lunar
occultation of radio sources. First, an estimate of the electron
density of the lunar atmosphere can be obtained from accurate comparison
of the predicted times of occultation of radio source behind the moon
[Link and Neuzil, 195^] • The presence of a lunar atmosphere will cause
refraction of the radio waves received from the occulted source during
the times of emersion and immersion and will thus cause the observed
period of obscuration to be somewhat less than that expected in the
absence of refraction. In this way Elsmore and Whitfield [1955] and.
Elsmore [1958] observed occultations of ICMj-3 and Taurus A to derive
an upper limit for the density of the lunar atmosphere of about lO-13
that of the atmosphere of the earth.
The second reason for observing a lunar occultation is to determine
the precise position of the occulted source. From the known ephemerides
of the moon, the time of occultation of a source can be translated
directly into a very precise position of right ascension and declination.
Since the accurate determination of the positions of radio sources is a
major problem in the present techniques of radio astronomy, this method
is of unique value. It is, of course, rather limited since only sources
that lie within the path of the moon are available. Furthermore, most
occultations give information on only the right ascension of the source;
information on declination is afforded only in the case of a grazing
occultation by the upper or lower limb of the lunar disk. Hazard [1961]
has determined the right ascension of the source 3C212 to within 5" of
arc by this method and more recently [Bolton, 1963] ^as fixed- the
position of the distant source 3C273 "to within about 1".
Finally, occultations permit the study of the angular structure or
brightness distribution of occulted sources with unusually high
resolution. The method involved is precisely that employed in the study
-2-
of solar eclipses: the disk of the moon acts as a screen that progres-
sively isolates the emission from incremental areas of the source. For
most sources observed at the higher frequencies in the radio case, the
moon is a more intense emitter than the occulted source, but this does
not exclude the method provided the emission from the occulted source
is within the range of detection of the radio receiver. Resolution is
limited only by the Fresnel diffraction pattern at the lunar limb and
by the time and intensity resolution of the radiometer. Recently,
Scheuer [1962] has shown that the Fresnel diffraction limitation (about
18" of arc at 100 Mc/s and 6" at 1000 Mc/s) may be overcome by analytical
reduction, so that the strip distribution of the occulted source may in
practice be completely recovered from the diffraction records. The
angular distribution of brightness across the Crab Nebula was studied
by this method during the 1955-5& occultation by Costain, Elsmore, and
Whitfield [1956]; Boischot, Blum, and LeRoux [195^]; Tuominen and Karras
[1957]j ai*d Seeger and Westerhout [1957]. The last occultation of
10^3 was observed by Elsmore and Whitfield [1955] an(i by Rishbeth
[1956], whose reconstruction of its angular distribution confirmed the
tentative identification of the source that had been proposed by Baldwin
and Dewhirst [195^]- Other sources whose angular distributions have
been studied by lunar occultations are Kepler's Star [Rishbeth and Little,
1957; Talen, 1963] , Jupiter, and 3C273 [Bolton, 1963] . The method again
is limited to objects that lie near the ecliptic and whose flux densities
are within the sensitivity range of the radiometer employed. It seems a
particularly happy circumstance that the rather strong and interesting
sources Taurus A and ICU4-3 are included in these conditions.
-3-
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Table I summarizes some previous observations of lunar occultations
which have been reported in the radio astronomy literature.
III. OCCULTATIONS OF TAURUS A AND IC^3 IN 1963-196)+
It was stated in Section I that the occultations of Taurus A and
IC^3 recur in a period of eight to nine years, with the next occultation
season beginning late in 1963. The explanation for this cycle of
recurrence is given simply by the relation of the orbit of the moon to
the ecliptic. The angle of inclination of the lunar orbit to the
ecliptic is 5 9'j so "that the area of the sky traversed by the moon is
a band 10° 18* wide centered on the ecliptic. Within this band the
position and shape of the apparent lunar orbit continually changes,
following a major cycle of about 19 years that is fixed by the period of
the regression of nodes. The interval between occultations of sources
within the lunar band is thus determined by the 19 year cycle and the
particular ecliptic latitude of the source. Thus Taurus A and IC^3j
found within about 1° of the ecliptic, are occulted about every half
period, or every eight or nine years. The geometry of an occultation is,
however, much different on alternate half-cycles. Figure 1 shows the
path of the moon during the times of an occultation of IC^3 in January
1956 and a half-cycle later in March I96U. The lunar band about the
ecliptic is shown as a shaded area.
Figure 1 is constructed from geocentric coordinates taken from the
American Ephemeris. Because of the significant horizontal parallax of
the moon (about 1°) an exact calculation of a lunar occultation must be
corrected for the geographic position of the observer, i.e., topocentric
coordinates must be obtained.
-5-
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-6-
Table II lists the predicted occultations of Taurus A and ICU^3
during the I963-6U season calculated for the geographic position UO.1^8
N, 105° 231 W (the midpoint of the T-22 interferometer at Table Mesa,
north of Boulder). The source coordinates (epoch I96U) used in the
calculations were the following:
Taurus A 5*1
32m
19s
+21° 59!
8
ICH1+3 6h
15m
03S
+22° 36 !0
Taurus A (the Crab Nebula) is a source whose angular diameter is
about U' of arc; thus any occultation by the moon (angular diameter 31'
)
is likely to be a total occultation. Occultations of this sort occur
at Boulder on 13 May and 7 July 196^. ICM4-3, on the other hand, is a
more extended source whose radio diameter (about ^5') i-s larger than
that of the moon; thus an assortment of partial occultations is possible.
The principal area of interest in IC^U3^ at least at the longer wave-
lengths, is the north-following edge. Rishbeth [195&], i-n observing the
partial occultation of the northern half of the nebula visible in
Australia, noted that though only one tenth of the optical nebula was
obscured, the 3«5m radio radiation was reduced by over one fifth, and
concluded that the optical bright arch on the north-following edge is
the source of strongest radio emission. Observations of an occultation
of the sourthern part of the nebula visible in England [Elsmore and
Whitfield, 1955] seemed to bear this out, for an obscuration of a large
part of the southern half resulted in little change in radio flux. The
occultations to occur in the 1963-6^ season present a nice opportunity
to check these earlier results, since a rather complete range of IC^-3
occultations is predicted.
-7-
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-8-
The predictions given above were calculated on the 709O computer
using a program, RSL007, written by R. S. Lawrence. For a given span
of time at a given geographic location, the program computes (l) the
geocentric altitude (a) and azimuth (A) of the moon, (2) the topocentric
altitude of the moon (a 1
) using the formula
a' = a - rr cos a
where rr = the interpolated equatorial horizontal parallax for the moon,
and (3) the topocentric right ascension [a(a',A)] and declination
[5 (a 1, A)] of the moon. Asphericity of the earth is neglected. As an
aid to observation, the program also calculates the altitude and azimuth
of a given grid of points that describe the position of an occulted
source. The program is sufficiently general to predict the lunar
occultation of any fixed source at any geographic location on the earth.
One of the predictions was checked by hand using the rigorous formulae
given in section 2F of the Explanatory Supplement to the Ephemeris and
was found to be accurate to within less than 0l5 of arc, indicating that
the RSL007 predictions are satisfactory for predicting an occultation
to within a few seconds of time. However, their use for data reduction
purposes is not recommended. Completely precise predictions for
individual occultations are probably best obtained from the Nautical
Almanac Office; at least this has been the practice of previous observers
[Rishbeth, 1956; Costain, Elsmore, and Whitfield, 195&] . Moreover,
schedules of lunar occultations of a selected group of sources are
furnished without charge on a routine basis by requesting this service
from H. M. Nautical Almanac Office, Royal Greenwich Observatory.
-9-
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<*
Figure 2. Path of the center of the moon at times of conjunction with
the radio source ICU*4-3 as seen from Table Mesa from June
1963 through December I96I+. The small lunar symbol denotes
moonrise or moonset. Altitude and azimuth of the moon are
given above each track for the time of closest approach to
the radio source. Coordinates are topocentric right
ascension and declination.
-10-
.*>:*H5
zr'•-^l(Moy^
22'
. ^. •&}**,
21*
2Cf
TAJJRUS ft
size ofLUNAR DVSfc.
.— 21 S6PT64
7 ^64.
-<J>— i^343
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•*<FSB6i
4f*>v63
'29 06c 63
'OQ-63 ^.
— 5&L 42-2<ft
14 ^0^ 63
49 JX/ty63
5*36" ^34* %*»**5*ai &w 5%2Vc*
Figure 3« Path of the center of the moon at times of conjunction with the radiosource Taurus A as seen from Table Mesa from July 19^3 through NovemberI96U. The small lunar symbol denotes moonrise or moonset. Altitude andazimuth of the moon are given above each track for the time of closestapproach to the radio source. Coordinates are topocentric right ascensionand declination.
-11-
Figures 2 and 3 show the positions of the center of the moon and
the sources Taurus A and ICJ+J+3 for the series of occultations and near-
occultations that are in the visible hemisphere at Boulder from June
1963 through December 196k; they indicate the general march of the moon
through these two sky regions throughout this period. Coordinates are
topocentric right ascension and declination for the geographic position
of Table Mesa. Pairs of numbers (e.g. 67-225) given alongside the
tracks indicate the approximate altitude and azimuth of the center of
the moon at closest approach to the occulted source. A lunar symbol at
either end of a track indicates moonrise and moonset. Figure k compares
the geocentric and topocentric coordinates of the path of the center of
the moon for the ICMi-3 occultation of 20 March I96J+ to demonstrate the
effect of the coordinate conversion. The occultation on this date
occurs at a rather high altitude (about 67°); the conversion effect
would be more severe for events at lower altitudes.
IV. SUMMARY
Lunar occultations of the radio sources Taurus A and ICU^3 will
be observable at Boulder during late 1963 and early 196^ in step with
a cycle of recurrence of about &§• years. Taurus A will be occulted on
two occasions, both at elevations below 30° in the western sky. ICi+^3^
a more extended source, will undergo a series of very favorable partial
occultations.
Figure 5 illustrates the onset and duration of the coming occulta-
tion season on a time scale that covers a period of about If- years. The
ordinate is an importance scale, obtained rather subjectively by
weighting together (a) the fraction of the source that is occulted by
-12-
: ,/////////////,
i
> oO 03
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(oU) <D
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"<* MD TJ <Do\ a aH aS
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-13-
the moon, and (b) the observability of the particular occultation at
Boulder. Thus, the optimum occultation of ICM4-3 on 20 March 196^ is
rated perfect at 1.0, while the partial occultation of the same source
on k August 196^ that is interrupted by moonset is rated 0.1.
Observations of the Taurus A and ICh.U.3 occultations made during
their last occurrence provided the first accurate determination of the
angular extent of these two sources. The more favorable series of
occultations coming up should be even more profitable, especially if
observed on a variety of frequencies and at different polarizations.
The opportunity will not come again in this decade.
10-WWJSA
\CA4b
VWr
J J A S Q N t> .3 P*\A«*JJASOND
Figure 5. Graphical presentation of the data in Table 2, demonstrating
the nature of the Occultation Season of 1963-I96J+. The
ordinate is a scale of relative importance.
-Ik-
V. REFERENCES
Bakulin, P. I., and I. S. Shklovski (1955) "Eclipses by the Moon of Two
Discrete Sources of Radio Emission," Astron. Journal USSR, 32,
29-32; NRL Translation 526 by A. Pingell, Washington D. C. 1955-
Baldwin, J. E., and D. W. Dewhirst (195^) "Position and Identificationof a Bright Extended Radio Source in Gemini," Nature, 173 ; I6U-I65.
Boischot, A., E. J. Blum, M. Ginat, and E. LeRoux (1956) "Observationd'une Eclipse de la Nebuleuse du Crabe," Comptes Rendus 2^2 , 18^9-1851.
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-16-GPO 836-591
OF COMMERCELuther H.
NATIONAL BUREAU OF STANDARDSA.'V. Astin, Director
THE NATIONAL BUREAU OF STANDARDS
itional Bureau of Standards at its major laboratories in Washington, D.C., andin the following listing of the divisions and sections engaged in technical work.
search, development, and engineering in the field indicated byiption of the activities, and of the resultant publications, appears on the inside of the
• IINGTON. D.C.
Electricity. Resistance and Reactance. Electrochemistry. Electrical Instruments. Magnetic Measurements,tries. High Voltage. Absolute Electrical Measurements.
Metrology. Photometry and Colorimetry. Refractometry. Photographic Research. Length. Engineering Metrology.- and Volume.
Heat. Temperature Physics. Heat Measurements. Cryogenic Physics. Equation of State. Statistical Physics.
Radiation Physics. X-ray. Radioactivity. Radiation Theory. High Energy Radiation. Radiological Equipment.
eonic Instrumentation. Neutron Physics.
Analytical and Inorganic Chemistry. Pure Substances. Spectrochemistry. Solution Chemistry. Standard Refer-
rals. Applied Analytical Research. Crystal Chemistry.
Mechanics. Sound. Pressure and Vacuum. Fluid Mechanics. Engineering Mechanics. Rheology. CombustionControls.
Polymers. Macromolecules: Synthesis and Structure. Polymer Chemistry. Polymer Physics. Polymer Charac-terization. Polymer Evaluation and Testing. Applied Polymer Standards and Research. Dental Research.
Metallurgy. Engineering Metallurgy. Metal Reactions. Metal Physics. Electrolysis and Metal Deposition.
Inorganic Solids. Engineering Ceramics. Glass. Solid State Chemistry. Crystal Growth. Physical Properties.
Crystallography.
Building Research. Structural Engineering. Fire Research. Mechanical Systems. Organic Building Materials.
Codes and Safety Standards. Heat Transfer. Inorganic Building Materials. Metallic Building Materials.
Applied Mathematics. Numerical Analysis. Computation. Statistical Engineering. Mathematical Physics. Op-erations Research.
Data Processing Systems. Components and Techniques. Computer Technology. Measurements Automation.
Engineering Applications. Systems Analysis.
Atomic Physics. Spectroscopy. Infrared Spectroscopy. Far Ultraviolet Physics. Solid State Physics. ElectronPhysics. Atomic Physics. Plasma Spectroscopy.
Instrumentation. Engineering Electronics. Electron Devices. Electronic Instrumentation. Mechanical Instru-
ments. Basic Instrumentation.
Physical Chemistry. Thermochemistry. Surface Chemistry. Organic Chemistry. Molecular Spectroscopy. Ele-
mentary Processes. Mass Spectrometry. Photochemistry and Radiation Chemistry.
Office of Weights and Measures.
BOILDER, COLO.
CRYOGENIC ENGINEERING LABORATORY
Cryogenic Processes. Cryogenic Properties of Solids. Cryogenic Technical Services. Properties of CryogenicFluids.
CENTRAL RADIO PROPAGATION LABORATORY
Ionosphere Research and Propagation. Low Frequency and Very Low Frequency Research. Ionosphere Re-search. Prediction Services. Sun- Earth Relationships. Field Engineering. Radio Warning Services. VerticalSoundings Research.
Troposphere and Space Telecommunications. Data Reduction Instrumentation. Radio Noise. Tropospheric
Measurements. Tropospheric Analysis. Spectrum Utilization Research. Radio-Meteorology. Lower AtmospherePhysics.
Radio Systems. Applied Electromagnetic Theory. High Frequency and Very High Frequency Research. Fre-
quency Utilization. Modulation Research. Antenna Research. Radiodetermination.
Upper Atmosphere and Space Physics. Upper Atmosphere and Plasma Physics. High Latitude Ionosphere
Physics. Ionosphere and Exosphere Scatter. Airglow and Aurora. Ionospheric Radio Astronomy.
RADIO STANDARDS LABORATORY
Radio Standards Physics. Frequency and Time Disseminations. Radio and Microwave Materials. Atomic Fre-
quency and Time-Interval Standards. Radio Plasma. Microwave Physics.
Radio Standards Engineering. High Frequency Electrical Standards. High Frequency Calibration Services. High
Frequency Impedance Standards. Microwave Calibration Services. Microwave Circuit Standards. Low Frequency
Calibration Services.
Joint Institute for Laboratory Astrophysics- NBS Group (Univ. of Colo.).
NBS