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OCT 2 9 1963

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LUNAR OCCULTATIONS OF TWO DISCRETE

RADIO SOURCES IN 1963-1964

JOHN A. EDDY

*^Vv:

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS

technical ^Yiote fS4

Issued 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 Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

For sale by the Superintendent of Documents, U. S. Government Printing Office Washington, D.C. 20402

£BLGE 2 CENTS
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25, D. C. Such permission is not needed, however, by the Government agency for which the Report has been 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. and

ICUU3 lie close to the ecliptic and are occulted by the

moon about every &§• years, providing rare opportunities to

study their angular brightness distribution with very high

resolution. The nature of the next series of occultations

of these sources, which will occur for observers at Boulder

during 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 progressively 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.

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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.

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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

19

s +21° 59!

ICH1+3 6

h

15

m

03

S +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 wavelengths, 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.

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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-
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c* Figure 3« Path of the center of the moon at times of conjunction with the radio

source Taurus A as seen from Table Mesa from July 19^3 through November

I96U. 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.

-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 nearoccultations 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 occultation 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-
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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

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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 Identification

of a Bright Extended Radio Source in Gemini," Nature, 173 ; I6U-I65.

Boischot, A., E. J. Blum, M. Ginat, and E. LeRoux (1956) "Observation

d'une Eclipse de la Nebuleuse du Crabe," Comptes Rendus 2^2 , 18^9-

1851.

Bolton, J. G. (1963) "The Australian 210' Reflector and Its Research

Program," Proc. IRE Australia, 2k, 106-112.

Costain, C. H., B. Elsmore, and G. R. Whitfield (1956) "Radio Observations

of a Lunar Occultation of the Crab Nebula." M.N.R.A.S. Il6, 38O-385.

Elsmore, B. (1957) "Radio Observations of the Lunar Atmosphere," Phil.

Mag. 2, 101+0, No. 20.

Elsmore, B. (1958) "Radio Observations of the Lunar Atmosphere," Paris

Symposium on Radio Astronomy, R. N. Bracewell, ed. , Stanford

University Press, Stanford, lj-7-^8.

Elsmore, B., and G. R. Whitfield (1955) "Lunar Occultation of a Radio

Star and the Derivation of an Upper Limit for the Density of the

Lunar Atmosphere," Nature, 176 , I1.57-I1.58.

Hazard, C. (1961) "Lunar Occultation of a Radio Source," Nature 191, 58.

Link, F. , and L. Neuzil (195^0 "Occultation des Radiosources par la Lune

en 1955-1957," Bull. Astr. Insts. Csl. 5,. 112-115.

Rishbeth, H. (1956) "An Investigation of the Radio Source 06N2A in

Gemini," Aust. J. Phys. 9, I4-9I4-- 50U.

Rishbeth, H., and A. G. Little (1957) "A Lunar Occultation of the Radio

Source Associated with Kepler's Supernova," Observatory, 77> 71-7^

Scheuer, P. A. G. (1962) "On the Use of Lunar Occultations for

Investigating the Angular Structure of Radio Sources," Aust. J. Phys. 15, 333-3^3-

Seeger, C. L., and G. Westerhout (1957) "Observations of Two Occultations

of the Crab Nebula by the Moon at 400 Mc/s." B.A.N. 13, 312-316.

-15-
Talen, J. (1963) (University of Michigan Radio Observatory) private

commuaication

.

Tuominen, J. , and M. Karras (1957) "A Lunar Occultation of the Crab

Nebula at Wavelength 3-7 Meters," Suomalainen Tiedeakatemia,

Series A, VT, Physica, 6.

-16-

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