Photographic measurements of Saturn and its ring

Reese, E. J.

English

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https://ntrs.nasa.gov/search.jsp?R=19710018508 2020-03-11T19:59:22+00:00Z
tNEW MEXICO STATE UNIV,
LAS CRUCES, NEW MEXICO 88001
AREA 505 646-2614
Observatory
MEASUREMENTS OF SATURN IN 1969
E. J. Reese
N71-27984
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(PAG 5)
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(CODE)_
a (NASA CR OR TMX OR AD NUhil3ER) ----`^^`^--(CATEGORY)
The Observatory*
New Mexico State University
Las Cruces, NewexM :- o 88001
May 1970
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*TN-701-70-33 is supported by NASA Grant NGL-32-003-0
i
MEASUREMENTS OF SATURN IN 1969
:;
INTRODUCTION
Some recent measurements of Saturn and its rings are summarized in
this note. The results are based on measurements of good quality photo-
graphs with a Mann measuring machine. The photographic plates were
exposed at the f:75 Cassegranian focus of the NMSU 61-cm reflector.
Photographs of Saturn used in measuring the latitudes of the belts
and the dimensions of rings A and B were taken in orange light. Stan-
dard exposures produced images of the planet having an optical density
of about 0.65 above the plate background. Images of Saturn having an
optical density of 0.9 above the background were selected for measuring
the equatorial and polar diameter of the planet, and 1.2 above the back-^q	 p	 p	 ,
ground for measuring the inner edge of ring C.
The measurements reported here involved a slight change in our usual
technique. Diffraction at the edges of the cross wires in the measuring
machine impairs definition as a wire approaches the edge of a feature such
as a delicate belt. This uncertainty in the measurements has been eliminated
by using as a reference an exceedingly small, well .-defined black spot on
the reticle having a diameter of about 10 microns. This reference spot is
comparable in size to the grain in the emulsion of our plates and, although
easily visible at all times, the spot does not appear to modify delicate
detail in its vicinity.
2.
LATITUDES OF SATURN'S BELTS
The latitudes of the edges of Saturn's belts were measured on nine
photographic plates of excellent quality taken in orange light between 30
September and 5 November (Fig. 1). The results of these measurements are
tabulated in Table I, while the adopted nomenclature of the belts is
explained in Fig. 2.
It is inconvenient that different kinds of latitude are in general use
for Saturn and Jupiter. Planetographic latitude is so generally used for
Jupiter that a change would not appear to be desirable. In the case of
Saturn, however, planetocentric latitude is more commonly used in the belief
that it is more significant from the terrestrial observer's point of view.
Actually, as pointed out by B. M. Peek (1958), mean eccentric latitude is
more convenient than either of the other two kinds of latitude. Not only
is the sine of the mean eccentric latitude the measured fraction of the
polar semidiameter when the Earth is overhead at the planet's e quator, but
the cosine of the mean eccentric latitude multiplied by the equatorial
radius of the planet gives the radius of rotation at that latitude. The
three kinds of latitude mentioned above are computed as follows:
Mean eccentric latitude: 	 S' = 0 + D'
Planetocentric latitude: tan R = tan s' a R
Planetographic latitude: tan a" = R tan a'
where 0 is the angle whose sine is the measured fraction of the polar semi-
diameter, R is the ratio of equatorial diameter to polar diameter, and
i3.
D' is the angle whose tangent is equal to R times the tangent of the plane-
tocentric declination of Earth at the time of observation.
Table I gives our results in all three kinds of latitude.
The aspect of Saturn's belts was considerably different in ultra-
violet light (Fig. 3) where the Equatorial Zone was very dark and extended
southward to Saturnigraphic latitude -15°3. There was a dark shading in
the South Polar Region which extended northward to latitude -64° Between
these two dark shadings were two conspicuous belts, the STB extending from
-27°8 to -36°4 and the SSTB extending from -41°9 to -49°4. A remarkable
feature in ultraviolet light was the excessive brightness of the northern
hemisphere of the planet north of the rings. We might speculate that the
shadow of the ring system to the south of this brilliant zone, and perpetual
night in the North Polar Region to the north might chill the upper atmo-
sphere and produce a highly reflecting cloud layer near the tropopause.
DIAMETER OF SA'T'URN (OPPOSITION, 1969)
Wavelength
0
6200 A (orange)
Equatorial diameter 	 Polar diameter
Unit	 Unit
distance
	
Kilometers distance. Kilometers
1657997	 120,393 ±220 148 1;306	 107,562 ±193
Oblateness
0.1066
6.
4.
DIAMETER OF SATURN'S RINGS (30 SEP 69-5 NOV 69)
Number of Diameter at Diameter in
Object measurements unit distance kilometers
Ring A, outer edge 49 377:'137 273,527 t 68
Ring A, inner edge 38 336.066 243,739 ± 94
Cassini's Division 66 329,783 239,182 g 44
Ring B, outer edge 43 324.557 235,392 ± 55
Ring B, inner edge 40 252.770 183,327 + 83
Ring C, inner edge 15 206.883 150,046 ±175
Photographs used in measuring rings A and B were taken in orange light
and given standard exposures. Measurements of ring C, however, were made
only on photographs taken in blue light and given three times the standard
exposure (Fig. 4). From these measurements the width of Cassini's Division
is 4,174 km.
BRIGHT SPOT IN SATURN'S SOUTHERN HEMISPHERE
Only on rare occasions does a discrete marking in Saturn's atmosphere
become conspicuous enough to provide a good reference for determining the
rotation period of the planet (Dollfus, 1963). The 1969-70 apparition of
Saturn was made unusually interesting by the appearance of a small but
well-defined bright spot at Saturnigraphic latitude 57°S, which persisted
for several months.
Our first record of the bright spot is on a photograph taken by A. S.
Murrell on 25 October 1969 (Fig. 5) which indicates that the spot was on
S.
the planet's central meridian at 0743 UT. Photographs taken during the next
few months (Fig. 6) revealed the spot to be nearly fixed in latitude and
rotating once every 0.4421 days. Little change was observed in the size or
brightness of the spot: measurements gave it a mean length of 6000 km and
a mean width of 8000 km. Photographs taken in orange, green, and blue light
indicate that the spot was yellowish in color. Our last record of the spot
was on 7 February 1970 when the spot was on Saturn's central meridian at
0219 UT. Even on this last date of observation, the spot still appeared
very much as it did three months previously; however, the S.S.S. Temperate
Belt apparently was fading during the interval of observation. It was in
the south edge of this belt that the bright spot produced a small notch or
gap.
Although the bright spot required only slightly better than average
seeing to be recorded photographically with the 24-inch reflector, it proved
to be a very elusive object for visual observers. R. B. Minton was unable
to see it with an 8-inch reflector in very good seeing, the writer was un-
able to see it with a 16-inch reflector in good seeing, and J. C. Robinson
could not see it visually with the 24-inch reflector even though 11e knew
where the spot should be and described the view as one of the best he had
ever had of Saturn. Clyde Tombaugh, however, was able to see the spot by
glimpses with his 16-inch reflector during moments of very good seeing.
Photographically the spot was ideal for measuring since it was very small,
completely isolated, and well-defined when seeing was good.
The longitude of the bright spot was measured and computed in a special
system of longitude which we call System III, System III was given an
l
j
6.
arbitrary sidereal rotation period of exactly 10 11 38 th , and an ephemeris was
constructed using the formula and epoch suggested by M. B. B. Heath (1953).
The longitude of the spot gradually decreased with time as shown in Table
II and Fig. 7.
A weighted linear least squares analysis indicates that the spot drifted
from longitude 148:9 on 2S October 1969 to 322°8 on 7 February 1970 with a
rotation period of 10 h36m36s53 ±0!48.
A second degree polynomial curvefit indicates that the spot drifted from
longitude 148°4 on 25 October 1969 to 321°6 on 7 February 1970 with a mean
rotation period of 10 h36m36520 ±0S47. The parabolic solution gives the
following equation for the longitude of the spot in System Ill.
X = 148 0 89 - 1 0 72825 T - 0°00052 T2
where T is the time in days from 25 October 1969 at 0h UT. The indicated
acceleration relative to System III corrected for latitude was 8.5 x 10 -8 m/sect.
It can be shown that statistically the standard deviation of the residuals
for the parabolic solution is not significantly better than that for the
linear solution.
SUPERIOR GEOCENTRIC CONJUNCTIONS OF TITAN
Photographs of Saturn were taken by Thomas B. Kirby near the times of
four superior geocentric conjunctions of Titan (Fig. 8). The observed
time of each conjunction was obtained by measuring the distance of the
satellite from the extension of the planet's central meridian.
Since the orbital plane of Titan was inclined about 17 0 to our line
of sight, the satellite was nearly one minute of arc south of the center of
i
7,
Saturn's disk at the time of conjunction. An error of one half degree in
setting Saturn's central meridian on the photographic plate parallel to the
Y-axis of the measuring machine would result in an error of 8,6 minutes in
the observed time of conjunction. By measuring the position angle of a
fixed fiducial line relative to many equatorial star trails, the position
angle of the fiducial line was determined with a standard deviation of 1,4
minutes of arc.
The central meridian of Saturn was found by measuring the outer edge
of Ring A or the center of Cassini's Division in each ansa. This method
should give a much more accurate value for the position of the central meridian
than could be obtained by measuring the east and west limbs of the planet and
applying a correction for phase.
The result of the measurements are tabulated below:
Observed UT Standard Number of Corrected UT Deviation
Date of conjunction deviation measurements of conjunction (0-C)
14 Dec 69 5h 4m32 +Om25 31 5h 3m48 +Om84
15 Jan 70 2	 10.30 ±0.30 27 2	 9.48 +0.82
31 Jan 70 1	 25.37 ±0,27 36 1	 26,23 -0.86
16 Feb 70 1	 9,11 ±0,44 17 1	 9,94 -0,83
The corrected times of conjunction represent the best fit which can be
obtained from the four observed times making allowances for changes in light
time and the planetocentric right ascension of Earth, but ignoring pertur-
bations, The sidereal period of Titan was assumed to be 15,945448 days.
8. i
The shift in the position of Titan relative to Saturn's central
meridian caused by atmospheric refraction was negligible at the greatest
hour angle at which a photograph was taken.
ACKNOWLEDGMENTS
The writer wishes to express his thanks to R. L. Fritz for his work
in composing and printing images of Saturn, and to Mrs. E. M. Ingraham
for her work in preparing some of the figures. The work reported here was
supported by the National Aeronautics and Space Administration under Grant
NGL-32-003-001.
REFERENCES
Dollfus, A. (1963). Plouvements dans 1'atmosphere de Saturne en 1960.
Icarus 2, 109-114.
Heath, M. B. B. (1953). The longitude of Saturn's central meridian. J.
Brit. Astron. Assoc. 63, 111-113.
Peek, B. M. (1958). "The Planet Jupiter." Macmillan, New York, 269-271.
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Note; The standard deviation of the mean, and the number of images measured (in
parentheses) are included. The mean Saturnigraphic latitude of the bright spot
from 24 measurements was -57°3 f 0.3•
AFi;, 1.	 Saturn in orange light, 5 November 1969, 0358 UT. Photograph by
Claude Knuckles. All of the Photographs in this report were taken at the
f:75 focus of the 61-cm reflector and are oriented with south at the top.
1Fig 2.	 Diagram of Saturn indicating the nomenclature of the belts used
in this report. South is at the top, and rotation is from right to left.
1. South Polar Cap
2. South Polar Belt (SPB)
3. S.S.S. Temperate Belt (SSSTB)
4. S.S. Temperate Belt (SSTB)
S. South Temperate Belt (STB)
6. South Equatorial Belt (SEB)
7. Equatorial Band (F.B)
Fig. 3.	 Saturn in ultraviolet light, S November 1969, 0440 UT, showing
a very dark Equatorial Zone and a brilliant north limb. Photograph by
Claude Knuckles.
Fig. 4.	 Saturn in blue light, 25 October 1969, 0720 IIT, with three times
standard exposure to show ring C. Photograph by A. S. Murrell.
Fig. S.	 Saturn in green light, 25 October 1969, 0708 11T, discovery photo-
graph of a small bright spot in the southern hemisphere which persisted
for several months. Photograph by A. S. Murrell.
Fig. 6.	 Three photographs of Saturn in orange light showing later views
of a small bright spot in the southern hemisphere. Top, 5 November 1969,
0812 UT, photograph by J. C. Robinson showing bright spot to right of central
meridian. Center, 16 December 1969, 0135 UT, photograph by Claude Knuckles
showing bright spot to left of central meridian. Bottom, 7 February 1970,
0226 UT, photograph by J. C. Robinson showing bright spot near central
meridian.
V
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♦
♦
I
Oct. 2 5
Nov. 4
14
24
Dec. 4
14
24
Jan. 3
13
23
Fe b. 2
12
3100	 00	 500	 1000	 1500
System ffi Longitude
Fig. 7.
	 Motion of a small bright spot in Saturn's southern hemisphere in
an arbitrary system of longitude from 25 October 1969 to 7 February 1970.
Fig. 8.
	
Portion of one of the photographic plates used to measure the
distance of Titan from superior geocentric conjunction. A star trail,
fiducial lines, and the satellites Titan, Rhea, Tethys, and Dione are shown.
Photogtaph by Thomas B. Kirby on 15 January 1970.


Measurements of Saturn in 1969

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