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ABSTRACT

Forty-five photoelectric magnitude measurements of Saturn were made between Oct. 31, 2002 and Feb. 24, 2003. The normalized magnitudes, at a ring tilt angle of B = 26.5°, were: B(1,0) = -8.75±0.02, V(1,0) = -9.77±0.01, R(1,0) = -10.43±0.01 and 1(1,0) = -10.65±0.01 while the corresponding solar phase angle coefficients were: c^sub B^ = 0.045±0.007, c^sub V^ = 0.035±0.005, c^sub R^ - 0.03±0.002 and c^sub I^ = 0.027±0.005. At least four oval storms on Saturn were imaged in late 2002 and their average area was 18 x 10^sup 6^ km^sup 2^. Storms and other features on Saturn did not have an impact on that planet's brightness and color.

Key Words: Saturn, Photometry, Saturn storms.

INTRODUCTION

Three highlights of Saturn in 2002-03 were: 1) the planet reached maximum brightness in late 2002, 2) the rings opened up to their maximum angle as seen from the Earth (27°) in April 2003 and 3) white oval storms were imaged on Saturn. Three factors determine the time when Saturn reaches maximum brightness; these are: 1) distance, 2) ring tilt and 3) solar phase angle. Although Saturn will be closer to the Earth and Sun in late 2003, the rings were tilted at a greater angle in late 2002 thus showing a greater area as seen from the Earth. The ring tilt more than compensates for the slight difference in distance; therefore, Saturn reached its maximum brightness for the decade in late 2002.

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The measurements presented here are part of an ongoing photometric study of Saturn that the author began in 1991. The primary reasons for continuing the photometric study of Saturn are: 1) determine how the changing orientation of the rings affect the magnitude, color and solar phase angle coefficient of Saturn, 2) determine how storms, belts and zones affect the brightness and color of Saturn and 3) determine if there is a difference in brightness between the northern and southern hemispheres of Saturn and its rings.

Several people have taken images of Saturn that show white ovals, and a series of bright and dark belts. The writer is interested in the impact that these features have on Saturn's brightness.

METHODS AND MATERIALS

An SSP-3 solid-state photometer along with filters transformed to the Johnson B, V, R and I system were used in making all magnitude measurements (1). A 0.09 m Maksutov telescope was also used in data collection. All magnitude data was corrected for both atmospheric extinction and transformation in the same way that is described in (2). The comparison and check stars used were epsilon-Taurus and iota-Gemini respectively. The magnitudes used for epsilon-Taurus were B = 4.55, V = 3.54, R = 2.81 and I = 2.31 (3). The writer made all measurements in Tables I and II, but made his measurements from images made by Alan Chu, Ed Grafton, Eric Ng, Donald Parker and Gu Yu. Alan Chu and Eric Ng made their images with a 0.25 m Newtonian from Hong Kong, Ed Grafton used a 0.36 m Schmidt Casegrain from Texas, Don Parker used a 0.41 m Newtonian from Florida and Gu Yu used a 0.23 m Schmidt Casegrain from West Virginia. The author also used his SSP-3 photometer mounted on his 0.09 m Maksutov telescope to make all photoelectric magnitude measurements in Table III with the assistance of his students Jared Huckaby, Jason Lyles and Lance Murphy.

All video and charged coupled device (CCD) images analyzed in this report were taken in visible light (~550 nanometers) and had resolutions of 0.3 to 0.5 arc-seconds. Two of the reasons for the high resolution are the short exposure times used in making images (less than 0.1 seconds) and the stacking of several images to make one very sharp image. This stacking reduces much of the noise and it improves resolution (4). Images recorded by Alan Chu, Ed Grafton, Eric Ng, Donald Parker and Gu Yu were all used in the measurement of belt latitudes and of oval sizes. Belt latitudes were measured from the images and were computed from the formulas on p. 450 of (5).

Images showing the white spots on Saturn have been published elsewhere (6) and can be viewed at: http://www.lpl.arizona.edu/alpo. The areas of the spots were computed in the same was as in (7).

RESULTS

The magnitude measurements are listed in Table III. The average measured magnitudes of iota-Gemini were: B = 4.83±0.01, V = 3.79±0.01, R = 3.04±0.01 and I = 2.51±0.01, which compare well with the literature values of: B = 4.82, V = 3.79, R = 3.03 and I = 2.52 (3). This good agreement between measured and literature magnitudes for the check star attest to the quality of the photoelectric magnitude data.

Equation 3 is based on measurements made between 1877 and 1891 by Muller (9). Muller's measurements were not made through the Johnson V band since standard V magnitude stars were not established until the mid twentieth century. Because of the lack of a true V-filter magnitude, the author has decided to begin rewriting equation (3) for the Johnson V pass band.

The X(1,[alpha]) values for B = 26.5° are listed in Table III and plots of X(1,[alpha]) versus a are shown in Figure 1. All data with [alpha]>1.5° (solid circles in the figure) were fitted to a linear equation of the form: X(1,[alpha]) = c^sub x^ [alpha] + X(1,0) where c^sub x^ is the solar phase angle coefficient, and X(1,0) is the normalized magnitude of Saturn + rings at [alpha] = 0° and B = 26.5°. The values of c^sub x^ and X(1,0) are summarized in Table IV. Figure 2 shows B and V magnitudes measured on Dec. 21, 2002. This figure covered the longitude range from 98° to 226°. One oval was at 130° but it had no affect on Saturn's brightness.


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