THE ROYAL ASTRONOMICAL SOCIETY OF CANADA
            ________________________________________

                                                             Bulletin No. 14

                     PLANETARY SECTION


POSSIBLE MAGNITUDE VARIATIONS OF SATURN'S SATELLITES:


For many years it has been known that the magnitude of Iapetus, Saturn's
eighth moon, is variable. Recent observations have indicated that some
of the other moons may vary in brightness too (ref. 1, 2). This was pointed
out in Planetary Bulletin No.9. However, observers were left to their own
devices to work out the positions of the satellites.

Now that Saturn is approaching a favourable period for observation, it may
be possible that the observers of the RASC may be able to measure the
variations and arrive at some conclusion. Therefore I have constructed
the following diagrams which, when combined with the ephemerides will
indicate the approximate position of the satellite on any given night for
the remainder of 1970. (These diagrams may also be used in future years
when the ephemerides are released.)

The procedure is relatively simple. One has to calculate the elapsed time
from Eastern Elongation as given in the accompanying ephemeris. The
satellite will appear in the eyepiece of an inverting telescope in the
position corresponding to the position for that elapsed time on the diagram.

Magnitude estimates may be made in two ways: (a) by the "variable star"
od, comparing the magnitudes to those of background stars as found
in a good star atlas, or (b) assuming that Titan is of constant magnitude
8.3 and working from there.

There are a few other factors to remember while attempting this work. The
observer should keep in mind that the scale of these diagrams differs with
the individual satellite. Saturn's rings are approximately 40" of arc across,
so that Tethys and Dione will be difficult to see due to glare, as they will
never appear to be very far from the outer extremities of the rings. If
possible, an occulting apparatus should be used.

Estimating the magnitudes of these moons will make an interesting project
for amateur astronomers with moderate-sized telescopes throughout the
autumn of the year.

Ref. 1--Moore, Patrick, J.Brit.Ast.Assoc. Vol. 79, No.2, p.121.
Ref. 2--Delano, Rev. K., J.Brit.Ast.Assoc.Vol. 79, No.2, p.124.


                                                Kenneth E. Chilton,
                                              National Co-ordinator,
                                                Planetary Section,
                                   93 Currie Street, Hamilton 57, Ontario.
17 July, 1970.                               Phone: 416-388-0586

                                                                                -2-
                           Hours after Eastern Elongation                TETHYS

(Diagram here.)

Times of Eastern Elongation (U.T.)
     d  h	      d  h            d    h         d    h          d    h
Aug. 5  21.8    Sept. 1	08.2     Oct. 1  12.9   Nov. 2  15.9    Dec. 2  19.5
     7  19.1          3	05.5          3  10.2        4  12.1         4  16.8
     9  16.4          5	02.8          5  07.5        6  09.4         6  14.1
    11  13.8          7	00.1          7  04.8        8  06.7         8  11.3
    13  11.1          8	21.4          9  02.1       10  04.0        10  08.6
    15  08.4         10	18.7         10  23.4       12  01.3        12  05.9
    17  05.7         12	16.0         12  20.7       13  22.6        14  03.2
    19  03.0         14	13.3         14  18.0       15  19.9        16  00.5
    21  00.3         16	10.6         15  15.3       17  17.2        17  21.8
    22  21.6         17	07.9         18  12.6       19  14.5        19  19.1
    24  18.9         20	05.2         20  09.9       21  11.7        21  16.4
    26  16.2         22	02.5         22  07.2       23  09.0        23  13.7
    28  13.6         23	23.8         24  04.4       25  06.3        25  11.0
    30  10.9         25	21.1         26  01.7       27  03.6        27  08.3
                     27	18.4         27  23.0       29  00.9        29  05.6
                     29	15.7         29  20.3       30  22.2        31	02.9
                                     31  17.6

                                                                              -3-
                                                                     DIONE

                     Days and Hours after Eastern Elongation

(Diagram here.)

Times of Eastern Elongation (U.T.)
     d    h           d   h          d    h           d    h          d   h
Aug. 5  15.1    Sept. 2	00.1    Oct. 2  02.5     Nov. 1  04.6    Dec. 1  06.7
     8  08.8          4	17.8         4  20.1          3  22.3         4	 00.3
    11  02.5          7	11.4         7  15.8          6  15.8         6	 18.0
    13  20.2         10	05.1        10  07.4          9  09.5         9	 11.6
    16  13.9         12	22.8        13  01.1         12  03.1        12	 05.3
    19  07.6         15	16.5        15  18.7         14  20.8        14	 22.9
    22  01.3         18	10.1        18  12.4         17  14.4        17	 16.6
    24  19.0         21	03.8        21  06.0         20  08.1        20	 10.3
    27  12.7         23	21.4        23  23.7         23  01.7        23	 04.0
    30  06.4         26	15.1        26  17.3         25  19.4        25	 21.6
                     29	08.8        29  11.0         28  13.0        28  15.3
                                                                     31  08.9

                                                                                 -4-

                                                                          RHEA
Days and Hours from Eastern Elongation

(Diagram here.)

Times of Eastern Elongation (U.T.)
     d    h            d    h           d    h           d    h           d    h
Aug. 3  09.7     Sept. 4  00.9     Oct. 1  03.1     Nov. 1  17.4     Dec. 3  07.6
     7  22.2           8  13.3          5  15.5          6  05.7          7  19.9
    12  10.6          13  01.7         10  03.8         10  18.0         12  08.2
    16  23.1          17  14.0         14  16.1         15  06.3         16  20.6
    21  11.6          22  02.4         19  04.5         19  18.6         21  08.9
    26  00.0          26  14.8         23  16.8         24  06.9         25  21.3
    30  12.4                           28  05.1         28  19.2         30  09.7

                                                                             -5-

                                                                  TITAN
                  Days and Hours from Eastern Elongation

(Diagram here.)

Times of Eastern Elongation (LJ.T.)
     d	    h                d    h
Aug. 15   09.9          Nov. 3  00.4
     31   09.0              18  21.6
Sept.16   01.5          Dec. 4  18.9
Oct.  2   05.6              20  16.5
     18   03.1

                                                                           -6-

                                                               IAPETUS
(Diagram here.)

Iapetus is brighter at Western Elongations than at Eastern Elongations.
The variation is quite noticeable.

Western Elongations - Sept. 29
                      Dec.  16

Eastern Elongations - Aug.  20
                    - Nov.   6