S A T U R N : Part 6
OBSERVATIONS of SATURN
During 2003 to 2005 was one of the best times to observe Saturn, as the planet is closest to the Earth and the rings are wide open. Such favourable circumstances occur once every fourteen years or so. This is partially due to the slightly eccentric Saturnian orbit, meaning opposition distances can be anywhere between 1.20 and 1.65 billion kilometres. Like Jupiter, Saturn appears like a bright star to the naked eye, whose visual magnitudes varies anywhere between −0.4 and +0.8. Such differences are mainly dependent on the orientation of the rings. Successive oppositions, the time interval called the synodic period, occurs every 378 days or roughly about two weeks later each year.
Often each month or two, either side of opposition, Saturn disk can place shadows close to the planet and across the far side of the rings. This can appear quite pronounced, being often visible in 10.5cm or 15cm telescopes, and can be deceiving regarding the true planet’s shape. The best time to see this phenomenon is at quadrature, when the planet viewed from the Earth is 90° to the Sun’s position along the ecliptic. The angle between the Earth and the Sun at this time is at its maximum, and for Saturn and this corresponds to about 6° either side of the line of sight. Quadrature also slightly affects the appearance and brightness of the rings.
Disk features often require at least 20cm or 25cm telescopes to see, but brighter features rare exceedingly rare. Interestingly, the bands and the depth of colour seen at the poles change over the years. This is usually best to observe when the rings are wide open or alternatively when the rings appears edgewise and these bands become much less conspicuous.
Near the times of the ring crossing the rings will disappear in small apertures, but above 25cm, they remain just visible as a thin streak of light. Often the rings appear as two or more thin little lines.
Drawings of Saturn can easily be made. This is achieved by using outline on a pre-prepared blank form whose outline is made more complicated by the continuous changing aspects of the rings. A standardised method often used by many amateur planetary observers is to make a blank outline of the planet to the widest ring exactly 10.0 cm. across. I.e. Out to the outer edge of the telescopically visible A-ring. Figure 2 shows the current aspects of the rings for between 2003 and 2004, which are tilted by +26°, +25° and +24°. The geocentric angle of the rings ‘Bo’ and is listed within the Saturn Ephemeris 2009-2010 and Saturn Ephemeris 2009-2010 Tables.
If we need to prepare an Saturn observation blank, being easily created in many of the computer graphic programs, the following widths for of each ellipses can be easily proportioned from Table 3a &. 3b. A graphic representation of these values appears in Figure 3.
The width of the rings ‘W’, can be determined using the formulae;
W (cm.) = sin A × 10.00
Calculated value for W are as follows;
Depending on the tilt of Saturn to the ecliptic, the minor polar diameter will slightly vary, whose tilt decreases negligibly by only about 0.03cm. For most apparitions only two or three standard blanks are normally required.
Also even though the apparent diameter of the planet and rings change depending on the distance where Saturn is from both the Sun and Earth, a 10cm blank is always used. Those who need to know the true diameter of the planet or the individual rings (measured in arc seconds) will find these listed in Saturn’s Ring Sizes.
Mutual transits and occultations do occur when the rings appear edgewise, but these require large telescopes above 25cm. Only Titan can cast its definite shadow across Saturn girth and is visible in 25cm with care. The current series of mutual transit and occultation events began in 2008 and 2009. Prediction of all satellite positions and other interesting phenomena are often given during this time. All other satellite can also display similar phenomena, but regrettably, do this nearer the north or south poles but rather than across Saturnian equator. Transits do readily occur but are difficult to see mainly because the shadows sizes are quite small.
Useful ephemerides are often calculate for the moons in the same phenomena terms used for the inner planets of Mercury and Venus. This includes successive inferior or superior conjunctions, and greatest elongations east or west of the planet.
Occasionally faint stars will be occultated by the rings, causing both dimming or twinkle, but most of the time the star will still remain visible. Observations of transits and of occultations do occur when the rings appear edgewise, but these require large telescopes above 25cm. Only Titan can cast definite shadows across Saturn girth, being readily visible in 25cm. with care. The next series of events did not begin until 2008 and 2009.
Prediction of the satellite’s positions and other interesting phenomena are often given during this time. The other satellites can also display similar phenomena, but regrettably, do this nearer the north or south poles rather than across Saturn's equator. Transits do occur but are difficult to see mainly because the shadows are so small.
Ephemerides are often calculated for the moons in the same terms as often used for the inner planets of Mercury and Venus. This includes inferior and superior conjunctions, and greatest elongations east and west of the planet.
Another interesting observation that occurs from time to time is Saturn being occultated by the Moon. Often these events come in series, where the planet looks like some kind of spacecraft hovering above the lunar surface. An two articles features in April 2002 “Sky and Telescope” of two Northern American observed events that occurred on the 20th February and 21st May 2002. No predictions of any Lunar-Saturn events will occur over Australia or New Zealand skies for many years to come.
Disclaimer : The user applying this data for any purpose forgoes any liability against the author. None of the information should be used for either legal or medical purposes. Although the data is accurate as possible some errors might be present. Onus of its use is placed solely with the user.
Last Update : 29th September 2012
Southern Astronomical Delights © (2012)
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