Saturn’s rings to become invisible

Press, 27 February 1980, Page 14

Saturn’s rings to become invisible

PA " Wellington Saturn presents an unusual appearance this month, when at opposition on March 14. as its rings will be invisible. This will, however, enable the eclispses and occultations of some of the satellites to be seen. The equator of Saturn makes an angle of nearly 27 degrees with the plane of its orbit. Thus, during its 291-year orbit around the Sun, the planet alternatively turns its north and south poles towards Sun and Earth. The plane of the ring system coincides with that of the planet’s equator. This results in the north face of the rings being in sunlight for 15 J years and the southern face for 13? years. The difference in length of these two intervals results from Saturn’s moving faster when at perihelion. The rings disappear at the times of the Saturnian equinoxes; when the Sun is crossing the planet’s equator. Earth itself can also cross the plane of the rings rendering them invisible to us.

Earlier this year the rings have been invisible because their northern side turned to Earth has not been illuminated by the Sun. This state persists until Saturn’s spring equinox on March 3 when the rings are inclined at 0.35 degrees. . They can then be seen in large telescopes as a thin bright line until March. 13 by which time their inclination has decreased so that they are edgewise on. Again they become invisible as. Sun and Earth are then on opposite sides. The inclination of the rings to Earth’ increases until-mid-May, after which it slowly decreases jto once again become edgewise on July 23. Soon after that date Saturn will be too close to the Sun to be visible until it reappears in the.morning sky in October. Then the inclination of the rings will be 5.4 degrees. This will steadily increase over the next seven years until the rings’ southern face is at its widest viewed from Earth.

The rings have a diameter of 77,000 kilometres at their inner edge and 136,000 kilometres at their outer edge. Estimates of their thickness range downwards from ..a few kilometres. They consist of millions of particles, which’ are covered ’ with water ice.

Gaps occur in the rings where there appear to be no particles. These gaps are so’situated that if any particle entered it would orbit the planet in a period that is a precise ratio to the period of one of Saturn’s inner satellites. Particles cannot remain in such orbits because the attraction of a satellite will draw them inwards or push them outwards into one of the rings. There are two theories as to how the rings formed. The first states that they represent material which never condensed to form a satellite and therefore they have always existed in much the same position and form as now.

The other theory states that a satellite came too close to Saturn. Its nearness to the planet would result in the inner face being under the influence of a stronger gravitational pull than the outer face. This would cause the inward side to travel faster than the opposite side. The result would be that the satellite would be steadily pulled apart and its fragments drawn out into a system of rings. The recent discovery of a thin ring around Jupiter has been advanced as proof of the second theory. This ring appears to be associated with Jupiter’s innermost satellite, Amalthea, and is believed to consist of frag- . ments pulled off this sa- , tellite. However, one' would expect the same to ..happen to Phobos, Mars’s inner satellite. While there are arguments as to whether Phobos is being fractured by the' greater pull on its inward face, there is no sigji of any ring around Mar's.'“ ; <s.. x One advantage of thb>invisibility of Saturn's rings this year is that some of its satellites can be seen being eclipsed in the planet’s shadow or oc-

culted by the planet itself. The brightest of these satellites is Titan, visible in even the smallest telescopes as an eighth magnitude object. It is best seen at. eastern elongation on March 13 and 29, or at ■western elongation on March 21.

Telescopes over 60mm in aperture will show Rhea, which is of tenth magnitude. On March 9 at 8 p.m. it will be first eclipsed and then occulted by Saturn. Four hours will elapse before it is again visible. On March 18 at 8.49 p.m. and on March 27 at 9.29 p.m., Rhea will be first occulted and then eclipsed, the order of occurrence being reversed compared to March 9 as these latter events occur after opposition.

March will see three bright planets — Mars, Jupiter and Saturn —• clustered together in the constellation Leo. There Mars and Jupiter will be 3 deg. apart on March 3. The Moon will be close to Saturn on the evening of March 29 and on the morning of March 3.

Venus can be seen low in the western evening sky after sunset and remains a brilliant object. Mercury is at inferior conjunction on March 6, after which it passes-into the morning sky. By the end of the month it will rise about half an hour before dawn. It is then moving into its most favourable western elongation for our latitudes. This occurs in April which will be the best time this year to find this elusive planet in the morning .sky.. : On the ‘ evening of March 28, -Jupiter will be very close to the Moon. A very brief occultation of 'the planet will be visible from the far south of the South Island commencing around 6.20 p.m. While.. Comet Bradfield failed to become as bright as predicted it was an easy object to see by the naked eye during, late January and part of February. It ...then was at a high altitude in the far southern sky. Its. extremely rapid motion was noticeable - in an . hour’s watching. At its brightest it reached magnitude 4.8 and a very short tail was visible on .some evenings.