BASIC OBSERVING SERIES

by Greg Burnett (gburnett)

Planetary Observing

[This article was first published in Star Stuff, the newsletter of the Ford Amateur Astronomy Club, in March, 1994.]

Planetary observing presents a variety of challenges to the amateur astronomer. The planets are constantly changing in appearance and position. With few exceptions (e.g. the Sun and the Moon), this is a unique characteristic among astronomical objects, many of which are the very definition of constancy and changelessness. The continuous movement of the planets requires planning and some ability to navigate in the sky for successful observations. Several of the planets present a level of detail in their appearance that will challenge the resolving ability of the amateur scope and the observing capabilities of the amateur.

Only three of the planets, Mars, Jupiter, and Saturn, are really interesting to "look at." The others are interesting mainly to locate and to follow their movements. However, Mercury and Venus do exhibit interesting phases, similar to the Moon, that add to their attraction even though no surface features may be visible. In what follows, we will look at each planet in turn and discuss its noteworthy features.

Mercury is a difficult planet to observe because it is never very far from the Sun. It is best seen at maximum elongation, and even then it is very small and reveals no surface features in amateur scopes. It does exhibit phases, but is usually observed near half phase because at other times it is just too close to the Sun. It can be challenging even to locate Mercury, and it is worth your while to do so when favorable presentations occur. It is said that Johannes Kepler, who studied planetary motions in great detail, went to his grave without ever seeing Mercury.

Venus is much more easily observed than Mercury, and exhibits very interesting phases and variations in size, which can be followed through nearly a complete cycle. Strangely enough, Venus is seen best during daylight, when its bright glare is suppressed. Through the telescope it looks like a beautiful miniature Moon against the blue of the sky. It is often bright enough to be seen with the naked eye during full daylight, if you know exactly where to look. There have been some claims of seeing surface features on Venus, but such observations are very doubtful in amateur scopes.

Mars provides many interesting surface features and surface activity. Landform markings are easily visible near opposition, and can be compared with the detailed surface maps provided by orbiting spacecraft. The polar caps are visible most of the time, and can bee seen to shrink and grow with the Martian seasons. From time to time huge dust storms will obscure the surface features, and their progress can be followed from night to night. The apparent size of Mars varies greatly with its position, from less than 10" (arc seconds) to almost 24" during a favorable opposition. Due to the relative orbital geometry of the Earth and Mars, some oppositions are much more favorable than others. Mars' apparent diameter at opposition varies from 13.8" to 23.8", almost a 2 to 1 ratio, which can make a significant difference in the amount of visible detail.

Jupiter also presents interesting surface detail, although it's not really a surface we are seeing, but the tops of Jupiter's gas clouds. The appearance of Jupiter is fairly constant, although in the past several years significant changes in the Great Red Spot and one of the major belts have been observed. Some short-lived features can also be observed at times. A major attraction of Jupiter is the set of four ever-changing Galilean moons, which present an almost continuous parade of transits, eclipses and appulses of every sort. The apparent size of Jupiter varies only a little. At opposition it can be from 44.7" to 49.8" in diameter.[Since this article was written, we have experienced the impact of comet Shoemaker-Levy 9 on Jupiter, creating unprecedented interest in the planet and truly unique observing opportunities. --G.B.]

Saturn's main attraction is, of course, its famous ring system. The rings show a great deal of detail that will challenge the best telescopes. They also vary their angle to the Earth, disappearing completely as the Earth crosses the ring plane about every twelve years or so. Saturn is probably the most exciting sight for a non-astronomer to see through a telescope. Saturn varies only slightly in size at any time. Several of Saturn's satellites are visible, but they are constantly up-staged by the wonderful rings and consequently are not nearly as popular as Jupiter's four visible moons.

Uranus and Neptune are something of a challenge to locate and identify. They are too small to present any surface detail in amateur scopes, and indeed Neptune is sometimes easy to mistake for a star. Uranus is at times barely visible to the naked eye in very dark skies, reaching a maximum magnitude at opposition of 5.7. Neptune only reaches magnitude 7.8 at opposition. Neptune's moon Triton is visible in scopes of 6 inches of aperture or more.

Pluto is always a challenge to locate, never being brighter than magnitude 13 to 14. Right now Pluto is actually closer than Neptune, but since it is so small, it is a consistently difficult target. The attraction of observing Pluto is mainly the satisfaction of observing the outermost planet of our Solar System.

Since planets are generally bright (except for Pluto) aperture is not a factor in choosing planetary observing equipment. The name of the game is resolution! This makes refractors, unobstructed reflectors (e.g. tri-schifspieglers), and specialized long-focus Newtonians highly recommended for planetary observing. Apertures of six to eight inches and under are usually adequate. Larger apertures can suffer more from bad seeing than smaller ones, and owners of large reflectors often stop them down with an off-axis aperture mask for planetary observations.

Choosing the right magnification is important for planetary observing. For a particular set of conditions (i.e. target, scope, and seeing), the observer needs to select a magnification that provides sufficient image size for visibility of available detail, but not so much magnification that seeing effects or resolution limitations begin to dominate the image. Therefore, a wide range of eyepieces is desirable. To assist in seeing surface detail, some observers use colored filters. Their purpose is to enhance certain color contrasts, but their effects are subtle at best and their utility should not be overemphasized.

The planets provided the amateur with an endless variety of observing challenges. While deep-sky objects may hold more romance for some, our neighbors close to home should not be neglected.