Many amateur astronomers have a fascination for the objects of the Solar System. This is not surprising; some of these objects are bright enough to observe from urban skies and can look interesting even in small telescopes; also, unlike many vaster and further-distant objects, the bodies that we share our sun with move and change in appearance.

A person becomes an amateur ‘planetarian’ because the planets and their cousins the asteroids, meteors and comets interest that person. By definition, amateurs receive no pay, neither do they need formal certificates to pursue their interest. These self-motivated amateur students of the Solar System belong to at least three categories (often overlapping) and I think that The planet observer's handbook offers something for each:

General

We live at the bottom of a vast ocean of air, several miles deep, that envelops the surface of the Earth. Though essential for the continuation of all animal and human life it is nothing but a nuisance to the practical astronomer. All telescopic observation has to be through this mass of air which rarely, if ever, is quite still. Apart from the obvious obstructions of thick clouds and fog which prevent observation altogether, telescopic planetary images are often ruined by atmospheric turbulence even when the air is perfectly clear. Air currents and differences of temperature at different levels of the upper atmosphere all conspire to cause irregular refraction of light rays reaching us from the planets. This causes shimmering or ‘boiling’ and telescopic planetary images oscillate and ripple. Fine planetary detail is therefore difficult to see and ‘hold’; if the trembling is bad enough little or nothing of disc markings can be seen. These are what is meant by ‘bad seeing’.

When this happens there is nothing that you can do but to wait until conditions improve. However, there is no need to stop observing in the usually somewhat poor seeing conditions prevailing most of the year at most observing sites. On a night of tremulous seeing occasional steady intervals occur lasting a second or two during which the definition of the telescopic planetary image can be astonishingly good.

General

Like Uranus its close relative, Neptune is a gas giant planet, a huge rapidly rotating largely fluid world with a density more than twice that of water, greater than that of Uranus, and with a considerable atmosphere. The equatorial diameter is 31 403 miles (50 538 km) and the polar diameter is 30 589 miles (49 229 km) so that the ratio of the polar to the equatorial diameter is 0.98. Neptune is about four times bigger than the Earth (fig. 12.1) and is slightly smaller than Uranus. The rotational axis is inclined 29.6° to the plane of its orbit so that it does not share the remarkable axial tilt of Uranus. The axial rotation period is 16 hours 3 minutes (16 hours 7 minutes for the magnetic field). Neptune's distance from the sun varies from an aphelion distance of 2819.2 million miles (4537.0 million km) to a perihelion distance of 2771.4 million miles (4460.2 million km).

The mean orbital velocity is 3.37 miles (5.43 km) per second and the orbital eccentricity is 0.009. The orbit is inclined 1° 46′ to the plane of the ecliptic. Neptune's orbital (sidereal) revolution period is 164.8 Earth years and its synodic period is 367.5 Earth days.

Neptune is encircled by eight satellites and a system of rings. Two of the satellites, Triton and Nereid, were discovered by Earth-based observation, the remainder by the Voyager 2 spacecraft. The orbits of Triton and Nereid around Neptune are shown in fig 12.2.

General

Pluto, the outermost planet of the Solar System, has an estimated diameter of 1431 miles (2302 km) and a mass equal to 0.0025 Earth masses. It has an apparent visual magnitude of 13.7. Its axial rotation period is 6 days 9 hours and 17 minutes. The rotational axis is tilted at an angle of 118° to the plane of its orbit. Pluto orbits the sun at an average distance of 3674.48 million miles (5913.52 million km). Its orbital eccentricity of 0.25 is so high that Pluto will be closer to the sun than Neptune between 1979 and 1999. The orbit is inclined at an angle of 17.1° to the plane of the ecliptic. Pluto makes one complete orbital revolution around the sun in 248.54 Earth years and moves with an average speed of 2.95 miles (4.7 km) per second.

Pluto has one satellite, Charon, that revolves around its primary at a distance of 12 204 miles (19 636 km). It makes one revolution around Pluto in the same time that Pluto rotates once on its axis. The minimum diameter given for Charon is 739 miles (1190 km). It has an apparent visual magnitude of 16.8. The plane of the orbit of Charon around the primary is tilted at a large angle (98.8° to the plane of Pluto's orbit. This means that twice in every orbital revolution of Pluto, Charon's orbital plane will be presented edgewise to the Earth and Pluto and Charon will show mutual transit and occultation phenomena.

Astronomy has always been a popular hobby with all kinds of people, especially since the arrival of the ‘Space Age’. Telescope sales are brisk and books on astronomy abound. Astronomy is a hobby that can be enjoyed even if the only optical instrument that you have is a pair of binoculars or a very small telescope. Books have even been written on naked eye astronomy.

There is something for everyone in observational astronomy. Some like to study either the Sun or Moon, which are especially suitable for those owning only modest telescopic equipment. Others prefer ‘deep sky’ observing and love to probe the depths of space with the largest telescopes that they can afford and enjoy the satisfaction of locating and identifying bright and faint star clusters, galaxies and nebulae that abound in our universe. Still others like to hunt for comets or keep track of the brightness changes in variable stars, or plot the paths of meteors (‘shooting stars’). Many who are also keen photographers couple their cameras to their telescopes and delight in taking portraits of their favourite celestial objects.

To those who enjoy deep sky observing and the mind-boggling immensities of outer space, planetary observing must seem a little tame. The planets of the Solar System must seem like mere pebbles in their back yards when compared to the immensities of the universe beyond the Solar System.

General

The development of photoelectric light measuring instruments (photometers) which have electronic light detectors and current amplifiers has made it possible for the weak current generated by the light of planets, their satellites and the asteroids focused by the telescope objective on the detector to be amplified sufficiently to cause deflection of a galvanometer needle or to give a digital readout as in more recent equipment.

Photoelectric photometry dates from about the latter part of the nineteenth century. The first ever photoelectric measurement of a star's light was made in 1892 with a 24-inch telescope belonging to an amateur. The observation was made by a professional astronomer, C. M. Minchin and two amateurs. Subsequent to this first amateur venture into photoelectric photometry, professional astronomers took it over completely during the next 60 years and it became widely available to them during the 1930s. Photoelectric measuring instruments are now available at prices affordable by most amateurs who are thus enabled to contribute much valuable data in this area of research.

Although astronomical photometry is done by professional astronomers using large telescopes, there are several advantages in amateur pursuit of this type of investigation:

1. There are more amateurs than professionals available to do this kind of work.

2. Amateurs don't have to ‘book’ time on large observatory telescopes which are available for short periods only. There is greater freedom and flexibility from owning their own telescopes.

3. Amateurs are able to spend more time on prolonged studies of selected objects.

General

Mercury, the first of the two ‘inferior’ planets, is the nearest planet to the sun and has a diameter of 3010 miles (4843 km) which is slightly more than one third of the Earth's diameter (fig. 5.1). Its mean distance from the sun is 36.0 million miles (57.9 million km) but it varies from 28.6 million miles (46.0 million km) at perihelion to 43.4 million miles (69.8 million km) at aphelion, owing to the great orbital eccentricity of 0.206 which is exceeded only by the orbital eccentricities of Pluto and several of the asteroids. This means that the sun is 7.5 million miles (11.9 million km) from the centre of Mercury's orbit. Owing to Mercury's nearness to the sun, its orbital speed is greater than that of any other planet, varying from a maximum of about 35 miles (56.3 km) per second when nearest and about 23 miles (37.0 km) per second when farthest from the sun. Mercury has no satellites.

Because it is an inner planet Mercury exhibits phases like our moon (fig. 5.2). It is ‘full’ at superior conjunction when on the opposite side of the sun to us, roughly half-moon shaped at its elongations, i.e., when at its greatest apparent angular distance from the sun on either side and ‘new’ when at inferior conjunction when between us and the sun.

General

Although visible to the naked eye as a sixth magnitude star, Uranus appears to have been unknown to the ancients. The planets previously described are all brighter than most naked eye stars and move rapidly among them and so attracted the attention of the peoples of old. Uranus, on the other hand, being faint and moving very slowly among the fixed stars did not attract attention. In the dark unpolluted skies of the ancient world it would be quite lost amongst the myriad stars that peppered the clear night skies of long ago. Uranus was, in fact, the first planet to be discovered, by William Herschel in 1781, who at first thought that it was a comet. He made the discovery with a home-made 6.2-inch reflector.

Uranus, like Jupiter and Saturn, is classed as a ‘gas giant7. It is an enormous rapidly rotating world with a low density and a thick atmosphere. Its equatorial diameter is 31763 miles (51 111 km) and the polar diameter is 30 811 miles (49 575 km). Uranus is therefore noticeably ellipsoidal, the ratio of the polar to equatorial diameter being 0.97. Compared to the Earth, Uranus is about four times larger (fig. 11.1).

Four of the Solar System planets have their rotational axes tilted from the perpendicular to their orbital planes by between 20° and 30°. The axis of Uranus is tilted 98° to the plane of the orbit and this makes it unique among the planets.

General

Provided that you have sufficient observational drawings of a planet that comprise one complete axial rotation, you can combine them all into a map or planisphere. Mars is especially suitable for this because of its well-marked surface features. Maps of Mercury have been constructed by several amateurs. Jupiter is also well suited because of its prominent atmospheric features, some of which are fairly permanent.

With some, Mars and Mercury for example, the surface features may take months or even years to gather whereas with Jupiter a complete axial rotation of 10 hours may be charted in a single night when the planet is at or near to opposition.

When making a chart of a planetary surface we are faced with the same problem as with making terrestrial maps – the representation of a curved spherical surface on a flat sheet of paper. The more of the planetary or terrestrial surface that is charted the greater the difficulty of rendering a distortion-free picture – or at least of minimising distortion. Various projections have therefore been devised to achieve the representation of the surface of a planetary or the terrestrial globe on a flat sheet as accurately as possible, all of which have advantages and disadvantages,

The horizontal orthographic projection

The horizontal orthographic projection is the one usually employed in making planetary maps.