The Inner Planets

                                              [ Talk given before 1986 ]   


 As everyone knows, these comprise Mercury, Venus, Earth / Moon, Mars, and Minor Planets.


     Mercury. Good eyesight is needed for naked eye observations, with a low western and eastern horizon, a very clear sky, and the ability to enjoy a challenge. You may feel that time on the Space telescope could be added to the list for all the difference it will make to most of us, but don’t despair. Nobody forgets their first naked eye observation of Mercury. Occasionally it passes very close to Venus (when viewed from earth) and then, using Venus as a marker, it is easy to find. That is how I first found it.

    Mercury is available as an evening object from early November to early December, its best appearance being in late November. Then, as a morning object from mid December to mid January, rising 2 hours before sunrise around Christmas. Once found, it resembles a faint orange object, visible at dusk, with the characteristic planet steadiness. Binoculars depict it as a brighter point of light, and a small telescope shows it as a small slightly orange crescent. A larger telescope simply depicts a larger crescent, while a space probe shows it cratered like the Moon. (2 Mercury slides).    


     Venus. Easy to recognise as the brightest celestial object after the Moon. At present it sets 2 hours after sunset and will continue being later until by the end of January it will set at about 9 o’ clock, while the Sun will have set by 5 o’clock. Binoculars show a half-moon or crescent and a small telescope a larger crescent, shimmering and wobbling and presenting all sorts of colours due to its light passing through a large amount of atmosphere. Larger telescopes can depict vague markings, and by changing filters changes in the crescent shape. The whole disc is just visible when the crescent is in sunlight, probably due to the sun being refracted round the whole planet in its heavy atmosphere. A space probe shows Venus as a swirling mass of cloud, but no surface detail. Soft landing probes show an atmospheric pressure of 100, and sulphuric acid rain on a rock strewn surface. (2 Venus slides).


     Although the Earth is not exactly the planet for amateur observers, space probes and photographic satellites have made some remarkable discoveries, from the Van Allen belts to previously unknown geological features.


    Moon. A splendid and rewarding object for amateurs. Even with the naked eye the phases and [mara?] can be seen along with the occasional eclipse and occultation. Binoculars. These show some craters, seas and more occultations. A small telescope is ideal for graze occultations. With large amateur instruments ‘the sky is the limit’!  Not only thousands of craters but features down to 1 km across can be seen. However serious mapping of the Moon, on both sides, has been done by the space shots, and serious amateur work concerning TLP’s. For this you not only need a large telescope but a thorough knowledge of the Moon, willingness to observe frequently for long periods, and a telephone to quickly alert like minded observers at any hour of the night if you think you’ve found a TLP.

       Of course the ultimate observations have been taken by the numerous probes, and the manned Apollo series 11 to 17. (3 Moon slides).


      Mars.  Like Mercury, a challenge, not because it is as elusive, but it rarely comes really close to the Earth. It is visible to the naked eye as an evening object, setting at about 9 o’clock for the next two or three months. Its most feature is its steady orange / red glow, but the Earth / Mars distance is increasing, so with time it will become fainter. Binoculars show Mars as a bright red point of light, and only during the best oppositions does it show as a tiny disc, using standard 10 X 50’s. a small telescope depicts a small disc, but at or near opposition details such as a few vague markings and possibly the polar caps may be observes. A large telescope in a good elevated position with a southern horizon (eg. Failand) is what is needed for useful work on Mars. Surface detail is now covered by the Viking probes but dust storms occur at unpredictable intervals, and a dedicated observer with the right equipment can detect these events as soon as they happen.

     Owners of smaller instruments can still do useful work by plotting the Mars orbit.

     Mars mapping , and other discoveries have been brought forward by the probes. During the next opposition, in 1986, the best for about 15 years, even the modestly equipped amateur will see as much of Mars as anyone with Earth based equipment. Like the Olympics, its not the gold medal of an unlikely discovery, but the pleasure of taking part in observing, that amateur astronomy is all about. (% Mars slides).


     Minor planets.  Nobody has seen the surfaces of these, and the closest approach is the photographs of Phobus and Demios. However there is scope for the amateur on two fronts, with a reasonably large telescope and dedication. Firstly, the larger of the minor planets can be tracked photographically. This requires a dark location and a large aperture, low power telescope and necessitates taking photographs of areas of the sky, and repeating them at a later date. By comparing the photos for the movement of any point of light an asteroid can be picked up. Another photo, giving a third point, can theoretically plot its position. Secondly, amateurs, if really well organised, can measure asteroid diameters using an occultation technique. .

    Some time back a talk was given on this subject. You start by finding your asteroid, and wait until it occults a star. When this happens the asteroid casts a shadow of the star upon the Earth. This is moving fairly fast and is detected by a line-up of telescopes  

hundreds of miles long, placed so as to intercept the shadow. Ideally the telescopes at the ends of this line-up miss the occultation, while those in the middle see a long occultation. From this the size and the shape of the asteroid can be deduced.

    Commitment, enthusiasm, and a willingness to travel, plus organisation, are essential in creating this, but it is original amateur research.


                                              Thank You.