HALF-HOURS WITH THE PLANETS.
In observing the stars, we can select a part of the heavens which may be conveniently observed; and in this way in the course of a year we can observe every part of the heavens visible in our northern hemisphere. But with the planets the case is not quite so simple. They come into view at no fixed season of the year: some of them can never be seen by night on the meridian; and they all shift their place among the stars, so that we require some method of determining where to look for them on any particular night, and of recognising them from neighbouring fixed stars.
The regular observer will of course make use of the 'Nautical Almanac'; but 'Dietrichsen and Hannay's Almanac' will serve every purpose of the amateur telescopist. I will briefly describe those parts of the almanac which are useful to the observer.
It will be found that three pages are assigned to each month, each page giving different information. If we call these pages I. II. III., then in order that page I. for each month may fall to the left of the open double page, and also that I. and II. may be open together, the pages are arranged in the following order: I. II. III.; III. I. II.; I. II. III.; and so on.
Now page III. for any month does not concern the amateur observer. It gives information concerning the moon's motions, which is valuable to the sailor, and interesting to the student of astronomy, but not applicable to amateur observation.
[
] Plate VI
We have then only pages I. and II. to consider:—
Across the top of both pages the right ascension and declination of the planets Venus, Jupiter, Mars, Saturn, Mercury, and Uranus are given, accompanied by those of two conspicuous stars. This information is very valuable to the telescopist. In the first place, as we shall presently see, it shows him what planets are well situated for observation, and secondly it enables him to map down the path of any planet from day to day among the fixed stars. This is a very useful exercise, by the way, and also a very instructive one. The student may either make use of the regular maps and mark down the planet's path in pencil, taking a light curve through the points given by the data in his almanac, or he may lay down a set of meridians suited to the part of the heavens traversed by the planet, and then proceed to mark in the planet's path and the stars, taking the latter either from his maps or from a convenient list of stars.[9] My 'Handbook of the Stars' has been constructed to aid the student in these processes. It must be noticed that old maps are not suited for the work, because, through precession, the stars are all out of place as respects R.A. and Dec. Even the Society's maps, constructed so as to be right for 1830, are beginning to be out of date. But a matter of 20 or 30 years either way is not important.[10] My Maps, Handbook and Zodiac-chart have been constructed for the year 1880, so as to be serviceable for the next fifty years or so.
Next, below the table of the planets, we have a set of vertical columns. These are, in order, the days of the month, the calendar—in which are included some astronomical notices, amongst others the diameter of Saturn on different dates, the hours at which the sun rises and sets, the sun's right ascension, declination, diameter, and longitude; then eight columns which do not concern the observer; after which come the hours at which the moon rises and sets, the moon's age; and lastly (so far as the observer is concerned) an important column about Jupiter's system of satellites.
Next, we have, at the foot of the first page, the hours at which the planets rise, south, and set; and at the foot of the second page we have the dates of conjunctions, oppositions, and of other phenomena, the diameters of Venus, Jupiter, Mars, and Mercury, and finally a few words respecting the visibility of these four planets.
After the thirty-six pages assigned to the months follow four (pp. 42-46) in which much important astronomical information is contained; but the points which most concern our observer are (i.) a small table showing the appearance of Saturn's rings, and (ii.) a table giving the hours at which Jupiter's satellites are occulted or eclipsed, re-appear, &c.
We will now take the planets in the order of their distance from the sun: we shall see that the information given by the almanac is very important to the observer.
Mercury is so close to the sun as to be rarely seen with the naked eye, since he never sets much more than two hours and a few minutes after the sun, or rises by more than that interval before the sun. It must not be supposed that at each successive epoch of most favourable appearance Mercury sets so long after the sun or rises so long before him. It would occupy too much of our space to enter into the circumstances which affect the length of these intervals. The question, in fact, is not a very simple one. All the necessary information is given in the almanac. We merely notice that the planet is most favourably seen as an evening star in spring, and as a morning star in autumn.[11]
The observer with an equatorial has of course no difficulty in finding Mercury, since he can at once direct his telescope to the proper point of the heavens. But the observer with an alt-azimuth might fail for years together in obtaining a sight of this interesting planet, if he trusted to unaided naked-eye observations in looking for him. Copernicus never saw Mercury, though he often looked for him; and Mr. Hind tells me he has seen the planet but once with the naked eye—though this perhaps is not a very remarkable circumstance, since the systematic worker in an observatory seldom has occasion to observe objects with the unaided eye.
By the following method the observer can easily pick up the planet.
Across two uprights (Fig. 10) nail a straight rod, so that when looked at from some fixed point of view the rod may correspond to the sun's path near the time of observation. The rod should be at right-angles to the line of sight to its centre. Fasten another rod at right angles to the first. From the point at which the rods cross measure off and mark on both rods spaces each subtending a degree as seen from the point of view. Thus, if the point of view is 9½ feet off, these spaces must each be 2 inches long, and they must be proportionately less or greater as the eye is nearer or farther.
Now suppose the observer wishes to view Mercury on some day, whereon Mercury is an evening star. Take, for instance, June 9th, 1868. We find from 'Dietrichsen' that on this day (at noon) Mercury's R.A. is 6h. 53m. 23s.: and the sun's 5h. 11m. 31s. We need not trouble ourselves about the odd hours after noon, and thus we have Mercury's R.A. greater than the sun's by 1h. 41m. 52s. Now we will suppose that the observer has so fixed his uprights and the two rods, that the sun, seen from the fixed point of view, appears to pass the point of crossing of the rods at half-past seven, then Mercury will pass the cross-rod at 11m. 52s. past nine. But where? To learn this we must take out Mercury's declination, which is 24° 43' 18" N., and the sun's, which is 22° 59' 10" N. The difference, 1° 44' 8" N. gives us Mercury's place, which it appears is rather less than 1¾ degree north of the sun. Thus, about 1h. 42m. after the sun has passed the cross-rod, Mercury will pass it between the first and second divisions above the point of fastening. The sun will have set about an hour, and Mercury will be easily found when the telescope is directed towards the place indicated.
It will be noticed that this method does not require the time to be exactly known. All we have to do is to note the moment at which the sun passes the point of fastening of the two rods, and to take our 1h. 42m. from that moment.
This method, it may be noticed in passing, may be applied to give naked-eye observations of Mercury at proper seasons (given in the almanac). By a little ingenuity it may be applied as well to morning as to evening observations, the sun's passage of the cross-rod being taken on one morning and Mercury's on the next, so many minutes before the hour of the first observation. In this way several views of Mercury may be obtained during the year.
Such methods may appear very insignificant to the systematic observer with the equatorial, but that they are effective I can assert from my own experience. Similar methods may be applied to determine from the position of a known object, that of any neighbouring unknown object even at night. The cross-rod must be shifted (or else two cross-rods used) when the unknown precedes the known object. If two cross-rods are used, account must be taken of the gradual diminution in the length of a degree of right ascension as we leave the equator.
Even simpler methods carefully applied may serve to give a view of Mercury. To show this, I may describe how I obtained my first view of this planet. On June 1st, 1863, I noticed, that at five minutes past seven the sun, as seen from my study window, appeared from behind the gable-end of Mr. St. Aubyn's house at Stoke, Devon. I estimated the effect of Mercury's northerly declination (different of course for a vertical wall, than for the cross-rod in [fig. 8], which, in fact, agrees with a declination-circle), and found that he would pass out opposite a particular point of the wall a certain time after the sun. I then turned the telescope towards that point, and focussed for distinct vision of distant objects, so that the outline of the house was seen out of focus. As the calculated time of apparition approached, I moved the telescope up and down so that the field swept the neighbourhood of the estimated point of apparition. I need hardly say that Mercury did not appear exactly at the assigned point, nor did I see him make his first appearance; but I picked him up so soon after emergence that the outline of the house was in the field of view with him. He appeared as a half-disc. I followed him with the telescope until the sun had set, and soon after I was able to see him very distinctly with the naked eye. He shone with a peculiar brilliance on the still bright sky; but although perfectly distinct to the view when his place was indicated, he escaped detection by the undirected eye.[12]
Mercury does not present any features of great interest in ordinary telescopes; though he usually appears better defined than Venus, at least as the latter is seen on a dark sky. The phases are pleasingly seen (as shown in Plate [6]) with a telescope of moderate power. For their proper observation, however, the planet must be looked for with the telescope in the manner above indicated, as he always shows a nearly semi-circular disc when he is visible to the naked eye.
We come next to Venus, the most splendid of all the planets to the eye. In the telescope Venus disappoints the observer, however. Her intense lustre brings out every defect of the instrument, and especially the chromatic aberration. A dark glass often improves the view, but not always. Besides, an interposed glass has an unpleasant effect on the field of view.
Perhaps the best method of observing Venus is to search for her when she is still high above the horizon, and when therefore the background of the sky is bright enough to take off the planet's glare. The method I have described for the observation of Mercury will prove very useful in the search for Venus when the sun is above the horizon or but just set. Of course, when an object is to be looked for high above the horizon, the two rods which support the cross-rods must not be upright, but square to the line of view to that part of the sky.
But the observer must not expect to see much during his observation of Venus. In fact, he can scarcely do more than note her varying phases (see Plate [6]) and the somewhat uneven boundary of the terminator. Our leading observers have done so little with this fascinating but disappointing planet, that amateurs must not be surprised at their own failure.
I suppose the question whether Venus has a satellite, or at any rate whether the object supposed to have been seen by Cassini and other old observers were a satellite, must be considered as decided in the negative. That Cassini should have seen an object which Dawes and Webb have failed to see must be considered utterly improbable.
Leaving the inferior planets, we come to a series of important and interesting objects.
First we have the planet Mars, nearly the last in the scale of planetary magnitude, but far from being the least interesting of the planets. It is in fact quite certain that we obtain a better view of Mars than of any object in the heavens, save the Moon alone. He may present a less distinguished appearance than Jupiter or Saturn, but we see his surface on a larger scale than that of either of those giant orbs, even if we assume that we ever obtain a fair view of their real surface.
Nor need the moderately armed observer despair of obtaining interesting views of Mars. The telescope with which Beer and Mädler made their celebrated series of views was only a 4-inch one, so that with a 3-inch or even a 2-inch aperture the attentive observer may expect interesting views. In fact, more depends on the observer than on the instrument. A patient and attentive scrutiny will reveal features which at the first view wholly escape notice.
In Plate [6] I have given a series of views of Mars much more distinct than an observer may expect to obtain with moderate powers. I add a chart of Mars, a miniature of one I have prepared from a charming series of tracings supplied me by Mr. Dawes. The views taken by this celebrated observer in 1852, 1856, 1860, 1862, and 1864, are far better than any others I have seen. The views by Beer and Mädler are good, as are some of Secchi's (though they appear badly drawn), Nasmyth's and Phillips'; Delarue's two views are also admirable; and Lockyer has given a better set of views than any of the others. But there is an amount of detail in Mr. Dawes' views which renders them superior to any yet taken. I must confess I failed at a first view to see the full value of Mr. Dawes' tracings. Faint marks appeared, which I supposed to be merely intended to represent shadings scarcely seen. A more careful study shewed me that every mark is to be taken as the representative of what Mr. Dawes actually saw. The consistency of the views is perfectly wonderful, when compared with the vagueness and inconsistency observable in nearly all other views. And this consistency is not shown by mere resemblance, which might have been an effect rather of memory (unconsciously exerted) than observation. The same feature changes so much in figure, as it appears on different parts of the disc, that it was sometimes only on a careful projection of different views that I could determine what certain features near the limb represented. But when this had been done, and the distortion through the effect of foreshortening corrected, the feature was found to be as true in shape as if it had been seen in the centre of the planet's disc.
In examining Mr. Dawes' drawings it was necessary that the position of Mars' axis should be known. The data for determining this were taken from Dr. Oudemann's determinations given in a valuable paper on Mars issued from Mr. Bishop's observatory. But instead of calculating Mars' presentation by the formulæ there given, I found it convenient rather to make use of geometrical constructions applied to my 'Charts of the Terrestrial Planets.' Taking Mädler's start-point for Martial longitudes, that is the longitude-line passing near Dawes' forked bay, I found that my results agreed pretty fairly with those in Prof. Phillips' map, so far as the latter went; but there are many details in my charts not found in Prof. Phillips' nor in Mädler's earlier charts.
I have applied to the different features the names of those observers who have studied the physical peculiarities presented by Mars. Mr. Dawes' name naturally occurs more frequently than others. Indeed, if I had followed the rule of giving to each feature the name of its discoverer, Mr. Dawes' name would have occurred much more frequently than it actually does.
On account of the eccentricity of his orbit, Mars is seen much better in some oppositions than in others. When best seen the southern hemisphere is brought more into view than the northern because the summer of his northern hemisphere occurs when he is nearly in aphelion (as is the case with the Earth by the way).
The relative dimensions and presentation of Mars, as seen in opposition in perihelion, and in opposition in aphelion, are shown in the two rows of figures.
In and near quadrature Mars is perceptibly gibbous. He is seen thus about two months before or after opposition. In the former case, he rises late and comes to the meridian six hours or so after midnight. In the latter case, he is well seen in the evening, coming to the meridian at six. His appearance and relative dimensions as he passes from opposition to quadrature are shown in the last three figures of the upper row.
Mars' polar caps may be seen with very moderate powers.
I add four sets of meridians (Plate [6]), by filling in which from the charts the observer may obtain any number of views of the planet as it appears at different times.
Passing over the asteroids, which are not very interesting objects to the amateur telescopist, we come to Jupiter, the giant of the solar system, surpassing our Earth more than 1400 times in volume, and overweighing all the planets taken together twice over.
Jupiter is one of the easiest of all objects of telescopic observation. No one can mistake this orb when it shines on a dark sky, and only Venus can be mistaken for it when seen as a morning or evening star. Sometimes both are seen together on the twilight sky, and then Venus is generally the brighter. Seen, however, at her brightest and at her greatest elongation from the sun, her splendour scarcely exceeds that with which Jupiter shines when high above the southern horizon at midnight.
Jupiter's satellites may be seen with very low powers; indeed the outer ones have been seen with the naked eye, and all are visible in a good opera-glass. Their dimensions relatively to the disc are shown in Plate 7. Their greatest elongations are compared with the disc in the low-power view.
Jupiter's belts may also be well seen with moderate telescopic power. The outer parts of his disc are perceptibly less bright than the centre.
More difficult of observation are the transits of the satellites and of their shadows. Still the attentive observer can see the shadows with an aperture of two inches, and the satellites themselves with an aperture of three inches.
The minute at which the satellites enter on the disc, or pass off, is given in 'Dietrichsen's Almanac.' The 'Nautical Almanac' also gives the corresponding data for the shadows.
The eclipses of the satellites in Jupiter's shadow, and their occultations by his disc, are also given in 'Dietrichsen's Almanac.'
In the inverting telescope the satellites move from right to left in the nearer parts of their orbit, and therefore transit Jupiter's disc in that direction, and from left to right in the farther parts. Also note that before opposition, (i.) the shadows travel in front of the satellites in transiting the disc; (ii.) the satellites are eclipsed in Jupiter's shadow; (iii.) they reappear from behind his disc. On the other hand, after opposition, (i.) the shadows travel behind the satellites in transiting the disc; (ii.) the satellites are occulted by the disc; (iii.) they reappear from eclipse in Jupiter's shadow.
Conjunctions of the satellites are common phenomena, and may be waited for by the observer who sees the chance. An eclipse of one satellite by the shadow of another is not a common phenomenon; in fact, I have never heard of such an eclipse being seen. That a satellite should be quite extinguished by another's shadow is a phenomenon not absolutely impossible, but which cannot happen save at long intervals.
The shadows are not black spots as is erroneously stated in nearly all popular works on astronomy. The shadow of the fourth, for instance, is nearly all penumbra, the really black part being quite minute by comparison. The shadow of the third has a considerable penumbra, and even that of the first is not wholly black. These penumbras may not be perceptible, but they affect the appearance of the shadows. For instance, the shadow of the fourth is perceptibly larger but less black than that of the third, though the third is the larger satellite.
In transit the first satellite moves fastest, the fourth slowest, the others in their order. The shadow moves just as fast (appreciably) as the satellite it belongs to. Sometimes the shadow of the satellite may be seen to overtake (apparently) the disc of another. In such a case the shadow does not pass over the disc, but the disc conceals the shadow. This is explained by the fact that the shadow, if visible throughout its length, would be a line reaching slantwise from the satellite it belongs to, and the end of the shadow (that is, the point where it meets the disc) is not the point where the shadow crosses the orbit of any inner satellite. Thus the latter may be interposed between the end of the shadow—the only part of the shadow really visible—and the eye; but the end of the shadow cannot be interposed between the satellite and the eye. If a satellite on the disc were eclipsed by another satellite, the black spot thus formed would be in another place from the black spot on the planet's body. I mention all this because, simple as the question may seem, I have known careful observers to make mistakes on this subject. A shadow is seen crossing the disc and overtaking, apparently, a satellite in transit. It seems therefore, on a first view, that the shadow will hide the satellite, and observers have even said that they have seen this happen. But they are deceived. It is obvious that if one satellite eclipse another, the shadows of both must occupy the same point on Jupiter's body. Thus it is the overtaking of one shadow by another on the disc, and not the overtaking of a satellite by a shadow, which determines the occurrence of that as yet unrecorded phenomenon, the eclipse of one satellite by another.[13]
The satellites when far from Jupiter seem to lie in a straight line through his centre. But as a matter of fact they do not in general lie in an exact straight line. If their orbits could be seen as lines of light, they would appear, in general, as very long ellipses. The orbit of the fourth would frequently be seen to be quite clear of Jupiter's disc, and the orbit of the third might in some very exceptional instances pass just clear of the disc. The satellites move most nearly in a straight line (apparently) when Jupiter comes to opposition in the beginning of February or August, and they appear to depart most from rectilinear motion when opposition occurs in the beginning of May and November. At these epochs the fourth satellite may be seen to pass above and below Jupiter's disc at a distance equal to about one-sixth of the disc's radius.
The shadows do not travel in the same apparent paths as the satellites themselves across the disc, but (in an inverting telescope) below from August to January, and above from February to July.
We come now to the most charming telescopic object in the heavens—the planet Saturn. Inferior only to Jupiter in mass and volume, this planet surpasses him in the magnificence of his system. Seen in a telescope of adequate power, Saturn is an object of surpassing loveliness. He must be an unimaginative man who can see Saturn for the first time in such a telescope, without a feeling of awe and amazement. If there is any object in the heavens—I except not even the Sun—calculated to impress one with a sense of the wisdom and omnipotence of the Creator it is this. "His fashioning hand" is indeed visible throughout space, but in Saturn's system it is most impressively manifest.
Saturn, to be satisfactorily seen, requires a much more powerful telescope than Jupiter. A good 2-inch telescope will do much, however, in exhibiting his rings and belts. I have never seen him satisfactorily myself with such an aperture, but Mr. Grover has not only seen the above-named features, but even a penumbra to the shadow on the rings with a 2-inch telescope.
Saturn revolving round the sun in a long period—nearly thirty years—presents slowly varying changes of appearance (see Plate [7]). At one time the edge of his ring is turned nearly towards the earth; seven or eight years later his rings are as much open as they can ever be; then they gradually close up during a corresponding interval; open out again, exhibiting a different face; and finally close up as first seen. The last epoch of greatest opening occurred in 1856, the next occurs in 1870: the last epoch of disappearance occurred in 1862-63, the next occurs in 1879. The successive views obtained are as in Plate [7] in order from right to left, then back to the right-hand figure (but sloped the other way); inverting the page we have this figure thus sloped, and the following changes are now indicated by the other figures in order back to the first (but sloped the other way and still inverted), thus returning to the right-hand figure as seen without inversion.
The division in the ring can be seen in a good 2-inch aperture in favourable weather. The dark ring requires a good 4-inch and good weather.
Saturn's satellites do not, like Jupiter's, form a system of nearly equal bodies. Titan, the sixth, is probably larger than any of Jupiter's satellites. The eighth also (Japetus) is a large body, probably at least equal to Jupiter's third satellite. But Rhea, Dione, and Tethys are much less conspicuous, and the other three cannot be seen without more powerful telescopes than those we are here dealing with.
So far as my own experience goes, I consider that the five larger satellites may be seen distinctly in good weather with a good 3½-inch aperture. I have never seen them with such an aperture, but I judge from the distinctness with which these satellites may be seen with a 4-inch aperture. Titan is generally to be looked for at a considerable distance from Saturn—always when the ring is widely open. Japetus is to be looked for yet farther from the disc. In fact, when Saturn comes to opposition in perihelion (in winter only this can happen) Japetus may be as far from Saturn as one-third of the apparent diameter of the moon. I believe that under these circumstances, or even under less favourable circumstances, Japetus could be seen with a good opera-glass. So also might Titan.
Transits, eclipses, and occulations of Saturn's satellites can only be seen when the ring is turned nearly edgewise towards the earth. For the orbits of the seven inner satellites lying nearly in the plane of the rings would (if visible throughout their extent) then only appear as straight lines, or as long ellipses cutting the planet's disc.
The belts on Saturn are not very conspicuous. A good 3½-inch is required (so far as my experience extends) to show them satisfactorily.
The rings when turned edgewise either towards the earth or sun, are not visible in ordinary telescopes, neither can they be seen when the earth and sun are on opposite sides of the rings. In powerful telescopes the rings seem never entirely to disappear.
The shadow of the planet on the rings may be well seen with a good 2-inch telescope, which will also show Ball's division in the rings. The shadow of the rings on the planet is a somewhat more difficult feature. The shadow of the planet on the rings is best seen when the rings are well open and the planet is in or near quadrature. It is to be looked for to the left of the ball (in an inverting telescope) at quadrature preceding opposition, and to the right at quadrature following opposition. Saturn is more likely to be studied at the latter than at the former quadrature, as in quadrature preceding opposition he is a morning star. The shadow of the rings on the planet is best seen when the rings are but moderately open, and Saturn is in or near quadrature. When the shadow lies outside the rings it is best seen, as the dark ring takes off from the sharpness of the contrast when the shadow lies within the ring. It would take more space than I can spare here to show how it is to be determined (independently) whether the shadow lies within or without the ring. But the 'Nautical Almanac' gives the means of determining this point. When, in the table for assigning the appearance of the rings, l is less than l' the shadow lies outside the ring, when l is greater than l' the shadow lies within the ring.
Uranus is just visible to the naked eye when he is in opposition, and his place accurately known. But he presents no phenomena of interest. I have seen him under powers which made his disc nearly equal to that of the moon, yet could see nothing but a faint bluish disc.
Neptune also is easily found if his place be accurately noted on a map, and a good finder used. We have only to turn the telescope to a few stars seen in the finder nearly in the place marked in our map, and presently we shall recognise the one we want by the peculiarity of its light. What is the lowest power which will exhibit Neptune as a disc I do not know, but I am certain no observer can mistake him for a fixed star with a 2-inch aperture and a few minutes' patient scrutiny in favourable weather.
