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.