Stargazing: Past and Present

by Sir Norman Lockyer

The planets also come in for their share of examination. Mercury is so near the sun as seldom to be seen. Venus in small telescopes is only interesting with reference to her changes, like the moon, but in larger ones with great care the spots are visible. Mars is interesting as being so near a counterpart of our own planet. On it we see the polar snows, continents and seas, partially obscured by clouds, and these appearances are brought under our view in succession by the rotation of the planet. With a good six-inch glass and a power of 200 when the air is pure and the opposition is favourable, there is no difficulty in making out the coast-lines, and the various tones of shade on the water surface may be observed, showing that here the sea is tranquil, and there it is driven by storms. Up to very lately it was the only planet of considerable size further off the sun than Venus that was supposed to have no satellite; two of these bodies have however been lately discovered by Hall with the large Washington refractor of twenty-six inches diameter, and they appear to be the tiniest celestial bodies known, one of them in all probability not exceeding 10 miles in diameter. Jupiter and Saturn are very conspicuous objects, and the eclipses, transits, and occultations of the moons, and the belts of the former and rings of the latter, are among the most interesting phenomena revealed to us by our telescopes, while the delicate markings on the third satellite of Jupiter furnish us with one of the most difficult tests of definition. Uranus and Neptune are only just seen in small telescopes, and even in spite of the use of larger ones, we are in ignorance of much relating to these planets. The amateur will do well to attack all these with that charming book, the Rev. T. W. Webb’s Celestial Objects for Common Telescopes, in his hand.

To observe the fainter satellites of the brighter planets, or, indeed, faint objects generally, near very bright ones, the bright object may be screened by a metallic bar, or red or blue glass placed in the common focus.

So much with regard to our own system. When we leave it we are confounded with the wealth of nebulæ, star-clusters, and single or multiple systems of stars, which await our scrutiny. With the stars, not much can be done without further assistance than the eyepiece alone. The colours of stars may however be observed, and for this purpose a chromatic scale has been proposed, and a memoir thereon written, by Admiral Smyth, for comparison with the stars. The colour of a star must not be confused with the colours—often very vivid—produced by scintillations, these rapid changes of brightness and colour depending on atmospheric causes. Of the large stars, Sirius, Vega and Regulus are white, while Aldebaran and Betelgueux are red. In many double and multiple stars however the contrast of colours shows up beautifully; in β Cygni for instance we have a yellow and blue star, in γ Leonis, a yellow and a green star; and of such there are numerous examples.

Interesting as all these observations are, a new life and utility are thrown into them when instead of using a simple eyepiece the wire micrometer is introduced. This, as we have before stated, generally consists of one wire, or two parallel wires, fixed, and one or two other wires at right angles to these, movable across the field. This micrometer is used in connection with a part of the eyepiece end of the telescope, which has now to be described. This is a circle, the fineness of the graduation of which increases with the size of the telescope, read by two or four verniers. The circle is fixed to the telescope, while the verniers are attached to the eyepiece, carrying the micrometer, which is rotated by a rack and pinion.

The whole system of position circle (as it is called) and wire micrometer, is in adjustment when (1) the single or double fixed wires and the movable ones cross in the centre of the field, and (2) when with a star travelling along the single fixed or between the two fixed wires, the upper vernier reads 180 and the lower one reads zero.

This motion across the field gives the direction of a parallel of declination; that is to say, it gives a line parallel to the celestial equator, and, knowing that, one will be able at once, by allowing the object to pass through the field of view, to get this datum line. For instance, supposing the whole instrument is turned round on the end of the telescope, so that one of the two wires x and y, Fig. [104], at right angles to the thin wires for measuring distance, shall lie on a star during all its motion across the field of view; then those two wires, being parallel to the star’s motion, will represent two parallels of declination; and we use the direction of the parallels of declination to determine the datum point at right angles to them, that is, the north point of the field. We have then a position micrometer, that is, one in which the field of view is divided into four quadrants, called north preceding, north following, south preceding, and south following, because if there be an object at the central point it will be preceded and followed by those in the various quadrants. The movable wires lie on meridians and the fixed ones on parallels when adjusted as above.

Fig. 158.—Position Circle.

The position circle is often attached to, and forms part of, the micrometer instead of being fixed to the telescope, and in screwing it on from time to time, the adjustment of the zero changes, and the index error must be found each time the micrometer is put on the telescope.

In practice it is usual to take the north and south line as the datum line, and positions are always expressed in degrees from the north round by east 90°, south 180°, and west 270°, to north again in the direction contrary to that of the hands of a clock.