Part 6

The point A on the equinoctial (Fig. 19) is called the vernal equinox, or the first point of Aries. The hour circle, APP', which passes through it, is called the equinoctial colure.

14. Declination and Right Ascension.--The declination of a heavenly body is its distance north or south of the celestial equator. The polar distance of a heavenly body is its distance from the nearer pole. Declination and polar distance are measured on hour circles, and for the same heavenly body they are complements of each other.

The right ascension of a heavenly body is its distance eastward from the first point of Aries, measured from the equinoctial colure. It is equal to the arc of the celestial equator included between the first point of Aries and the hour circle which passes through the heavenly body. As right ascension is measured eastward entirely around the celestial sphere, it may have any value from 0° up to 360°. Right ascension corresponds to longitude on the earth, and declination to latitude.

15. The Meridian Circle.--The right ascension and declination of a heavenly body are ascertained by means of an instrument called the meridian circle, or transit instrument. A side-view of this instrument is shown in Fig. 20.

It consists essentially of a telescope mounted between two piers, so as to turn in the plane of the meridian, and carrying a graduated circle. The readings of this circle are ascertained by means of fixed microscopes, under which it turns. A heavenly body can be observed with this instrument, only when it is crossing the meridian. For this reason it is often called the transit circle.

To find the declination of a star with this instrument, we first ascertain the reading of the circle when the telescope is pointed to the pole, and then the reading of the circle when pointed to the star on its passage across the meridian. The difference between these two readings will be the polar distance of the star, and the complement of them the declination of the star.

To ascertain the reading of the circle when the telescope is pointed to the pole, we must select one of the circumpolar stars near the pole, and then point the telescope to it when it crosses the meridian, both above and below the pole, and note the reading of the circle in each case. The mean of these two readings will be the reading of the circle when the telescope is pointed to the pole.

16. Astronomical Clock.--An astronomical clock, or sidereal clock as it is often called, is a clock arranged so as to mark hours from 1 to 24, instead of from 1 to 12, as in the case of an ordinary clock, and so adjusted as to mark 0 when the vernal equinox, or first point of Aries, is on the meridian.

As the first point of Aries makes a complete circuit of the heavens in twenty-four hours, it must move at the rate of 15° an hour, or of 1° in four minutes: hence, when the astronomical clock marks 1, the first point of Aries must be 15° west of the meridian, and when it marks 2, 30° west of the meridian, etc. That is to say, by observing an accurate astronomical clock, one can always tell how far the meridian at any time is from the first point of Aries.

17. How to find Right Ascension with the Meridian Circle.--To find the right ascension of a heavenly body, we have merely to ascertain the exact time, by the astronomical clock, at which the body crosses the meridian. If a star crosses the meridian at 1 hour 20 minutes by the astronomical clock, its right ascension must be 19°; if at 20 hours, its right ascension must be 300°.

To enable the observer to ascertain with great exactness the time at which a star crosses the meridian, a number of equidistant and parallel spider-lines are stretched across the focus of the telescope, as shown in Fig. 21. The observer notes the time when the star crosses each spider-line; and the mean of all of these times will be the time when the star crosses the meridian. The mean of several observations is likely to be more nearly exact than any single observation.

18. The Equatorial Telescope.--The equatorial telescope is mounted on two axes,--one parallel with the axis of the earth, and the other at right angles to this, and therefore parallel with the plane of the earth's equator. The former is called the polar axis, and the latter the declination axis. Each axis carries a graduated circle. These circles are called respectively the hour circle and the declination circle. The telescope is attached directly to the declination axis. When the telescope is fixed in any declination, and then turned on its polar axis, the line of sight will describe a diurnal circle; so that, when the tube is once directed to a star, it can be made to follow the star by simply turning the telescope on its polar axis.

In the case of large instruments of this class, the polar axis is usually turned by clock-work at the rate at which the heavens rotate; so that, when the telescope has once been pointed to a planet or other heavenly body, it will continue to follow the body and keep it steadily in the field of view without further trouble on the part of the observer.

The great Washington Equatorial is shown in Fig. 22. Its object-glass is 26 inches in diameter, and its focal length is 32-1/2 feet. It was constructed by Alvan Clark & Sons of Cambridge, Mass. It is one of the three largest refracting telescopes at present in use. The Newall refractor at Gateshead, Eng., has an objective 25 inches in diameter, and a focal length of 29 feet. The great refractor at Vienna has an objective 27 inches in diameter. There are several large refractors now in process of construction.

19. The Wire Micrometer.--Large arcs in the heavens are measured by means of the graduated circles attached to the axes of the telescopes; but small arcs within the field of view of the telescope are measured by means of instruments called micrometers, mounted in the focus of the telescope. One of the most convenient of these micrometers is that known as the wire micrometer, and shown in Fig. 23.

The frame AA covers two slides, C and D. These slides are moved by the screws F and G. The wires E and B are stretched across the ends of the slides so as to be parallel to each other. On turning the screws F and G one way, these wires are carried apart; and on turning them the other way they are brought together again. Sometimes two parallel wires, x and y, shown in the diagram at the right, are stretched across the frame at right angles to the wires E, B. We may call the wires x and y the longitudinal wires of the micrometer, and E and B the transverse wires. Many instruments have only one longitudinal wire, which is stretched across the middle of the focus. The longitudinal wires are just in front of the transverse wires, but do not touch them.

To find the distance between any two points in the field of view with a micrometer, with a single longitudinal wire, turn the frame till the longitudinal wire passes through the two points; then set the wires E and B one on each point, turn one of the screws, known as the micrometer screw, till the two wires are brought together, and note the number of times the screw is turned. Having previously ascertained over what arc one turn of the screw will move the wire, the number of turns will enable us to find the length of the arc between the two points.

The threads of the micrometer screw are cut with great accuracy; and the screw is provided with a large head, which is divided into a hundred or more equal parts.

These divisions, by means of a fixed pointer, enable us to ascertain what fraction of a turn the screw has made over and above its complete revolutions.

20. Reflecting Telescopes.--It is possible to construct mirrors of much larger size than lenses: hence reflecting telescopes have an advantage over refracting telescopes as regards size of aperture and of light-gathering power. They are, however, inferior as regards definition; and, in order to prevent flexure, it is necessary to give the speculum, or mirror, a massiveness which makes the telescope unwieldy. It is also necessary frequently to repolish the speculum; and this is an operation of great delicacy, as the slightest change in the form of the surface impairs the definition of the image. These defects have been remedied, to a certain extent, by the introduction of silver-on-glass mirrors; that is, glass mirrors covered in front with a thin coating of silver. Glass is only one-third as heavy as speculum-metal, and silver is much superior to that metal in reflecting power; and when the silver becomes tarnished, it can be removed and renewed without danger of changing the form of the glass.

The Herschelian Reflector.--In this form of telescope the mirror is slightly tipped, so that the image, instead of being formed in the centre of the tube, is formed near one side of it, as in Fig. 24. The observer can then view it without putting his head inside the tube, and therefore without cutting off any material portion of the light. In observation, he must stand at the upper or outer end of the tube, and look into it, his back being turned towards the object. From his looking directly into the mirror, it is also sometimes called the front-view telescope. The great disadvantage of this arrangement is, that the rays cannot be brought to an exact focus when they are thrown so far to one side of the axis, and the injury to the definition is so great that the front-view plan is now entirely abandoned.

The Newtonian Reflector.--The plan proposed by Sir Isaac Newton was to place a small plane mirror just inside the focus, inclined to the telescope at an angle of 45°, so as to throw the rays to the side of the tube, where they come to a focus, and form the image. An opening is made in the side of the tube, just below where the image is formed; and in this opening the eye-piece is inserted. The small mirror cuts off some of the light, but not enough to be a serious defect. An improvement which lessens this defect has been made by Professor Henry Draper. The inclined mirror is replaced by a small rectangular prism (Fig. 25), by reflection from which the image is formed very near the prism. A pair of lenses are then inserted in the course of the rays, by which a second image is formed at the opening in the side of the tube; and this second image is viewed by an ordinary eye-piece.

The Gregorian Reflector.--This is a form proposed by James Gregory, who probably preceded Newton as an inventor of the reflecting telescope. Behind the focus, F (Fig. 26), a small concave mirror, R, is placed, by which the light is reflected back again down the tube. The larger mirror, M, has an opening through its centre; and the small mirror, R, is so adjusted as to form a second image of the object in this opening. This image is then viewed by an eye-piece which is screwed into the opening.

The Cassegrainian Reflector.--In principle this is the same with the Gregorian; but the small mirror, R, is convex, and is placed inside the focus, F, so that the rays are reflected from it before reaching the focus, and no image is formed until they reach the opening in the large mirror. This form has an advantage over the Gregorian, in that the telescope may be made shorter, and the small mirror can be more easily shaped to the required figure. It has, therefore, entirely superseded the original Gregorian form.

Optically these forms of telescope are inferior to the Newtonian; but the latter is subject to the inconvenience, that the observer must be stationed at the upper end of the telescope, where he looks into an eye-piece screwed into the side of the tube.

On the other hand, the Cassegrainian Telescope is pointed directly at the object to be viewed, like a refractor; and the observer stands at the lower end, and looks in at the opening through the large mirror. This is, therefore, the most convenient form of all in management.

The largest reflecting telescope yet constructed is that of Lord Rosse, at Parsonstown, Ireland. Its speculum is 6 feet in diameter, and its focal length 55 feet. It is commonly used as a Newtonian. This telescope is shown in Fig. 27.

The great telescope of the Melbourne Observatory, Australia, is a Cassegranian reflector. Its speculum is 4 feet in diameter, and its focal length is 32 feet. It is shown in Fig. 28.