Stargazing: Past and Present. Sir Norman Lockyer
instruments must and actually do fulfil.
It is true that the present idea of the earth’s place in the system is different. Euclid imagined the earth to be at the centre of the universe. It is now known that the earth is one of various planets which revolve round the sun, and further, that the journey of the earth round the sun is so even and beautifully regulated that its motion is confined to a single plane. Year after year the earth goes on revolving round the sun, never departing, except to a very small extent, from this plane, which is one of the fundamental planes of the astronomer and called the Plane of the Ecliptic.
Fig. 4.—The Plane of the Ecliptic.
Suppose this plane to be a tangible thing, like the surface of an infinite ocean, the sun will occupy a certain position in this infinite ocean, and the earth will travel round it every year.
If the axis of the earth were upright, one would represent the position of things by holding a globe with its axis upright, so that the equator of the earth is in every part of its revolution on a level with this ecliptic sea. But it is known that the earth, instead of floating, as it were, upright, as in Fig. 4, has its axis inclined to the plane of the ecliptic, as in Fig. 5.
It is also known that by turning a globe round, distant objects would appear to move round an observer on the globe in an opposite direction to his own motion, and these distant objects would describe circles round a line joining the places pointed to by the poles of the earth, i.e., round the earth’s axis.
Fig. 5.—The Plane of the Ecliptic, showing the Inclination of the Earth’s Axis.
It is now easy to explain the observations referred to by Euclid by supposing the surface of the water in the tub to represent the plane of the ecliptic, that is, the plane of the path which the sun apparently takes in going round the earth; and examining the relative positions of the sun and earth represented by two floating balls, the latter having a wire through it inclined to the upright position; it will be seen at once by turning the ball on the wire as an axis to represent the diurnal motion of our earth, how Euclid finds the Bear which never sets, being the place in the heavens pointed to by the earth’s pole; and how all the stars in different portions of the heavens appear to move in complete circles round the pole-star when they do not set, and in parts of circles when they pass below the horizon. By moving the ball representing the earth round the sun and so examining their relative positions, during the course of a year it will be seen how the sun appears to travel through all the signs of the zodiac in succession in his yearly course, remaining a longer or shorter time above the horizon at different times of the year.
For it will be seen that if the spectator on the globe, when in the position that its inclined axis, as shown in Fig. 5, points towards the sun, were looking at the sun from a place where one can imagine England to be at midday, the sun would appear to be very high up above the horizon; and if he looked at it from the earth in the opposite part of its orbit it would be very low and near the horizon. When the earth, therefore, occupied the intermediate positions, the sun would be half way between the extreme upper position and the extreme lower position as the earth moves round the sun, and in this way the solstices, equinoxes, and the seasonal changes on the surface of our planet, are easily explained.
1. Corresponding to 20th July, 139 B.C.
2. Anaximander flourished about 547 B.C.
3. Quoted by Sir G. C. Lewis in his Astronomy of the Ancients, p. 187.
CHAPTER II.
THE FIRST INSTRUMENTS.
The ancients called the places occupied by the sun when highest and lowest the Solstices, and the intermediate positions the Equinoxes. The first instrument made was for the determination of the sun’s altitude in order to fix the solstices. This instrument was called the Gnomon. It consisted of an upright rod, sharp at the end and raised perpendicularly on a horizontal plane, and its shadow could be measured in the plane of the meridian by a north and south line on the ground. Whenever the shadow was longest the sun was naturally lowest down at the winter solstice, and vice versâ for the summer solstice.
Here then we leave observations on the horizon and come to those made on the meridian.
The Gnomon is said to have been known to the Chinese in the time of the Emperor Yao’s reign (2300 B.C.), but it was not used by the Greeks[4] till the time of Thales, about 585 B.C., who fixed the dates of the solstices and equinoxes, and the length of the tropical year—that is, the time taken by the sun to travel from the vernal equinoctial point round to the same point again.
The next problem was to discover the inclination of the ecliptic, or, what is the same thing, the amount that the earth’s equator is inclined to the ecliptic plane (represented by the surface of the water in our tub).
Now in order to ascertain this, the angular distance between the positions occupied by the sun when at the solstices must be measured; or, since one solstice is just as much below the equinoctial line as the other is above it, we might take half the angle between the solstices as being the obliquity required.
The first method of measuring the angle was to measure the length of the sun’s shadow at each solstice, and so, by comparison of the length of the shadow with the height of the gnomon, calculate the difference in altitude, the half of which was the angle sought. And this was probably the method of the Chinese, who obtained a result of 23° 38´ 11˝ in the time of Yao; and also of Anaximander in his early days, who obtained a result of 24°. But before trigonometrical tables, the first of which seem to have been constructed by Hipparchus and Ptolemy, were known, in order to find this angle it was constructed geometrically, and then what aliquot part of the circumference it was, or how much of the circumference it contained was determined; for the division of the circle into 360° is subsequent to the first beginning of astronomy—and hence it was that Eratosthenes said that the distance from the tropics was 11 83 of the circumference, and not that it was 47° 46´ 26˝.
The gnomon is, without exception, of all instruments the one with which the ancients were able to make the best observations of the sun’s altitude. But they did not give sufficient attention to it to enable it to be used with accuracy. The shadow projected by a point when the sun is shining is not well defined, so that they could not be quite certain of its extremity, and it would seem that the ancient observations of the height of the sun made in this manner ought to be corrected by about half the apparent diameter of the sun; for it is probable that the ancients took the strong shadow for the true shadow; and so they had only the height of the upper part of the sun and not that of the centre. There is no proof that they did not make this correction, at least in the later observations.
In order to obviate this inconvenience, they subsequently terminated the gnomon by a bowl or disc, the centre of which answered to the summit; so that, taking the centre of the shadow of this bowl, they had the height of the centre of the sun. Such was the form of the one that Manlius the mathematician erected at Rome under the auspices of Augustus.
But in comparatively modern times astronomers have remedied this defect in a still more happy manner, by using a vertical or horizontal plate pierced with a circular hole which allows the rays of the sun to enter into a dark place, and in fact to form a true image of the sun on a floor or other convenient receptacle, as we find is the case in many continental churches.
Of course at this early period the reference of any particular phenomenon to true time was out of the question. The ancients at the period we are considering used twelve