Eratosthenes (276-196 B.C.) used most probably a solstitial armilla for measuring the obliquity of the ecliptic. Hipparchus (160-125 B.C.) probably used an armillary sphere of four rings.
The chief conclusions of astronomers concerning the .spherical figure and dimensions of the earth, its relation to the heavenly bodies, and the great circles of the globe - the equator, the ecliptic and the tropics - were considered as well established.
The Solar Astronomical Year Is The Period Of Time In Which The Earth Performs A Revolution In Its Orbit About The Sun, Or Passes From Any Point Of The Ecliptic To The Same Point Again; And Consists Of 365 Days 5 Hours 48 Min.
Discarding those of Uranus, in which the orbits of the satellites are highly inclined to the ecliptic, and in which manifestly some exceptional influences have been at work, we find that the satellites revolve around the primaries also in the same direction (Exceptions are Saturn ix.
In the astral theology of Babylonia and Assyria, Anu, Bel and Ea became the three zones of the ecliptic, the northern, middle and southern zone respectively.
One of these is the position of the line MN through the sun at F in which the plane of the orbit cuts some fundamental plane of reference, commonly the ecliptic. This is called the line of nodes, and its position is specified by the angle which it makes with some fixed line FX in the fundamental plane.
Its weak point is that the apparent motion of the node depends partly upon the motion of the ecliptic, which cannot be determined with equal precision.
The great variety in the apparent motions of meteors proves that they are not directed from the plane of the ecliptic; hence their orbits are not like the orbits of planets and short-period comets, which are little inclined, but like the orbits of parabolic comets, which often have great inclinations.
It is bounded by two circles equidistant from the ecliptic, about eighteen degrees apart; and it is divided into twelve signs, and marked by twelve constellations.
These were called by the Greeks " decans," because ten degrees of the ecliptic and ten days of the year were presided over by each.
2 Projected upon the ecliptic, these were considerably unequal, and the tse accordingly differed essentially from the Chaldaean and Greek signs.
Here the " signs " and the " constellations " of the lunar zodiac form two essentially distinct systems. The ecliptic is divided into twenty-seven equal parts, called bhogas or arcs, of Boo' each.
They were called the " media of the whole circle of the zodiac "; 11 each ten-day period of the Egyptian year was consecrated to the decanal god whose section of the ecliptic rose at its commencement; the body was correspondingly apportioned, and disease was cured by invoking the zodiacal regent of the part affected.
Among his happy conjectures may be mentioned that of the sun's axial rotation, postulated by him as the physical cause of the revolutions of the planets, and soon after confirmed by the discovery of sun-spots; the suggestion of a periodical variation in the obliquity of the ecliptic; and the explanation as a solar atmospheric effect of the radiance observed to surround the totally eclipsed sun.
Each solstice is upon the ecliptic midway between the equinoxes, and therefore go from each.
The ecliptic, and since it is equal to the ratio of the velocity of light to the velocity of the earth, it is necessarily constant.
The "ways" appear in this instance to have been the designation of the ecliptic circle, which was divided into three sections or zones - a northern, a middle and a southern zone, Anu being assigned to the first, Bel to the second, and Ea to the third zone.
Jacques Cassini also proved that Arcturus had even since the time of Tycho Brahe shifted five minutes in latitude; for r l Bootis, which would have shared in the change, if it had been due to a motion of the ecliptic, had not moved appreciably.
Since the sun moves in the ecliptic, it is in the last-named sense the point of intersection of the ecliptic and the celestial equator.
He taught, if he did not discover, the obliquity of the ecliptic, is said to have introduced into Greece the gnomon (for determining the solstices) and the sundial, and to have invented some kind of geographical map. But his reputation is due mainly to his work on nature, few words of which remain.
ECLIPTIC, in astronomy.
The plane of the ecliptic is that plane in or near which the centre of gravity of the earth and moon.
The ecliptic itself is the great circle in which this plane meets the celestial sphere.
Owing to the action of the moon on the earth, as it performs its monthly revolution in an orbit slightly inclined to the ecliptic, the centre of the earth itself deviates from the plane of the ecliptic in a period equal to that of the nodal revolution of the moon.
Owing to the action of the planets, especially Venus and Jupiter, on the earth, the centre of gravity of the earth and moon deviates by a yet minuter amount, generally one or two tenths of a second, from the plane of the ecliptic proper.
Owing to the action of the planets, the position of the ecliptic is subject to a slow secular variation amounting, during our time, to nearly 47" per century.
The obliquity of the ecliptic is the angle which its plane makes with that of the equator.
The motion of the ecliptic produces a secular variation in the obliquity which is now diminishing by an amount nearly equal to the entire motion of the ecliptic itself.
The laws of motion of the ecliptic and equator are stated in the article Precession Of The Equinoxes.
Attempts have been made by Laplace and his successors to fix certain limits within which the obliquity of the ecliptic shall always be confined.
The formula for the obliquity of the ecliptic, as derived from the laws of motion of it and of the equator, may be developed in a series.
Finally there is the envelope of still vaster extent and of unknown constitution which gives the zodiacal light; its greatest extent is along the ecliptic, but it can also be certainly traced for 35° in a perpendicular direction.
Equator; inclination to ecliptic: 7° 15'.
Gunter's Quadrant, an instrument made of wood, brass or other substance, containing a kind of stereographic projection of the sphere on the plane of the equinoctial, the eye being supposed to be placed in one of the poles, so that the tropic, ecliptic, and horizon form the arcs of circles, but the hour circles are other curves, drawn by means of several altitudes of the sun for some particular latitude every year.
(3) In the system of heliocentric co-ordinates, the plane in which the earth moves round the sun, which is the plane of the ecliptic, is taken as the fundamental one.
The axis of the ecliptic is a line perpendicular to this plane.
In consequence of this motion the sun appears to us to describe annually a great circle, called the ecliptic, round the celestial sphere, among the stars, with a nearly uniform motion, of somewhat less than 1 0 in a day.
The ecliptic intersects the celestial equator at two opposite points, the equinoxes, at an angle of 23° 27'.
The vernal equinox is taken as the initial point on the sphere from which co-ordinates are measured in the equatorial and ecliptic systems. Referring to fig.
Latitudinal Co-ordinate; Latitude or Ecliptic Polar Distance.
In other words the equator would be drawn into coincidence with the ecliptic. Here, however, the same action comes into play, which keeps a rotating top from falling over.
Actually the curve in which it moves is nearly a circle; but the distance varies slightly owing to the minute secular variation in the position of the ecliptic, caused by the action of the planets.
The rate of motion is slightly variable from century to century owing to the secular motion of the plane of the ecliptic. Its period, with the present rate of motion, would be about 26,000 years, but the actual period is slightly indeterminate from the cause just mentioned.
If we conceive a pole to each of these orbits, determined by the points in which lines perpendicular to their planes intersect the celestial sphere, the pole of the satellite orbit will revolve around the pole of the planetary orbit precisely as the pole of the earth does around the pole of the ecliptic, the inclination of the two orbits remaining unchanged.
In or near Iioo B.C., Chou Kung, an able mathematician, determined with surprising accuracy the obliquity of the ecliptic; but his attempts to estimate the sun's distance failed hopelessly as being grounded on belief in the flatness of the earth.
From of old, in China, circles were divided into 3654 parts, so that the sun described daily one Chinese degree; and the equator began to be employed as a line of reference, concurrently with the ecliptic, probably in the second century B.C. Both circles, too, were marked by star-groups more or less clearly designated and defined.
Their sidereal year was 4zm too long, s and they kept the ecliptic stationary among the stars, making no allowance for the shifting of the equinoxes.
540-510 B.C.) learned on his travels in Egypt and the East to identify the morning and evening stars, to recognize the obliquity of the ecliptic, goras.
He invented, or improved armillary spheres, the chief implements of ancient astrometry, determined the obliquity of the ecliptic at 23° 51' (a value 5' too great), and introduced an effective mode of arc-measurement.
Conceiving the plane of the earth's motion, which is that of the ecliptic, to be represented by the surface of the paper, the orbit of the moon makes a small angle of a little more than 5° with this plane.
Conceiving the line NN' to be that of the nodes at any time, and the earth and lunar orbit to be moving in the direction of the straight arrows, the earth will be on one side of the ecliptic from M2 to M5, and on the other side from M6 to M 1, intersecting it at the nodes.
This excess is, however, subject to wide variation, owing to the obliquity of the ecliptic and of the lunar orbit to the equator, and therefore to the horizon.
The inclination to the ecliptic is a little more than 5°, and the line of nodes performs a revolution in the retrograde direction in 18.6 years.
Euler conceived the idea of starting with a preliminary solution of the problem in which the orbit of the moon should be supposed to lie in the ecliptic, and to have no eccentricity, while that of the sun was circular.
These appearances he referred with great acuteness to the slight inclination of the sun's axis of rotation to the plane of the ecliptic. Thus, when the earth finds herself in the plane of the sun's equator, which occurs at two opposite points of her orbit, the spots, travelling in circles parallel with that plane, necessarily appear to describe right lines; but when the earth is above or below the equatorial level, the paths of the spots open out into curves turned downwards or upwards, according to the direction in which they are seen.
Of the meridian circle, which gives 23° 51' " for the obliquity of the ecliptic. His astronomical poem Hermes began apparently with the birth and exploits of Hermes, then passed to the legend of his having ordered the heavens, the zones and the stars, and gave a history of the latter.
The theory of the ecliptic as representing the course of the sun through the year, divided among twelve constellations with a measurement of 30 to each division, is also of Babylonian origin, as has now been definitely proved; but it does not appear to have been perfected until after the fall of the Babylonian empire in 539 B.C. Similarly, the other accomplishments of Babylonian astronomers, such as their system or rather systems of moon calculations and the drawing up of planetary tablets, belong to this late period, so that the golden age of Babylonian astronomy belongs not to the remote past, as was until recently supposed, but to the Seleucid period, i.e.
See, whose result for the position of the invariable plane is inclination to ecliptic I° 35' 7" 74, longitude of node on ecliptic 106° 8', 46" 7 (Eq.
Trepidare, to tremble), a term meaning, in general, fear or trembling, but used technically in astronomy for an imagined slow oscillation of the ecliptic, having a period of 7000 years, introduced by the Arabian astronomers to explain a supposed variation in the precession of the equinoxes.
The orbital planes of earth and moon are inclined to each other at an angle of 50.8 ° and at two points only in its orbit can the moon be situated in the plane of the ecliptic: the line joining these two points is called the "line of nodes."
Each discovery in turn was, according to the prevailing custom, announced to the learned world under the veil of an anagram - removed, in the case of the first, by the publication, early in 1656, of the little tract De Saturni luna observatio nova; but retained, as regards the second, until 1659, when in the Systema Saturnium the varying appearances of the so-called "triple planet" were clearly explained as the phases of a ring inclined at an angle of 28° to the ecliptic. Huygens was also in 1656 the first effective observer of the Orion nebula; he delineated the bright region still known by his name, and detected the multiple character of its nuclear star.