Astronomy: A Text Book |
Common terms and phrases
altitude angular appear astronomic astronomic units atmosphere axis binary bright lines called celestial equator celestial sphere circle color comets constellation curve dark density determined diameter direction disk distance diurnal motion Earth eclipse equal eyepiece faint galactic heavenly bodies horizon hour angle hydrogen Jupiter known latitude length lens light light-years longitude lunar magnitude Mars mass mean measured meridian Messier meteors miles Milky Moon Moon's Mount Wilson Mount Wilson Observatory move nearly nebula nebulosity node novæ objective Observatory observed orbit Orion parallax parallel perihelion period photographed planet planetary Plate polar pole position prism proper motions radial velocity radiation radius rays reflector refraction revolve right ascension rotation satellites seen shown sidereal slit Solar System spectra spectral type spectrograph spectroscope spectroscopic binaries spectrum spiral star's stars Sun's surface telescope temperature variable Venus vernal equinox visible wave-length Yerkes Observatory zenith
Popular passages
Page 207 - ... directly as the distance, or inversely as the square of the distance, or according to any other law, or even capriciously; and the law of areas proved above must still hold. This theorem illustrates one application — the only one that we shall attempt to give in detail — that Newton made of his laws of motion to the study of mechanics. He proved also that the converse is true: If a body be found moving in such a way that the line joining it to any point passes over equal areas in equal intervals...
Page 257 - On it place a globe, two feet in diameter; this will represent the Sun; Mercury will be represented by a grain of mustard seed, on the circumference of a circle 164 feet in diameter for its orbit; Venus a pea, on a circle 284 feet in diameter; the Earth also a pea, on a circle of 430 feet; Mars a rather large pin's head, on a circle of 654 feet...
Page 11 - This is a constant angular measurement equal to the angle subtended at the centre of any circle by an arc equal in length to the radius of the circle as shown in Figure 1.5. n radians = 180° 180 1 rad'an - " 180 371476 : 57° 17' 44
Page 257 - Pallas, grains of sand, in orbits of from 1000 to 1200 feet; Jupiter a moderate-sized orange, in a circle nearly half a mile across; Saturn a small orange, on a circle of four-fifths of a mile; Uranus a full-sized cherry, or small plum, upon the circumference of a circle more than a mile and a half, and Neptune a good-sized plum on a circle about two miles and a half in diameter.
Page 173 - ... polarities in the northern and southern hemispheres. "As the cycle progresses the mean latitude of the spots in each hemisphere steadily decreases, but their polarity remains unchanged. The high-latitude spots of the next ns-year cycle, which begin to develop more than a year before the last lowlatitude spots of the preceding cycle have ceased to appear, are of opposite magnetic polarity.
Page 63 - It should be noted that longitude is defined as a dihedral, or wedge angle, and that it is measured by the arc of either the terrestrial or the celestial equator included between the two planes. The astronomic latitude of a place is the angle between the plane of the equator and the direction of gravity at the place. The geocentric latitude is the angle between the plane of the equator and a straight line passing from the place to the center of the Earth. The difference between them is caused by...
Page 114 - ... luminary. At one of them the planet passes from the south to the north of the ecliptic, and at the other from the north to the south. 2625. Nodes, ascending and descending. — Those points, where the centre of a planet crosses the ecliptic, are called its NODES ; that at which it passes from south to north being called the ASCENDING NODE, and the other the DESCENDING NODE. While the planet passes from the ascending to the descending node, it is north of the ecliptic ; and while it passes from...
Page 257 - Venus a pea, on a circle of 284 feet in diameter ; the Earth also a pea, on a circle of 430 feet ; Mars a rather large pin's head, on a circle of 654 feet ; the Asteroids, grains of sand, in orbits of from 1000 to 1200 feet; Jupiter a moderate-sized orange, in a circle nearly half a mile across...
Page 80 - The shortest distance between two points on the surface of a sphere is the portion or arc which they include of the circle passing through both the points and the centre of the sphere. Such a circle is called a great circle...
Page 267 - Brahe," as Halley calls him), supposing that Kepler was right, "complained that his calculations did not agree perfectly with the matter of fact in the heavens," and became aware that the path of a comet was curved toward the Sun. In 1680 appeared a great comet whose orbit was shown by Doerfel and by Newton to be a parabola. Later, by using the gravitational principles developed by Newton, and "by a prodigious deal of calculation," Halley determined parabolic orbits for twenty-four bright comets...