Gershom Bradford II

“ The planets and moon join the sun and stars in the daily parade past our meridian, but their apparent movements are not entirely an illusion, for they have motions of their own that somewhat affect the precision of their daily revolution. ”

“ The movement of the planets eastward or westward as described, constantly changes their right ascension; and as their orbits are inclined at different angles to the celestial equator, they are always changing their declination.
The planets whose orbits are smaller than that of the earth are called inferior, while those whose orbits are of greater dimensions are known as superior planets. Mercury and Venus are inferior planets and consequently are always nearer the sun; their comparatively close proximity making them appear to us as morning and evening stars. ”

“ Traverse sailing is a series of plane sailing courses made, for instance, by a sailing vessel beating to windward.
In parallel sailing the vessel pursues a true E. or W. course and runs along a parallel of latitude. Thus all her progress is in the terms of departure with no difference of latitude. As all meridians converge from the equator towards the poles the length in miles of a degree of of longitude keeps on diminishing as the poles are approached and, conversely, miles of departure have an increasing value in degrees of longitude. ”

“ In practical navigation a vessel commences her voyage and attempts to sail in a certain direction, but the well-known elements of compass error, variation and deviation, current, wind, seas and poor steering all divert the vessel from the projected course. In dead reckoning a navigator strives to keep track of his position by keeping a record of actual courses steered and distances run. ”

“ The rational horizon is marked by a plane, perpendicular to a plumb-line and passing through the earth’s center; while the sensible horizon is determined by a plane, also perpendicular to the plumb-line, but passing through the eye of the observer. ”

“ In fact, Mercury is so close that it is unavailable, owing to the brilliancy of the sun, for observation with a sextant; while Venus, on the other hand, a little more remote, is an excellent body to observe, and is always found in the east or west, conveniently near the prime vertical, the most favorable place for a time sight for longitude. ”

“ Middle latitude sailing is very similar to parallel sailing in that it is assumed, for the purpose of getting the difference of longitude, that the whole departure of the course or courses sailed has been made in the mean or middle latitude, because the greater value (in the northern hemisphere) in difference of longitude of a mile northward of the middle latitude is counteracted by the corresponding lesser value southward of the middle latitude. ”

“ It is necessary, in order to conveniently define the position of heavenly bodies to mark this celestial sphere with imaginary circles to serve as coordinates, as we mark the earth with meridians of longitude and parallels of latitude. Before going into the explanation of these coordinates, it may be well to consider a few facts concerning circles. A great circle is of course understood to be one whose plane passes through the center of a sphere, dividing it into two equal parts. ”

“ The vertical circle passing through the east and west points is called the prime vertical, and has a distinction above other vertical circles by virtue of its being the most favorable position for a body in observations for longitude. The heavens are further swept by an infinite number of parallels of altitude which are, as their name implies, parallel to the horizon. ”

“ The sun is the center of the solar system, with all the planets, including the earth, revolving around it, some with orbits greater and others less than that of the earth. The planets in some instances have satellites revolving in turn around them, as the moon in the case of the earth.
The movements of the earth will be, perhaps, more readily understood by assuming a position at the North Pole; here beneath the observer the earth is rotating lefthanded—against the hands of a watch, once each day ”

“ The positions of heavenly bodies are determined by two measurements—coordinates—-the distance north or south of the celestial equator, called the declination, and the distance east from the prime celestial meridian taken as a reference, called the right ascension, each of which will be subsequently treated at length under its individual heading. ”

“ We (the earth) , are at the Perihelion about January x, and consequently this angle at that date is 0°, but from this time on, the angle increases approximately one degree a day throughout the year.
he plane of the earth’s equator makes at all times an angle of about 23° 28’ with the plane of its orbit. This is a highly important angle to mankind, for upon it depends the climate of the world. The axis of the earth, if we can conceive it as represented by a slender imaginary staff, extends through the unlimited distance to a point in the heavens—the celestial pole ”

“ Her actual revolution around the earth, being from west to east, is contrary to the apparent diurnal motion and thereby each evening finds her farther to the eastward and consequently rising later. As a result of this change in the time of rising, the moon must of necessity rise in every hour of both day and night in the course of a month—the time she requires to revolve around the earth. ”

“ They all revolve around the sun in the same direction as the earth, but as their orbits are of vastly different sizes, they will be found in various positions relative to the sun; they overhaul and pass each other, but owing to their uniform direction of revolution they never meet. ”