« السابقةمتابعة »
OF THE SUN'S RISING AND SETTING, RIGHT ASCENSION,
DECLINATION, AND EQUATION OF TIME.
Day. | Moon. Mercury. Venus.
Venus. Mars. Jupiter. Saturn. Uranus.
A meridian line, or a line, which, if produced, would pass through both the poles of the earth, and the spot where the observer is placed, is the basis of all astronomical observation, and has, from the very earliest ages of the science, been considered of great importance. The following method of drawing a meridian line, is the simplest that can be adopted.
On a card, or any other convenient superficies, describe two, three, or more concentric circles, and in the .common centre, fix perpendicularly a wire, CB, having a well-defined point; this card should be fixed on a perfectly horizontal plane. When the sun shines in the morning, observe where the shadow of the top of the wire, as CD, touches one of the circles; and in the afternoon, mark where the extremity of the shadow C F, just touches the same circle: then through the centre C, draw the line S N. bisecting the arc D F, and S N will be a meridian line, as required. If the same be done with as many of the circles, as the shining of the sun will admit of, and the mean of all the bisecting lines, be chosen as a meridian, there will be no doubt of its accuracy, particularly if the observations be made about midsummer, which is the best time. In the preceding figure, NS represent the nurthern and southern points of the horizontal plane ; if the line joining NS (the meridian line) be bisected at right angles, E will represent the eastern, and W the western points of the plane.
So important, even in the early ages of astronomy, was the construction of a meridian line, that pillars were erected, on the shadows of which at noon, were marked the varying altitudes of the sun. A pillar of this kind was erected by Eratosthenes at Alexandria. Pliny also