Let us, then, imagine the ancient Egyptians, furnished with the natural astronomical circle which is provided whenever there is an extended plain, engaged in their worship at sunrise, praying to the "Lord of the two Horizons." The rising (and setting) of stars we will consider later; it is best to begin with those observations about which there is the least question. In the very early observations that were made in Egypt and Babylonia, when the sun was considered to be a god who every morning got into his boat and floated across space, there was no particular reason for considering the amplitude at which the supposed boat left or approached the horizon. But a few centuries showed that this rising or setting of the sun in widely varying amplitudes at different parts of the year depended upon a very definite law. We now, more fortunate than the early Egyptians, of course know exactly what this law is, and with a view of following their early attempts to grapple with the difficulties presented to them we must pass to the yearly path of the sun, in order to study the relation of the various points of the horizon occupied by the sun at different times in the year. Not many years ago Foucault gave us a means of demonstrating the fact that the earth rotates on its axis. We have also a perfect method of demonstrating that the earth not only rotates on its axis once a day, but that it moves round the sun once a year, an idea which was undreamt of by the ancients. As a pendulum shows us the rotation, so the determination of the aberration of light demonstrates for us the revolution of the earth round the sun. We have, then, the earth endowed with these two movements--a rotation on its axis in a day, and a revolution round the sun in a year. To see the full bearing of this on our present inquiry, we must for a time return to the globe or model of the earth. To determine the position of any place on the earth's surface we say that it is so many degrees distant from the equator, and also so many degrees distant from the longitude of Greenwich: we have two rectangular co-ordinates, latitude and longitude. When we conceive the earth's equator extended to the heavens, we have a means of determining the positions of stars in the heavens exactly similar to the means we have of determining the position of any place on the earth. We have already defined distance from the equator as north or south declination in the case of a star, as we have north latitude or south latitude in case of a place on the earth. With regard to the other co-ordinate, we can also say that the heavenly body whose place we are anxious to determine is at a certain distance from our first point of measurement, whatever that may be, along the celestial equator. Speaking of heavenly bodies, we call this distance right ascension; dealing with matters earthy, we measure from the meridian of Greenwich and call the distance longitude.