The question is sometimes asked how near does the telescope bring the Moon. It has been shown[1] that with a fifteen-inch telescope and perfect seeing a round black dot measuring 0.1″ in diameter can be seen on a white disk. This means that the telescope will show an object having only 1-300 the diameter that can be seen with the naked eye. This would be equivalent to bringing the Moon within 1-300 of its real distance, or about 800 miles, and this is probably as near as it has ever been practically brought by any instrument. An investigation by another method, made at Arequipa, indicated that the least distance for a twelve-inch telescope was about 1,000 miles,[2] As we have just seen, our largest telescopes are not located where the seeing is perfect, but if they were moved to a tropical climate they might perhaps occasionally, under the most favourable possible conditions, bring the Moon to within 300 miles.
With perfect seeing the fifteen-inch telescope would show an absolutely black object on the Moon that measured only 600 feet in diameter. If the seeing were such as we usually have in the northern United States and Europe, it might have to be half a mile in diameter. If it were not black it would have to be still larger. Under perfect seeing, a black object a mile long would be seen if it were only 100 feet in breadth. Since an astronomical telescope turns all objects upside down, and since for northern observers most of the interesting objects in the heavens pass to the south of the zenith, it was the custom of the earlier astronomers to represent all astronomical objects, with the exception of star maps drawn on a small scale, with south at the top. This custom is still maintained, and has been followed in the present volume. The irregular line separating the bright from the dark portion of the Moon is called the terminator; the smooth edge of the Moon is called the limb. The large dark plains on the Moon visible to the naked eye are called the maria, from the Latin mare, a sea. The edge of the Moon toward the western horizon is called the preceding or western limb; the other edge is called the following or eastern limb. This, it will be noted, brings the points of the compass in the reverse order from what they appear upon a terrestrial map. Longitude upon the Moon is reckoned east and west from a central meridian, west longitude being usually considered positive and east longitude negative. For many purposes, however, it is more convenient to reckon longitude from the central meridian east round through 360°. This, to distinguish it from the other, is known as the colongitude.
