Page:The Moon (Pickering).djvu/16

vii To indicate the portion of a lunar day at which an observation is made we must give the colongitude of the sunrise terminator. To make this a little more intelligible, we may apply it to the Earth and say that a given observation was made when the Sun was rising at longitude 70° W., measured on the equator. This statement clearly indicates a particular instant of time for the whole Earth and also for the whole universe. While this would not be a convenient method of expressing time for us, it would be entirely so for a lunar astronomer. From sunrise to sunset, for an object on the Moon, is about fifteen of our terrestrial days. As supplementary to the colongitude of the sunrise terminator, or as a substitute for it, we may state that a certain observation was made when the Sun had been shining on the given formation for so many terrestrial days. It is often convenient to give both these figures—thus on Plate B, facing page 42, under each figure is given the date of the observation, the number of terrestrial days that had elapsed since sunrise, and the colongitude of the sunrise terminator.

American and English astronomers when dealing with the Moon generally refer the time of their observations to the meridian of Greenwich. Thus, when it is stated that a certain observation was made at 14 h. 20 m. G. M. T. (Greenwich Mean Time), we must, if we wish to reduce it to Eastern Standard, subtract five hours, and we shall then find that the observation was made at twenty minutes past nine in the evening.

Most of the plates illustrating this volume, together with a portion of the text, are taken from the "Annals" of the Harvard College Observatory, Volumes XXXII. and LI. Much of this portion has, however, been rewritten, condensed, and expressed in a less technical and more popular form. Some of the illustrations and text here given are published by the courtesy of the Century Company, having first appeared in their magazine during the past two years. For the use of the illustrations representing the Moon as seen by the naked eye I am indebted to the officers of the Société Astronomique de France, these figures having first appeared in their Bulletin for 1900. The remainder, including several of the illustrations and a large part of the text, are entirely new. It should also be stated that in the establishment of the observing station in Jamaica and in the securing of the negatives I was most efficiently aided by my assistant, Mr. E. R. Cram.