This figure is viewed from the sun looking towards the earth on March 21st—the vernal equinox.
Fig. 12.
In the autumn, there occurs an interesting phenomenon regarding the moon called the Harvest Moon. This is the time of unusually fine moonlight nights in which the moon rises for three or four evenings at about the same time instead of the usual rapid retardation. The time the sun or moon is above our horizon depends upon its declination and our latitude. As the sun moves northward in declination from March to June, our days lengthen by the sun rising earlier and farther in the northeast, and setting later and farther to the northwest. Similarly the moon in September is moving northward in declination very rapidly and would be rising earlier each evening were it not for its own eastward movement of revolution which causes her to slip eastward an average of 51 minutes daily and causes her later rising at night. The result is that these two influences at work almost counteract each other and cause the moon to rise at about the same time for several days giving us three or four glorious moonlight nights called the Harvest Moon.
It will be seen by the foregoing that great care must be exercised in having the time of observation accurately determined owing to those rapid movements of the moon. It is also a matter of great difficulty to correct the observed altitude of this body on account of numerous errors that become considerable in amounts due to her proximity to the earth. And for these reasons this body is not popular for observations with the general run of navigators. In the case of the semi-diameter, considerable error is apparent and is fully described, with parallax, which is excessive, under “Corrections for Observed Altitudes.”
CHAPTER XI
Charts
A difficulty was encountered when the early cartographers attempted to represent the earth’s spherical surface on a flat sheet. It can not be done, of course, without distortion being introduced in some manner. There are various methods of taking care of this error and one is adopted for one certain purpose while another scheme is used in some other work. These methods of caring for the error or distortion are known as projections, the principal being the Mercator, the gnomonic and polyconic. The Mercator projection is almost universally used for navigational purposes; the gnomonic projection facilitates the use of great circle sailing, and the polyconic is used for surveying sheets.
The Mercator chart represents the earth as though it were a cylinder instead of a sphere.
If we take the skin of one-half an orange, and assume it to represent the northern hemisphere of the earth, an attempt to forcibly bring it flat upon a table will result in the tearing or stretching of the skin. It can, however, be brought flat to the table in a regular uniform way by cutting it in a saw tooth fashion from the stem (pole) to the edge (equator), as shown in the diagram.
