The earth’s graticule and Projections 113
graphic, the stereographic, the gnomonic,
the azimuthal equal-area, and the azi-
muthal equidistant projections. The appearance of these projections varies only in
the spacing of parallels (in the polar case). All can be constructed by geometric pro-
jection.
Orthographic Projection
If a light source is assumed to be at infinity, the light rays appear to be parallel. An
azimuthal projection produced in this way is called an orthographic projection. We
know that Ptolemy used this projection, which he called the “analemma,” for rep-
resenting the heavens, and may have developed it. The orthographic was also a fre-
quently used projection for early maps of the moon because when the moon is viewed
from
the earth, it appears to be on the orthographic projection (Figure 6.24).
Figure 6.25 shows that an orthographic projection cannot present more than one
hemisphere at a time. When centered on the pole, it has parallels that become closer
together as the equator is approached. Other than azimuths being correct from the
center and great circles passing through the center being straight lines, both of which
are true for all azimuthal projections, the orthographic has no outstanding properties
save appearance. The orthographic closely resembles the view we have when look-
ing at a globe, and therefore it is often used when one simply wants to create a good
visual appearance.
