6 U. S. COAST AND GEODETIC SURVEY 
celestial sphere are represented in figure 1. These circles intersect in 
six points, three of them being marked by symbols in the figure, 
namely, the vernal equinox T at the intersection of the celestial equator 
and ecliptic, the ascending lunar node & at the intersection cf the 
ecliptic and the projection of the moon’s orbit, and the lunar inter- 
section A at the intersection of the celestial equator and the projection 
of the moon’s orbit. For brevity these three points are sometimes 
called respectively “‘the equinox,” “the node,’”’ and “the intersection.” 
The vernal equinox, although subject to a slow westward motion of 
about 50’’ per year, is generally taken as a fixed point of reference for 
the motion of other parts of the solar system. The moon’s node has a 
westward motion of about 19° a year, which is sufficient to carry it 
entirely around a great circle in a little less than 19 years. 
20. The angle w between the ecliptic and the celestial equator is 
known as the obliquity of the ecliptic and has a nearly constant 
value of 233°. The angle 7 between the ecliptic and the plane 
of the moon’s orbit is also constant with a value of about 5°. 
FIGURE 1. 
The angle J which measures the inclination of the moon’s orbit to the 
celestial equator might appropriately be called the obliquity of the 
moon’s orbit. Its magnitude changes with the position of the moon’s 
node. When the moon’s ascending node coincides with the vernal 
equinox, the angle J equals the sum of w and 7, or about 281°, and when 
the descending node coincides with the vernal equinox, the angle 
I equals the difference between w and 7, or about 184°. This variation 
in the obliquity of the moon’s orbit with its period of approximately 
18.6 years introduces an important inequality in the tidal movement 
which must be taken into account. 
21. In the celestial sphere the terms “‘latitude’”’ and “longitude” 
apply especially to measurements referred to the ecliptic and vernal 
equinox, but the terms may with propriety also be applied to meas- 
urements referred to other great circles and origins, provided they 
are sufficiently well defined to prevent any ambiguity. For example, 
we may say “‘longitude in the moon’s orbit measured from the moon’s 
