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he second reason for the yearly variation of the Equation of Time has to do
with the fact that the Earth's equator is inclined to the plane of the Earth's
orbit around the Sun.
2. Inclination of the Ecliptic. Another element
enters the scene, causing the sundial to vary from the
clock. This effect is purely a geometrical one. The axis
of rotation of the Earth is not perpendicular to the plane
of its orbit around the Sun, but is tilted by an angle of
23½ o . So, as the Earth revolves around the Sun, the north
pole is tilted 23½ o toward the Sun on June 21, and 23½ o
away from the Sun on December 21, as illustrated in Fig. 3.
These are the dates of the summer and winter solstices as
recognized in the northern hemisphere. The result, as seen
from the northern hemisphere, is that the Sun crosses the sky at
noon much higher in June than in December, and if one were
to plot the path of the Sun during the year, as seen against
the background of the stars, it would appear as a line
crossing over the celestial equator on March 21 and September
21 the vernal and autumnal equinoxes. The annual apparent
path of the Sun against the background of the stars, called
the ecliptic, is shown in Fig. 4, along with the celestial
equator. The celestial equator is an imaginary line in the
sky directly above the Earth's equator. Thus, when the Sun
is on the celestial equator, it stands directly above the
Earth's equator We see that the path extends north and south
> of the equator by 23½ o.
Figure 3. Inclination of the Earth's axis with respect to the plane of its
orbit.
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