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Figure 35 shows the orbit of an inferior planet.
As we have already mentioned, the epicycle and deferent of such a planet
have the same elements as
the orbit of the planet in question around the sun, and the apparent orbit of the
sun around the earth, respectively. It follows that the epicycle and deferent of an inferior planet are, respectively, inclined and parallel to the ecliptic plane.
Let the plane of the epicycle cut the
ecliptic plane along the line . Here, is the point at which the
epicycle passes through the plane of the ecliptic from south to north, in the direction of the
mean planetary motion. This point is called the ascending node.
Note that the line must pass through the guidepoint, , since the
sun (which is coincident with the guidepoint) is common to the plane of the planetary orbit and the ecliptic plane.
Now, it follows from simple geometry that the elevation of the planet above the
guidepoint, , satisfies
, where is the length , the fixed inclination of the
planetary orbit (and, hence, of the epicycle) to the ecliptic plane, and the angle . The
angle is termed the argument of latitude. We can write (see Cha. 9)

(227) 
where is the mean argument of latitude, and the equation of center of the epicycle. Note that
increases uniformly in time: i.e.,

(228) 
Now, since the deferent is parallel to the ecliptic
plane, the elevation of the planet above the said plane is
the same as that of the planet above the guidepoint. Hence, from simple geometry, the ecliptic latitude of the planet satisfies

(229) 
where is the length , and
we have used the small angle approximation. However, it is apparent from Fig. 31 that

(230) 
where the length , and the
equation of the epicycle. But, according to the analysis in Cha. 9, , where is the planetary
major radius in units in which the major radius of the sun's apparent orbit
about the earth is unity, and is defined in Eq. (196).
Thus, we obtain

(231) 
where

(232) 
is termed the epicyclic latitude,
and

(233) 
the deferential latitude correction factor.
Figure 35:
Orbit of an inferior planet. Here, , , ,
, , and represent the earth, guidepoint,
planet, ascending node, descending node, and argument of latitude, respectively. View is from northern
ecliptic pole.

In the following, , , , , ,
, , , and are elements of the orbit of the planet in question
about the sun, and , , and are elements of the sun's apparent orbit
about the earth.
The requisite elements for all of the superior planets at the J2000 epoch (
JD)
are listed in Tables 30 and 66.
Employing a quadratic interpolation scheme to represent
(see Cha. 8), our procedure for determining the ecliptic latitude of a
superior planet is summed up by the following formuale:



(234) 



(235) 



(236) 



(237) 



(238) 



(239) 



(240) 



(241) 



(242) 



(243) 



(244) 



(245) 



(246) 



(247) 



(248) 
Here,
,
,
,
and
. The constants , , , and
for each of the inferior planets are listed in Table 44. Finally, the functions are tabulated in Table 45.
For the case of Venus, the above formulae are capable of matching NASA ephemeris data during the years 19952006 CE
with a mean error of and a maximum error of . For the case of Mercury, with the augmentations to the theory described in Cha. 9, the mean error is
and the maximum error .
Next: Venus
Up: Planetary Latitudes
Previous: Saturn
Richard Fitzpatrick
20100721