1) AGADAH: MOSHE, LIKE THE SUN; YEHOSHUA, LIKE THE MOON
The Gemara says that in the World to Come a person will experience great shame when he sees the reward that the other person receives. To demonstrate, the Gemara relates that the elders of the generation that left Mitzrayim commented, "The face of Moshe shines like the sun while the face of Yehoshua shines only like the moon!" The Gemara says, "Woe to one who bears such shame! Woe to one who bears such humiliation!"
What is meant by the analogy between Moshe and Yehoshua, and the sun and the moon? In order to attain a deeper understanding of this Agadah, one must first analyze the difference between Moshe's leadership and Yehoshua's leadership from the Torah's perspective.
(a) The Gemara in Sanhedrin (8a) says that Moshe told Yehoshua, "You will come with the Jewish people into the promised land (Devarim 31:7); the elders and you will lead the people together." Hash-m corrected him and instructed him to tell Yehoshua, "You will bring the Jewish people into the land which I have promised (Devarim 31:23); take a stick and beat the elders until they follow your orders! There can be only one leader in each generation; there cannot be a joint leadership." Yehoshua would not be able to lead the Jews properly if other elders or prophets would join him. The concept of joint leadership was foreign to him.
In fact, Yehoshua himself expressed this on another occasion. When Moshe appointed the seventy elders to their position of authority, he asked them to join him in front of the Mishkan. There he "kindled their spirits" and caused them to experience prophetic visions. Two others, Eldad and Meidad, prophesied without Moshe's help while they were still in the camp. When Yehoshua heard of this he came running to Moshe, calling for Moshe to put an end to the prophecy of Eldad and Meidad (Bamidbar 11:26-9). Yehoshua could not tolerate independent prophets declaring the word of Hash-m simultaneously. He therefore assumed that the two self-declared prophets were fabricating. Moshe Rabeinu, however, saw things differently. His reaction to Yehoshua was, "If only Hash-m would make all of His people into prophets by resting His spirit upon them!" As far as he was concerned, it was perfectly acceptable for numerous prophets to lead the people jointly. In fact, Hash-m explicitly told Moshe to appoint elders "so that they may bear the burden of the people together with you" (Bamidbar 11:17). This was probably why Moshe originally instructed Yehoshua to lead with the elders -- just as Moshe himself had done.
(b) Why, though, was Moshe Rabeinu different? Why was he exempt from the rule of "one leader per generation"? The RAMBAM describes the difference between Moshe's prophecy and that of all other prophets as follows: "All the other prophets received their prophecy through an angel. This is why they saw what they saw only as a metaphoric vision or a non-explicit statement (which needed to be interpreted before it was understood). Moshe, however, did not receive his prophecies through an angel.... He did not see a metaphoric vision, but rather Hash-m revealed His word to Moshe clearly.... [As a result of his unique closeness to the Divine word,] Moshe's face radiated light and he became as spiritual as the angels" (Hilchos Yesodei ha'Torah 7:6; see also chapter 7 of "Shemoneh Perakim.") This is also the meaning of the Gemara in Yevamos (49b) that states, "Moshe saw a clear reflection, while the others saw an unclear reflection" (see Insights there).
Normally, successful leadership requires that one person serve as the sole authority. This is because each leader has his own distinct personality, and the differences between the personalities of two leaders inevitably will be expressed in their leadership decisions. Even if the leaders base their decisions on Divine revelation (as was the case in the times of Yehoshua), they are still liable to have differences of opinion. As the Rambam explains, a prophet's vision is still open to interpretation. Since the prophet's own input is required, the interpretation of his prophecy inevitably will be affected by his own unique personality. This is why the Gemara says that "two prophets never prophesy with an identical phraseology" (Sanhedrin 89a). Even two prophets would not be perfectly compatible leaders, since their differing personalities would be reflected in their interpretation of their prophecies.
Moshe's leadership was different. He did not merely see a vision; he heard the actual words of Hash-m, which he was able to pass on to others. He kindled the prophetic spirit of the seventy elders "as one candle kindles another" (Bamidbar 11:17; see Rashi there). Since Moshe's prophecy was the source of the elders' prophecy, the elders would never reach a conclusion that was contrary to Moshe's own conclusion. All of their decisions would be unanimous. Similarly, on the occasion that Eldad and Meidad prophesied in the camp, they received the Divine word directly from Hash-m, due to the influence of Moshe. They did not simply see a vision, as did normal prophets. (This is why the two were able to prophesy with an identical phraseology, contrary to the rule cited above.) They, too, would be no threat to Moshe's leadership. Since they saw clearly the will of Hash-m, they could only agree to what Moshe, who experienced the exact same prophecy, would decide.
Yehoshua, on the other hand, was like any other prophet. He could not "bring down" prophecy for others; he only could receive his own personal prophecy which would be affected by his own personality. Therefore, he would have to lead single-handedly. For the same reason, Yehoshua did not believe that Eldad and Meidad were true prophets. He assumed that all other prophets were like himself and could not be granted prophecy independent of Moshe while Moshe was the leader.
This explains why Moshe told Yehoshua to lead with the elders. Moshe thought that his student Yehoshua would lead in the same manner that he himself led. He assumed that the elders could lead the generation together with Yehoshua, just as they had done with Moshe. Hash-m, however, knew that this was not the case. Yehoshua would not be able to "share" his perception with the elders, since he would perceive only a weak reflection of Hash-m's word. Hash-m therefore told Moshe, "He should take a stick and beat them until they obey!" Yehoshua must lead alone.
(c) Based on this, the Gemara's analogy between Moshe and the sun, and Yehoshua and the moon, is understood. The sun not only gives forth its own shine, it illuminates the other heavenly bodies as well. The moon and the planets, in contrast, shine only due to their reflection of the light of the sun. It is in this manner that Moshe resembled the sun and Yehoshua resembled the moon. Moshe was able to be a source of prophetic illumination to others, such as the seventy elders (which is what caused his facial radiance, as the Rambam mentions). Yehoshua, in contrast, could become illuminated with the Divine light only through a "reflection" of the original light ("an unclear reflection"). He could not illuminate others. (The Zohar (Shemos 215a) indeed explains that this is the meaning of the comparison between Moshe and Yehoshua and the sun and the moon.)
Perhaps it is for this reason that specifically the elders commented on Yehoshua's likeness to the moon. The comparison between Yehoshua and the moon was most evident to the elders. It was the elders who would suffer the consequences of Yehoshua's inferiority to Moshe. The elders who led the generation together with Yehoshua's master, Moshe, would not be able to lead together with Yehoshua. This is why it was specifically the elders who noted Yehoshua's likeness to the moon! (M. KORNFELD. See YA'AROS DEVASH of RAV YONASAN EIBESHITZ, Derush #11, and BIRCHAS AVRAHAM of RAV ABISH'L FRANKFURTER, Eruvin 63a.)
2) DOING "MESHICHAH" TO ACQUIRE AN ANIMAL
QUESTION: Rav and Shmuel argue with regard to how much a person must pull a boat in order to acquire it through the act of Meshichah. Rav says that one needs only to pull it a small amount. Shmuel says that one must pull the boat so that it moves entirely out of the place in which it was standing. The Gemara cites a Beraisa and suggests that Rav's view is supported by the opinion of the Tana Kama in the Beraisa. When the Tana Kama describes the way to acquire an animal through Meshichah, he states that when the animal moves a foreleg and a hind leg, it is considered a valid Meshichah even though the other two legs have not moved. This implies that the animal does not have to leave its original place entirely, but rather it suffices that it move a small amount. The Gemara responds that Shmuel might agree with the Tana Kama, because doing Meshichah to acquire an animal is different from doing Meshichah to acquire a boat. Once an animal has moved two legs, the other two legs certainly are going to follow, and thus as soon as it moves two legs it is considered as though it has moved all four legs.
How does the Gemara's explanation reconcile Shmuel's opinion with that of the Tana Kama? Although it is considered as though the animal has moved all four legs, even when the animal moves all four legs it is still partially in the place in which it was originally standing! It has not moved entirely out of its place, which Shmuel maintains is a necessary requirement for the Kinyan of Meshichah to be effective.
ANSWER: The RASHBA and RITVA explain that when an animal moves even a small amount by moving all four legs, it is comparable to a boat moving entirely out of its place. This also seems to be the intention of the RASHBAM and RABEINU GERSHOM (DH Idach).
The logic for this distinction seems to be that when a boat is pulled, it is constantly in contact with the water (or ground) underneath it. If it is partially in the place in which it was standing previously, then it has not yet been lifted off of its place. In contrast, the only part of an animal that rests on the ground is its hooves. As long as it moves each leg out of the place in which that leg was standing previously, it is considered Meshichah. According to this understanding, if the animal would place its hoof down only slightly forward such that the back part of the hoof is resting in the place that the front hoof was resting previously, it would not be a valid Meshichah.
Alternatively, when an animal lifts all four legs, there is a moment at which each leg is actually lifted off of the ground on which it stood. Therefore, it is considered as though it moves entirely from its place as soon as it places all four legs on different places than the places on which they stood previously. According to this understanding, if the animal would place its hoof down only slightly forward such that the back part of the hoof is resting in the place that the front hoof was resting previously, it would be a valid Meshichah.
According to both explanations, if a person physically drags an animal along the ground, he would have to drag it completely out of its resting place in order for the Meshichah to be valid, similar to the Meshichah required for a boat.