On this topic:
http://www.ou.org/index.php/jewish_action/article/54428/
MISCONCEPTION: Many religious Jews do not visit Har HaBayit (the Temple Mount) today. This is because we are all presumed to be in a state of tumat met (ritual impurity due to “contact” with the dead), and a tamei met is prohibited from ascending Har HaBayit. (Since the removal of tumat met requires the use of the ashes of a parah adumah, which are currently not available, every Jew is presumed to be in this state of impurity.)
FACT: Although individuals with certain forms of ritual impurities are barred from all of Har HaBayit, a tamei met may enter the peripheral areas of Har HaBayit surrounding the central holier region that included the Temple compound. Thus, although we currently lack the means to remove tumat met, this is not really a deterrent for ascending Har HaBayit. Those who refrain from ascending do so because of other halachic or political concerns or because of archeological uncertainties.1
BACKGROUND: Halachah recognizes different levels of kedushah (holiness) that relate to both time and place. Thus, Shabbat has more kedushah than yom tov, which in turn has more kedushah than chol hamoed. This is reflected, for example, in various differences among the holy days with regard to Havdalah texts and the number of aliyot in the Torah reading, as well as in the range of prohibited activities and the punishments associated with their violation. A similar hierarchy is relevant to the sanctity pertaining to space. The Mishnah (Keilim 1: 6-9), for example, delineates ten levels of spatial kedushah within the Land of Israel, which is holier than all other lands. The first three levels are the following: walled cities in Israel, which are holier than the rest of the Land; Jerusalem, which is holier than other walled cities; and the Temple Mount, which is holier than Jerusalem. The remaining seven levels are areas of increasing sanctity within the Temple precinct. The level of sanctity of an area is reflected in the kinds of mitzvot that may be performed there as well as in the laws regarding entry.
The sanctified areas in Jerusalem correspond to the Israelite desert encampment (Tosefta, Keilim 1:10; Sifri, Naso 11; Rambam, Beit Habechirah 7:11; see Aruch Hashulchan Ha’atid, Beit Hamikdash 14:17, 36:7). Three concentric levels of sanctity existed in the encampment: the innermost area called Machaneh Shechinah, the Divine Camp that contained the mishkan (Tabernacle); Machaneh Leviyah, the encampment of the Levites that surrounded Machaneh Shechinah, and Machaneh Yisrael, an area beyond Machaneh Leviyah where the rest of the Jews encamped. When the Jews settled the Land of Israel these “camps” were represented by the following sanctified areas: the Azarah (Temple Courtyard), which started at Sha’ar Nikanor (the Nikanor Gate) and included the Beit Hamikdash building and the altar (Machaneh Shechinah); Har HaBayit (Machaneh Leviyah) and the rest of Jerusalem (Machaneh Yisrael).
The verses in Bamidbar 5:2-4 that describe the laws pertaining to the desert encampment would seem to indicate that individuals with all types of tumah (ritual impurity) were removed from all three camps. However, Chazal explain (Sifri, Naso: 4; Pesachim 67a-68a; Rambam, Biat Mikdash 3:1-2) the specific rules:
At one extreme, a metzorah (one who is afflicted with tzara’at) is excluded from Machaneh Yisrael; thus, he is not permitted anywhere in Jerusalem. At the other extreme, a tamei met is barred from Machaneh Shechinah but is permitted within Machaneh Leviyah.2 Chazal derive this (Tosefta, Keilim 1:7; Pesachim 67a; Sotah 20b) from the fact that a corpse itself was brought into Machaneh Leviyah when Moshe, a Levite, transported Joseph’s bones from Egypt for burial in the Land of Israel (see Shemot 13:19; Nazir 45a; Rambam, Beit Habechirah 7:15 and Biat Mikdash 3:4). Thus, according to Biblical law, a tamei met may ascend Har HaBayit and proceed all the way up to the Azarah, until Sha’ar Nikanor. However, the rabbis added additional restrictions, and decreed that a tamei met may not go all the way to the Azarah but must stop at the Cheil,3 the same boundary that applied to a non-Jew.4 The Biblical prohibition of entering the Azarah and the Temple building itself for a tamei met incurs the severe punishment of karet (Bamidbar 19:13, 20; Makkot 14b; Rambam, Biat Mikdash 3:12-13 and Sefer Hamitzvot, negative 77, positive 31).
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