Simcha BabaevThe moch dachuk is a bedikah cloth a woman inserts before sunset and removes after nightfall, on the day she performed a hefsek taharah.
Neither the Mishnah nor the Gemara states an explicit obligation to perform a moch dachuk. But the Gemara describes something close to it, and the halachah grew out of that.
The background in the Gemara
The Mishnah in Masechet Niddah records a machloket about which day a woman may perform a hefsek taharah (as covered in an earlier article). The underlying question: from what point can we say a hefsek taharah genuinely proves the uterine bleeding has stopped?
The idea is that if the hefsek is performed too close to the onset of bleeding, too large a window remains between the hefsek and immersion, and bleeding may have occurred in that window without the woman's knowledge.
Rabbi Yehudah holds that a woman may perform a hefsek taharah only from the time of minchah on the seventh day from the onset of bleeding.
The Gemara challenges him: even if she performs a hefsek at midday on the seventh day, she may still bleed between the hefsek and sunset. Just as a morning hefsek leaves a window until sunset, so does a midday hefsek — a smaller window, but a window.
The Gemara frames the challenge this way: Rabbi Yehudah's position would make sense only in one case, if "yad'ah bein eineha kol bein hashemashot" (יָדְעָה בֵּין עֵינֶיהָ כָּל בֵּין הַשְּׁמָשׁוֹת) — if the woman held the bedikah cloth in place throughout bein hashemashot. The Gemara offers the idea as an absurdity: obviously a woman cannot examine herself that way.
The Rashba turns the absurdity into a solution
The Rashba takes that same idea and casts it as the moch dachuk. He says: there is indeed a window of time in which we want to confirm the woman had no bleeding, between sunset and nightfall. And it is in fact possible, through a moch dachuk: a bedikah cloth inserted before sunset and removed only after nightfall.
The Tur does not mention a moch dachuk at all. The Beit Yosef (author of the Shulchan Aruch) expresses astonishment that the Tur omitted the idea, and writes that the Rashba's proposal — that a woman place a moch dachuk from sunset to nightfall — is particularly precious in his eyes.
The ruling of the Shulchan Aruch — two different cases
The Shulchan Aruch distinguishes two cases.
1. A woman who bled for one day only
A woman who bled for one day only and wants to perform a hefsek taharah that same day must perform a moch dachuk. Without it she may not begin counting seven clean days the next day.
The Rema's novel ruling: a woman who bled for one day and omitted the moch may, b'dieved, begin counting seven clean days, provided the hefsek taharah was performed close to sunset. If the hefsek was performed in the morning, she may not begin the count. Whether the Shulchan Aruch agrees with the Rema here is unclear.
2. A woman who bled for two days or more
For a woman who bled for two days or more, the Shulchan Aruch describes the need for a moch in a distinctive way: "a person should always teach within his household that the woman... should have a moch dachuk, and it should remain there throughout bein hashemashot."
Whether the Shulchan Aruch regards the moch dachuk here as a full obligation is unclear. That is, if she omitted the moch, must she perform one the next day and postpone the seven clean days, or may she begin counting immediately? Most poskim, Rav Wosner and Chacham Ovadia Yosef among them, hold that the Shulchan Aruch regards the moch dachuk as a mitzvah but not as a precondition. If the woman did not perform a moch, she may still begin the count.
Practical halachah — how a moch dachuk is done
On the day a woman performs a hefsek taharah, she also performs a moch dachuk: she inserts a bedikah cloth and leaves it in place throughout bein hashemashot.
- If the hefsek was performed a few minutes before sunset, she can insert the moch immediately after the hefsek and remove it after nightfall.
- If the hefsek was performed earlier, she needs to remember to insert the moch a minute or two before sunset and remove it after nightfall.
Important for her health: the moch must not stay in place longer than required.
When it is difficult — do not trade away your health
- A woman who finds a moch difficult to perform should seek halachic guidance. She will usually be permitted to forgo it. A woman after childbirth is generally permitted not to perform one.
- A woman who forgot the moch, or even deliberately omitted it without guidance, may b'dieved still begin counting seven clean days.
- A woman who has to choose between forgoing the moch and leaving it in place longer than required — she knows she will be in the middle of a long bus ride during bein hashemashot with no way to remove it, say — should forgo the moch rather than harm her body.
Note: if the woman bled (rather than stained) for one day only, she is obligated to perform a moch. If she nonetheless did not, then as long as the hefsek taharah was performed after minchah ketanah, she may begin counting seven clean days the next day.
Bein hashemashot — exactly how long?
There is a machloket about the times of sunset and nightfall, and consequently differing views on the length of bein hashemashot and how long the moch must stay in place. Most Acharonim hold that bein hashemashot is between 13 and 18 minutes.
Unless you know your family follows a narrow view of bein hashemashot (such as the Rambam's or the Chazon Ish's), you can simply rely on the times printed in a local zmanim calendar or an online one such as the Chabad calendar.