The שיר for שבת we know; but what’s this מוסף one: הזיו לך, ”Yours is the aura“?
There is no תהילים called הזיו לך; זיו is not a Hebrew word. It is Aramaic (appears multiple times in דניאל) and was borrowed into Mishnaic Hebrew.
And what does כדרך שחלוקים כאן mean?
The Leviim would sing one section of שירת האזינו each week for מוסף, and that is reflected in how we divide the עליות today (we have one more עליה that is the narrative after the song). But we do not say האזינו every שבת, the way we say מִזְמוֹר שִׁיר לְיוֹם הַשַּׁבָּת.
I can’t answer the Aruch HaShulchan’s question: why not? I will ask, why? What was the connection between שירת האזינו and שבת? This was inspired by Rav Moshe Eisemann’s Shiras Ha’azinu.
What was שירת האזינו?
It is a song—easily learned, למדה את בני ישראל שימה בפיהם—that tells בני ישראל about all the terrible things that will happen. There is something unique about it. There is a poignant midrash about Moshe’s last day:
There is a principle that נבואה is an intimate interaction with הקב״ה that cannot be shared unless the נביא is given the mission to share it:
Moshe and Yehoshua were in the משכן together and had a common experience, but they experienced it differently and thus expressed it differently. That means that שירת האזינו was not הקב״ה מדבר מגרונו של משה. We’ve talked many times about ספר דברים as Moshe’s תורה שבעל פה that was incorporated into תורה שבכתב; this was Moshe’s נבואה, for which Moshe to compose the words, that would be incorporated into תורה שבכתב.
What does that have to do with שבת? You could equally well ask, what does מִזְמוֹר שִׁיר לְיוֹם הַשַּׁבָּת have to do with שבת? The gemara says (בבא בתרא יד,ב, as understood by Rashi) that תהילים צ-ק were written by Moshe, including תהילים פרק צב:
It’s all about theodicy and ultimate justice; nothing about resting on Saturdays. There is an insight of Rav Yaakov Kaminetzky that I have quoted before:
There is an agaddah that Moshe invented the weekend:
I would say that מַתְּנַת חֶלְקוֹ isn’t the gift that he got; it is the gift that he was privileged to give.
And that gift was hope.
That was at the beginning of his career. Now, in פרשת האזינו, we are at the end of Moshe’s career, and ה׳ tells Moshe that everything is going to fall apart.
But ה׳ tells Moshe: your first gift to your people was the hope of a better future, embodied in שבת. Your last gift to the people will also be the hope of a better future.
And that was the song of שבת afternoon, לעתיד לבוא ליום שכולו שבת מנוחה לחיי העולמים.