And he was there with the Lord forty days and forty nights (Exod. 34:28). How did Moses know when it was day? When the Holy One, blessed be He, taught him the Written Law he knew it was the day time, and when He taught him the Oral Law, the Mishnah and the Talmud, he was aware that it was night time. This was so because day and night are alike in the presence of the Holy One, blessed be He, as is said: And the light dwelleth with Him (Dan. 2:22). And as it is also written elsewhere: Even the darkness is not too dark for Thee, but the night shineth as the day; the darkness even as the light (Ps. 139:12). In what other way did he distinguish between night and day? When he beheld the constellations kneeling and bowing (descending), he realized that it was day, and when he saw the sun bowing, he knew that it was night, as it is written: The host of heave worship Thee (Neh. 9:6).
וַיְהִי שָׁם עִם ה' אַרְבָּעִים יוֹם וְאַרְבָּעִים לַיְלָה. מִנַּיִן הָיָה יוֹדֵעַ מֹשֶׁה אֵימָתַי יוֹם. אֶלָּא כְּשֶׁהַקָּדוֹשׁ בָּרוּךְ הוּא הָיָה מְלַמְּדוֹ תּוֹרָה בִּכְתָב, הָיָה יוֹדֵעַ שֶׁהוּא יוֹם. וּכְשֶׁהָיָה מְלַמְּדוֹ עַל פֶּה מִשְׁנָה וְתַלְמוּד, הָיָה יוֹדֵעַ שֶׁהוּא לַיְלָה. לְפִי שֶׁהַיּוֹם וְלַיְלָה שָׁוִין לִפְנֵי הַקָּדוֹשׁ בָּרוּךְ הוּא, שֶׁנֶּאֱמַר: וּנְהוֹרָא עִמֵּהּ שְׁרֵא (דניאל ב, כב). וּכְתִיב: גַּם חֹשֶׁךְ לֹא יַחְשִׁיךְ מִמֶּךָּ וְלַיְלָה כַּיּוֹם יָאִיר כַּחֲשֵׁיכָה כָּאוֹרָה (תהלים קלט, יב). וְעוֹד מִנַּיִן הָיָה יוֹדֵעַ בֵּין יוֹם וּבֵין לָיְלָה. כְּשֶׁהָיָה רוֹאֶה הַמַּזָּלוֹת כּוֹרְעִין וּמִשְׁתַּחֲוִין, הָיָה יוֹדֵעַ שֶׁהוּא יוֹם. וּכְשֶׁהָיָה רוֹאֶה הַחַמָּה מִשְׁתַּחֲוָה, הָיָה יוֹדֵעַ שֶׁהוּא לָיְלָה. דִּכְתִיב: וּצְבָא הַשָּׁמַיִם לְךָ מִשְׁתַּחֲוִים (נחמיה ט, ו).
And he was there with the Lord forty days and forty nights (Exod. 34:28). How did Moses know when it was day? When the Holy One, blessed be He, taught him the Written Law he knew it was the day time, and when He taught him the Oral Law, the Mishnah and the Talmud, he was aware that it was night time. This was so because day and night are alike in the presence of the Holy One, blessed be He, as is said: And the light dwelleth with Him (Dan. 2:22). And as it is also written elsewhere: Even the darkness is not too dark for Thee, but the night shineth as the day; the darkness even as the light (Ps. 139:12). In what other way did he distinguish between night and day? When he beheld the constellations kneeling and bowing (descending), he realized that it was day, and when he saw the sun bowing, he knew that it was night, as it is written: The host of heave worship Thee (Neh. 9:6).