Midrash & the Making of the Moon

Having just read (this past shabbat) Parshat Hachodesh on Rosh Chodesh, when we also say the Psalm “Barchi Nafshi” (Psalms 104), this is an ideal time to discuss a midrash connecting the two – with some interesting insights into the ever-popular topic of Genesis and Cosmology as well.

Shemot Rabbah 15:22:

החדש הזה לכם – הדא הוא דכתיב (תהלים קד, יט): עשה ירח למועדים שמש ידע מבואו. הרבה מעשים כתב משה בתורה סתומים, עמד דוד ופירשם. אנו מוצאין ממעשה בראשית, משברא שמים וארץ ברא האור, שנאמר (בראשית א, א): בראשית ברא אלהים ואחר כך      ( שם שם, ג): ויאמר אלהים יהי אור. ודוד פרשו מאחר שברא אור, ברא שמים, שנאמר (תהלים שם, ב): עוטה אור כשלמה. והדר נוטה שמים כיריעה, הרי למדנו, משברא אור, ברא שמים.

“This month is for you” This is what is written (Psalms 104:19) “He made the Moon for festivals [lit. ‘times’]; the Sun knows its coming [or ‘setting’]” There are many things [lit. deeds] Moses wrote in the Torah concealed, and David arose and explained them. We find from the Creation [story, i.e. Genesis], that after the heavens and earth were created, He created light, as it says “In the beginning God created…” (Genesis 1:1) and afterwards (1:3) “And God said ‘Let there be light’”. But David explained that after He created light, He created the heavens, as it says (Psalms 104:2) “Enrobed in light” and then “He spreads out the heavens like a curtain.” So we learn that after light was created, He created the heavens.

This section of Shemot Rabbah then continues to expound at length on this Psalm, and only towards the end returns to the initial verse in Exodus and connects it back to the “dibbur hamatchil” quoted from that verse. (This is actually a fairly common form of midrash, a “Petichta”.)

…אחר כך מה דוד אומר (שם שם, יט): עשה ירח למועדים

כך השמש נקרא גדול והלבנה נקראת קטן, לכך השמש נקרא גדול, שהוא גדול על הלבנה אחד עשר יום, לכך ברא הלבנה בשביל מועדות, שיהיו ישראל מרבין וממעטין כלבנה, ואינו רע לה בעבור תקנת המועדות, שכל השנה מונה לחמה…. וכל הימך לומר, שבשביל אלו המועדות עשה את הלבנה

  עמד דוד ופירש: עשה ירח למועדים

אמרו לו לדוד: עד שאנו במצרים נטלנו חדש של לבנה, הדא הוא דכתיב: החדש הזה לכם

Afterwards what did David say? (Ps. 104:19) “He created the Moon for festivals”…

… So the Sun is called “great” and the Moon “small”; the Sun is called “great” because it is greater than the Moon by eleven days [i.e. the solar year is eleven days longer than 12 lunar months]. Thus He created the Moon for festivals, that Israel should wax and wane like the Moon, and it is not bad for it because of the fixing of the festivals. For the whole year is counted by the Sun…

And it is thus said [ “וכל הימך”…] that for the sake of these festivals He made the Moon.

David stood and explained “He made the Moon for festivals.” They said to David: “Even while we were in Egypt, we took up the month of the Moon.” That is what is written “This month is for you”.

This is a very interesting and rich midrash. It seems to contain, among other themes, some thoughts about the likeness of Israel and the Moon; how that represents our national cyclic fate and the nation’s birth in Egypt being connected to those. However, here, I wish to focus on one particular aspect of this midrash that takes us back to a theme discussed in an earlier post on Genesis  and science. We discussed the creation of light as the first thing created in both the Torah and Cosmology. Also mentioned was the idea that the heavenly luminaries we see today are not that initial light created but were created later.

However, one thing that was neglected in discussing those luminaries was the order of their creation in Genesis which seems, on the face of it, to flat-out contradict astronomy. In Genesis, the heavenly luminaries were created on the fourth day, after the Earth/land and seas (and plants!) were created (or separated/delineated…) on the third day.

Modern astronomical knowledge of the evolution of the universe, galaxies, stars, and planets has the reverse order: After the Big Bang, stars and galaxies formed well before the Earth. Our Sun is roughly 4.5 billion years old. (How we know this is a topic for another time, but it is well established.) Our planet, and the other ones in our Solar System, formed roughly contemporaneously with the Sun and are about that old as well. However, the earliest generation(s) of stars (known as Population III stars; also to be discussed in future posts) and galaxies formed early in the universe’s history, roughly within a few hundred million years after the Big Bang.

The midrash we have just seen implies, based on Psalms, that the order of creation of things in Genesis is not always as clear as it might seem and is open to interpretation. If we stretch this idea, we can use the midrash’s example to understand that the cosmologically problematic order in Genesis may not be literally as presented.

Now, I admit that I am perhaps stretching the idea a good deal past the example in the midrash – which only explains (based on another biblical verse!) the question that might arise from the first verse about when the heavens were created, relative to light – but it is ultimately the same idea.

If you think that it is too much of a stretch, then here is another – perhaps more interesting – explanation, also involving the same midrash…

According to the prevailing theory, our Moon formed shortly after the Earth itself (see here) as a result of a large impact with a proto-planet roughly the size of Mars. The impact flung large amounts of Earth’s crustal material into orbit around the Earth, and the Moon coalesced from that material. This event would have left the Earth’s surface very hot, and evaporated away any water on the surface, if there was any.

Artist's depiction of the Moon's formation. (from Pearson, The Cosmic Perspective)
Artist’s depiction of the Moon’s formation. (from Pearson, The Cosmic Perspective)

The water currently found on Earth  (and other light “volatiles” that make up our atmosphere) was likely delivered to Earth after the Moon forming event in a period called the “late heavy bombardment”, during which greater than usual numbers of comets and asteroids impacted the Earth. That water stayed with us until today.

Thus even the Moon, the latest of the heavenly luminaries, formed before the seas and/or other terrestrial bodies of water! This too may seem problematic to reconcile.

One aspect of this midrash that it “brings to the table” is an idea helpful in explaining this problem.  In the midrash, David says that the Moon’s (whole?) purpose is for the establishment of the months, calendar and the festivals (ignoring nighttime illumination, which is also mentioned in Genesis…)

This idea is, at least in part, suggested by God’s own statement in Genesis 1:14-15:

(יד)  וַיֹּאמֶר אֱלֹהִים, יְהִי מְאֹרֹת בִּרְקִיעַ הַשָּׁמַיִם, לְהַבְדִּיל, בֵּין הַיּוֹם וּבֵין הַלָּיְלָה; וְהָיוּ לְאֹתֹת וּלְמוֹעֲדִים, וּלְיָמִים וְשָׁנִים  (טו)  וְהָיוּ לִמְאוֹרֹת בִּרְקִיעַ הַשָּׁמַיִם, לְהָאִיר עַל-הָאָרֶץ; וַיְהִי-כֵן.

14 And God said: ‘Let there be lights in the firmament of the heaven to divide the day from the night; and let them be for signs, and for seasons, and for days and years; 15 and let them be for lights in the firmament of the heaven to give light upon the earth.’ And it was so.

At least half of the stated purpose of the heavenly luminaries is to be “signs for seasons, days and years”. Thus one could make the argument – and I will – that their “creation” was only truly  “completed” when the Moon achieved its current orbital period of 29.5 days (approximately). That did not occur when the Moon first formed around 3.9 billion years ago, but considerably later in the history of the Solar System.

The Moon when it first formed was closer to the Earth than it is today (which is not surprising, considering that it formed partially from the Earth, as mentioned above), and so would have had a shorter orbital period in its orbit around the Earth.

What changed the Moon’s orbit? Tidal recession!

The Moon and tides on Earth are intricately connected. The Moon causes daily rising and falling of Earth’s seas (and more). As the Earth rotates on its axis, the Moon’s gravitational pull is slightly greater on the part of the Earth directly facing it, and less on the opposite side. This causes the well known effect of daily high and low tides. Due to the extra force of gravity (stronger at closer distances) felt by the water lined up with the Moon, the water level rises. On the opposite side of the Earth, at the same time, the Moon’s gravitational pull is weakest, and so the water “rises up” away from it. Conversely, the sides of the Earth at right angles to the line-up with the Moon experience the least effect from the Moon and are thus at low tide. (Note that this force is not strictly limited to water or other liquids. They just move more easily and quickly than solids, so the effect is only seen in fluids.)

Diagram of the Moon's differential gravitational pull on different sides (and center) of Earth. (from Pearson, The Cosmic Perspective)
Diagram of the Moon’s differential gravitational pull on different sides (and center) of Earth. (from Pearson, The Cosmic Perspective)

Note that tides are even higher when the Moon and Sun are lined up, as then the Sun’s pull increases the effect of the Moon’s pull along the same line. And the reverse; when the Moon, Earth and Sun form a 90 degrees angle, the Sun’s pull reduces the effect of the Moon’s a little bit. Thus New Moon and Full Moon tides tend to be the greatest every month.

Diagram of the Moon's and Sun's gravitational pull on Earth at different times of the (lunar) month. (from Pearson, The Cosmic Perspective)
Diagrams of the Moon’s and Sun’s gravitational pull on Earth at different times of the (lunar) month. (from Pearson, The Cosmic Perspective)

Now, here’s the even more amazing part: The tides that the Moon causes on Earth are pulling back on the Moon slightly! Since the Earth rotates faster than the Moon revolves around it (daily vs. monthly) and the deformation of the water is not instantaneous but lasts longer than the precise line-up, the tidal bulge that the Moon is raising on the side of the Earth facing it will “lead” the Moon slightly and exerts a pull back on the Moon! This extra gravitational pull is not aligned directly on the radius of the Moon’s orbit and thus pulls the Moon ever so slightly forward along its orbit and causes it to speed up. The increase in orbital speed causes the Moon-Earth orbital distance to increase, and thus the Moon is very gradually receding from the Earth! This is known as tidal recession. (The current rate is a couple of centimeters per year; but it is unlikely to have been a constant over time.)

Schematic diagram of Moon and tidal bulges on Earth. (from Pearson, Cosmic Perspective)
Schematic diagram of Moon and rotation of tidal bulges on Earth. (from Pearson, Cosmic Perspective)

And the reverse is also true: the Moon’s gravity is pulling the bulge back towards the line between the centers of the Moon and Earth, thus causing the Earth’s rotation to slow down! Note that this is conservation of angular momentum in action; angular momentum is transferred from the Earth’s rotation to the Moon’s orbital revolution, but the total of the combined system is conserved.

This process has been gradually ongoing for 3+ billion years. when it began, the Earth’s rotation would have been faster and a day considerably shorter – by some estimates only as little as 16 hours! (But quite possibly a bit closer to the modern value; exact calculations are very difficult as they also depend on the tidal dissipation rates which are greatly affected by the shapes of ancient bodies of water which cannot be precisely measured or calculated.)

So, not only was the orbital period of the Moon originally shorter, the length of a day on Earth was also shorter! Both of these cycles gradually increased to the present day values over the course of billions of years. Àll because of the existence of Earth’s seas; the tides that caused tidal recession require (liquid) seas to be possible!

Here, then, is another explanation for the order in Genesis. If the focus of the heavenly bodies is their use as timekeeping devices – as stated, עשה ירח למועדים שמש ידע מבואו – “He made the Moon for festivals/ ‘times’; the Sun knows its setting” –  then they were not “complete” in their creation until after the seas. This function of the heavenly luminaries was only “completed” in a more recent epoch when the Moon’s rotation reached 29.5 days and the Earth’s rotation 24 hours (approximately…) There was a need for seas to exist “first” in order to adjust the moon’s orbit and the length of a day, and thus make it into the “Moon for festivals” about which God later commanded the Israelites in Egypt.



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