Bar-Ilan University's Parashat Hashavua Study Center

Parashat Bereshit

Lectures on the weekly Torah reading by the faculty of Bar-Ilan University in Ramat Gan, Israel. A project of the Faculty of Jewish Studies, Paul and Helene Shulman Basic Jewish Studies Center, and the Office of the Campus Rabbi. Published on the Internet under the sponsorship of Bar-Ilan University's International Center for Jewish Identity.
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Parashat Bereishit 5760

(October 6, 1999)

"And God made the two great luminaries..."

by Prof. Moshe Kaveh

President, Bar-Ilan University

(Physics Dept.)

About two months ago, billions of people around the world observed the total solar eclipse. From certain places on our planet, the Moon was seen to completely block out the bright disk of the Sun. This dramatic visual effect could not help but enthrall every observer. Some viewers were impressed by the uniqueness of the event (which occurs only about once per century). Others were amazed by the scientific knowledge which permitted a precise prediction of the timing of the darkness as it moved across the globe. Yet others were filled with awe at the beauty produced by the natural laws laid down by the Creator.

No doubt, this intense worldwide focus of interest in the Sun and the Moon led to a series of questions about these heavenly bodies. Such questions are often asked by children in kindergarten, who generally receive the traditional answer of the kindergarten teacher: "When you grow up, then you will understand". Many of these children, now adults, are still awaiting understanding...

Let me list several "simple" questions:

1) How was the Sun formed?

2) When was the Sun formed?

3) Why is the color of the Sun yellow?

4) How and when was the Moon formed?

5) How does the "small luminary" manage to cover the "great luminary" during an eclipse?

When a child enters primary school, and finally begins to learn Rashi, he encounters Rashi's commentary on the verse "And God said: Let there be luminaries in the heavens" (Genesis 1:14). The Talmud (Hullin 60b) quotes Rabbi Shimon ben Pazi: "It is first written 'the two great luminaries,' and then it is written 'the great luminary and the small luminary.' How is this explained? The Moon said to the Almighty: 'Master of the World, is it possible for two kings to rule under one crown?' The Almighty replied: 'Go and diminish yourself' ".

Although this parable has an important moral lesson for all generations, it is also interesting to ask whether there is any relation between the Genesis text and science, or at least the science of the ancient world. The idea that the Earth was destined to revolve around the "two luminaries" generates great curiosity. Is such a thing possible?

Before providing answers for legions of kindergarten teachers and their pupils to the questions listed above, it is useful to survey briefly the history of the universe and the scientific theory of the formation of the Sun and the Moon.

The universe originated about 15 billion years ago in a violent explosion, popularly known as the "big bang". (See note 1 regarding a resolution between the Genesis time scale and the cosmological time scale). Immediately after the big bang, the entire universe was filled with radiation. Part of the radiant energy was immediately and spontaneously transformed into the elementary particles of matter (e.g., quarks, electrons, positrons), and after only one second (!), protons and neutrons were formed.

Three minutes after the "big bang", the first atomic nuclei were formed, primarily helium. At that time, the temperature of the universe was one billion degrees! Great drama!! The physics of electromagnetic waves, elementary particles, and nuclear physics, which constitute about 75% of the physics curriculum for the B.Sc. degree in every university, all originated within three minutes!

The rate of the subsequent development of the universe was greatly slowed. About

300,000 uneventful years were to pass after the "big bang" as the universe gradually cooled down. However, when the universe finally reached a temperature of about 6000 degrees, dramatic events again began to happen. The first signs appeared of the present-day structure of the universe, which consists of different materials.

These scientific findings aroused great excitement seven years ago, when the results of the COBE satellite reached the Earth. This satellite, launched into space in 1989 with the most sophisticated equipment on board, succeeded in measuring the radiation that reaches our planet from a distance of over 500 million light years. (One light year is the distance traveled by light in one year, nearly 10 trillion kilometers). The photographs taken by COBE showed the development of the universe starting from 300,000 years after its origin. How much time had to pass until the formation of atoms, the study of which constitutes the remaining 25% of the university physics curriculum? The first atoms were formed about 10 million years after the "big bang" (which is only a "short" time, in comparison with the 15-billion-year age of the universe).

When were the Earth, Sun, and Moon formed? Physicists have learned that the solar system came into being about 10 billion years after the "big bang", that is, about five billion years ago. The Sun and planets appeared first, but the Moon was not formed until about 50 million years after the planet Earth.

This concludes our brief summary of the history of the solar system.

It is clear that the Sun was formed before the planet Earth. How can this order be reconciled with the chronological order of Genesis, where the Sun is mentioned only on the Fourth Day of Creation, whereas the plants had already appeared on the Earth on the Third Day of Creation? It is obvious that green plants cannot exist without photosynthesis, which depends on sunlight. Rashi was apparently aware of this difficulty, and therefore, quite surprisingly, he changed the chronological order of Genesis regarding the appearance of the Sun.

On verse 10: "And God said: Let there be luminaries in the heavens," Rashi brings the words of the Talmud (Hagiga 12a) "The Sun was created on the First Day, but not commanded to hang in the heavens until the Fourth Day..." (Support for this Talmudic statement is given in note 2.) Thus, we see that the Talmud is in agreement with the scientific requirement that the Sun was formed before the Earth.

It is worth noting that the text of the Fourth Day first states that "there were luminaries in the heavens," and then suddenly changes topic with the words, "And God made the two great luminaries." Is there a difference between these two passages? Certainly! Our Sun is not a unique luminous object in the galaxy, which in fact consists of a great many "suns" at large distances from us. Indeed, our Sun is but one of the many stars that form our galaxy!

How many "suns" are there in our galaxy (called the Milky Way)? Scientists have discovered that the Milky Way consists of about 100 billion stars, that is, 100 billion "suns." Therefore, it seems to me there is a difference between verse 14 ("there were luminaries in the heavens") and verse 16 ("And God made the two great luminaries"). Verse 14 refers to the 100 billion luminaries (stars) of the galaxy, whereas verse 16 refers to the two luminaries that appear so bright from the Earth, namely, the Sun and the Moon.

This interpretation is in precise agreement with the words of Psalm 148 of King David: "Let the Sun and Moon praise Him; let all the stars of light praise Him." A distinction is being made between "the two luminaries" (Sun and Moon) and "all the stars of light," which refers to all the other "suns" in the galaxy. A similar distinction occurs in Psalm 136 between "the great lights" (verse 7), "the Sun to rule by day" (verse 8), and "the Moon and stars to rule by night" (verse 9). Each of these verses, which ends with the words "His mercy endures forever," refers to different heavenly bodies.

To complete the scientific description of "all the stars of light," we point out that stars whose mass is somewhat greater than that of the Sun, are not stable and tend to explode, whereas stars smaller than about 60 percent of the Sun's mass tend to eject their outer layers and become "white dwarfs," which no longer "shine." The Creator arranged the universe so that only stars of the size of our Sun are stable.

Let us now return to a question commonly asked by kindergarten children. Why is the color of the Sun yellow?

It turns outthat the scientist who first succeeded in answering this question correctly was awarded a Nobel Prize for his answer! In order to understand the answer, one must understand the source of the energy of the Sun. Many generations were puzzled by this question. Exactly what is burning in the Sun that generates such dazzling brightness and great heat? The physicist Hans Bethe calculated that if the solar fuel were petroleum, then the entire Sun would have burned out within 10,000 years, and the solar system would already be dark! After considerable advances in nuclear physics, Professor Bethe came to realize that the Sun must be "burning" nuclear fuel, through a process called a thermonuclear reaction. In this type of "burning," the Sun consumes hydrogen and forms helium as the "ashes." The lifetime of the Sun through this process is about 10 billion years.

The temperature of the outer layer of the Sun is about 6000 degrees. From this outer layer, sunlight streams forth in the form of electromagnetic waves. The color of the electromagnetic waves depends on the temperature of their source. Since the temperature of the surface of the Sun -- 6000 degrees -- corresponds to the color yellow in the spectrum, we see the Sun as bright yellow.

The Creator had arranged that the color of the Sun would fall in the visible region of the spectrum, thus enabling human beings to enjoy the sunlight. If, for example, the temperature of the surface of the Sun were several times hotter, then the "sunlight" would be primarily in the ultraviolet region of the spectrum, a "color" which the human eye cannot "see." The Sun would then appear black to us and completely invisible!

From the above discussion, one can understand the words "Let there be light!" (verse 3), as referring to the Creator arranging the laws of nuclear physics such that the temperature of the Sun has just the right value to cause its light to be in the visible region of the spectrum. This enables human beings to benefit from and enjoy the sunlight.

How was the Moon formed? Before 1970, there were three principal theories.

1) The material of the Moon was pulled from the Earth, as it rapidly rotated about its axis.

2) The Moon and Sun were formed together from the huge cloud of gases, which gathered in the vicinity of what would eventually become the solar system.

3) The Moon is an astronomical body that became "trapped" by the Earth when it happened to pass nearby.

Only in the middle 1970s did astronomers finally realize how the Moon was really formed. Most interesting is the fact that our Moon is unique, having been formed by a process quite different from that responsible for the formation of the many other moons in our solar system.

About 4.5 billion years ago, a planetary body about the size of Mars collided with the Earth. Enormous quantities of energy were released as result of this collision, causing the total destruction of the colliding body and the complete evaporation of the surface of the Earth. The Moon was formed shortly afterwards from a combination of the remnants of the colliding body and the debris of the Earth that was hurled into space.

Do any of these theories of lunar formation appear consistent with the talmudic statement in Tractate Hagiga, which speaks about the "diminishing" of the Moon? And had the Moon once been another "sun" previous to its diminution? To answer these questions, we must first understand the process of stellar formation.

A star is formed from the vast cloud of dust and gases that collects in certain regions of space. Such a cloud is called a "solar nebula." A solar nebula tends to rotate and shrink as its particles attract each other through the force of gravity. This "shrinking" continues until the nebula "collapses" to form a highly condensed gas. As the nebula collapses, it heats up, reaching a temperature so high that the hydrogen in the nebula is ignited. The thermonuclear reaction then begins and "a star is born."

As explained above, during the process of stellar formation, the nebula rotates. One of the principles of physics is that rotational motion cannot cease unless forces are applied. The rotational motion, called "angular momentum," must therefore continue during the collapse of the nebula. As a result, a newly-formed star will have considerable rotational motion.

However, the rotation of the star itself is usually not sufficient to account for the large amount of angular momentum that the shrinking nebula had produced. So what happens? In order to conserve the amount of angular momentum, a double-star system usually forms, consisting of two stars rotating about each other. Sometimes, even this is insufficient to account for all the angular momentum present in the initial nebula, and therefore, planets will also form and rotate about the double-star. The number of planets formed is determined by the amount of angular momentum available from the original solar nebula.

Is there any evidence available for this theory of the formation of double-star systems? An amazing piece of scientific evidence was discovered just a few weeks ago! Using an ultra-powerful telescope, a group of American and Australian astronomers discovered, for the first time, a Jupiter-sized planet rotating around a double star! The two stars were rotating about each other, separated by a distance of about 250 million kilometers! This discovery inspires one to proclaim, "How great are Thy works, O Lord!" (Psalms 92:6).

How many such double-star systems are there in the universe? It turns out that of the 100 billion stars in the Milky Way, most (about 60 percent) are members of double-star systems. Therefore, the idea of two stars together, first proposed by the Talmudic Sage, Rabbi Shimon ben Pazi, is not so fanciful after all! The most likely formation of a planetary system is one having a double "sun" in the center.

Moreover, these scientific discoveries enable one to understand the talmudic passage in tractate Hullin about the "conversation" between God and the Moon. Consider the following scenario. As the solar nebula was rotating, the Moon "asked" the Almighty not to form a double-star system, as usually occurs, because "Is it possible for two kings to rule under one crown?" Therefore, the Almighty formed only a single star from the solar nebula and used the excess rotational motion to form the nine planets that rotate around the Sun. If so, there was no possibility at all for the Moon to form as "the small luminary" around the Earth. Therefore, the Almighty brought in an external astronomical body to collide with the Earth and thereby form the Moon "to rule by night."

This explanation, based on modern physics, is also consistent with the words we recite

every Shabbat morning in the wonderful prayer that begins "God is the Lord of all creation..." This prayer contains the following praise of God: "He called to the Sun, and it shone; He saw and regulated the form of the Moon." That is, the Sun and the Moon were not formed together. Rather, first the Sun was formed, and only then, "He called to the Sun (from the solar nebula) and it shone." Subsequently, the Almighty "saw" that the night as dark, and therefore, "He...regulated the form of the Moon" (by means of the collision between the Earth and a large astronomical body).

In view of all this, how are we to understand the recent total solar eclipse, in which the Moon totally blocked out the solar disk? After all, the sun is 400 times larger than the moon that was 'regulated' later. Here enters another wonder of Divine creation. The astronomical body that collided with earth to form the moon that is 400 times smaller than the sun, formed the moon at a distance that is exactly 400 times closer to the earth than to the sun. This wondrous "accident" of nature is what causes the moon to appear to be exactly the same size as the sun to an observer on earth (called 'apparent size' by astronomers). In other words, the disk of the moon exactly covers the disk of the sun - being neither larger nor smaller. This "accident" of nature permits a total solar eclipse to , and is thus responsible for the dramatic visual effect that so enthralls every observer.

Such wonderful harmony in the laws of nature must surely inspire every person who contemplates the universe. This harmony finds marvelous expression in the Shabbat morning prayer, entitled "God is the Lord of all creation...", which contains the following words: "Glory and honor do they [the heavenly bodies] give to His name...All the hosts of the heavens give Him praise."

We join the heavenly host in praise of our Creator!


1. If we take into account the fact that the universe was initially compressed to an enormous density, then according to Einstein's general theory of relativity, all physical processes were greatly accelerated and "billions of years" can occur within a few seconds or minutes. See the detailed discussion of this point in Genesis and the Big Bang by Gerald Schroeder, who shows that 15 billion years can be the equivalent of the Six Days of Creation!

On the other hand, according to the tradition of the Talmudic Sages, there is no contradiction between the billion of years of the cosmological time scale and the Six Days of Creation, because the Genesis term yom (day) need not be understood as a literal 24-hour day. See the discussion of this point by Nathan Aviezer in his book In the Beginning, quoting Maimonides on this question (Guide for the Perplexed, Part II, Chapter 25). Still another approach is that of Leo Levi in Facing the Challenges of the Age, which challenges the cosmological assertion that the universe began from a beginning at time "zero." Perhaps, the universe began directly from a "later" state.

2. From the text of Genesis, one sees that the Torah skips almost all the physics involved in the development of the universe, and proceeds directly to the history of the planet Earth, which is of course the home of the receivers of the Torah. Already in the very first verse, "In the beginning, God created the heaven and the earth," the "earth" is mentioned. Thus, we see that the most important event of the First Day of Creation from the human point of view is already mentioned on verse 3: "And God said: Let there be light; and there was light." This requires one to understand the formation of the Sun in the way that is explained in Tractate Hagiga. Only with this explanation can one find meaning in the words of verse 5: "And there was evening and there was morning - one day." These words seem to refer to a time in which the Sun already exists, and there is already meaning to the "earthly" concepts of morning and evening. This analysis thus lends additional support to the statement in Hagiga that the Sun was formed on the First Day.

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