How Large Is the Sun?
Theme: The sun displays God’s majesty.
Bible Verse: Then God made two great lights: the greater light to rule the day, and the lesser light to rule the night. He made the stars also. (Genesis 1:16)
Materials Needed:
• Ruler with millimeter markings
• Calculator
• Paper clip or straight pin
• Several index cards
Bible Lesson
The creation week was filled with miracle upon miracle as God formed the physical universe. On the fourth day the sun, moon, and stars were set in place. At this time the heavens were filled with a great variety of light sources. The last part of Genesis 1:16 is especially majestic, “He made the stars also.” These stars now are known to number in the billions of trillions. In fact, they comprise more than 99.9 percent of the physical creation. The average nighttime star is as large and bright as the sun. In other words, the sun itself is a star. It dominates our sky and our lives simply because it is much closer to us than the many remote evening stars.
Consider the energy available in our sun. Solar energy is thought to be produced by nuclear fusion reactions. The result is continuous nuclear energy production on a scale vastly greater than all of the power plants on earth combined. In fact, every second the sun gives off much more energy than mankind has produced since the beginning of time. And the sun never stops shining; it is always sunrise somewhere on the earth. We benefit from this solar energy in the form of our pleasant morning light. The sun itself is not running out of energy in any perceptible way; its fuel gauge is still on “full.” The significance of Genesis 1:16 is clear: God has infinite amounts of energy available to himself, and he also placed unimaginable amounts in the sun and the stars. God is worthy of all our praise.
Science Activity
Participants will measure the actual size of the sun in a simple way. This activity must be done during the day and at a time when clouds do not hide the sun. First, a pinhole is punched in the center of an index card. This small hole can be made with a paper clip, pin, or pencil point. On a second index card, draw a small circle with a diameter of 2 millimeters, using a ruler and a pen or thin pencil. The circle should be near the center of the card and can be drawn freehand.
Now the punched card is held above the second card as shown in the illustration. Orient the cards in the direction of the sunlight, either while standing outdoors or inside near a window in the path of the sun’s rays. A small round image of the sun should appear on the lower card. The pinhole in the upper card acts somewhat like a lens to focus the sunlight. Now adjust the distance between the two cards until the sun’s image is roughly the same size as the 2-mm circle drawn on the lower card. While one person holds the cards steady, a second person then measures the cards’ separation, also in millimeters. Precision is not needed; let’s give this card separation distance the symbol x.
The actual diameter D of the sun can now be found by substituting the measured value of x into this formula,
If the x distance is in millimeters, the final answer for the sun’s diameter D will be in miles. As a check on the answer, the accurate values are
x = 215 mm (about 8.5 in.)
D = 864,000 miles (or 1.4 million km)
Participants will usually get a sun diameter within 10 to 20 percent of the correct value. Differences are mainly due to the difficulty in accurately producing the 2-mm image of the sun on the lower card.
Emphasize the participants’ accomplishment: they have measured the vast size of the sun using only a simple ruler! It may be worthwhile to show participants the origin of the formula used. It comes from the similar triangles shown in the figure. The large solar diameter, 864,000 miles, also is typical for many of the nighttime stars. This is a greater distance than many people travel in their entire lifetime. The creation is beyond our understanding, and truly it declares God’s great glory and also his great love for us.
Science Explanation
The formula used to calculate the sun’s diameter comes from a proportion based on similar triangles. From the figure,
Solving for D,
The sun-earth separation X averages 93,000,000 miles. If the image size d is adjusted to 2 millimeters, then
Sunlight passes through a small opening in the upper card, focusing as a small round image on the lower card. Note the narrow triangles with lengths X and x above and below the top card. The drawing is not to scale; the X distance actually is billions of times greater than x.
where x is measured in millimeters. Notice that the solar image size does not depend on the size of the pinhole but only on the distance between the cards.
One complicating factor is that the sun is about 2 percent closer to the earth during December and January, and 2 percent farther away during June and July. This variation in distance is due to the earth’s elliptical orbit. A slightly smaller sun diameter might therefore be measured in summer when the sun is more distant from earth, and a slightly larger diameter in winter when the sun is closer. This earth-sun distance variation has little effect on our weather. It is the earth’s tilt that causes our seasons, not the small change in earth-sun distance.
The pinhole measurement technique also can be used in the evening to measure the diameter of a bright full moon. The equation for moon diameter D with a 2-mm image is
The moon’s actual diameter is 2,160 miles (3,456 kilometers). For this correct answer, the card separation x is 221 millimeters, or about 8.7 inches. Our moon is about 4 times smaller than the earth and 400 times smaller than the sun.