Abbas Abdulrazaq Hashim Kareem

Department of English

“Measuring the Sun: From Ancient Astronomy to Modern Science”

1-Introduction 

If you search on Google for the volume of the Sun, you’ll find that it’s approximately 151×1018151 \times (1018) cubic kilometers. If you search for the Sun’s weight, you’ll find that it’s around 1.999×10301.999 \times 1030 kilograms. And if you look up the Sun’s composition, you’ll learn that it consists of approximately 94.9% hydrogen, 23.8% helium, and smaller amounts of other elements like oxygen (0.14%), carbon (0.03%), neon (0.02%), and iron (0.02%). But have you ever wondered how we’ve managed to determine the Sun’s size and composition with such precision, even though no one has ever reached or even come close to it? 

2- The size of the Sun

To calculate the Sun’s size and mass, we first need to determine the distance between the Earth and the Sun. Once we know that, we can figure out other details about the Sun. But how did we even calculate that distance in the first place?

The first person to answer this question was the Greek astronomer and mathematician Aristarchus of Samos, who lived between 310 BCE and 280 BCE. Aristarchus calculated the distance between the Earth and the Sun by first determining the distance between the Earth and the Moon and then measuring the Moon’s diameter using the phenomenon of a total lunar eclipse. Using these two pieces of information and trigonometry, he was able to estimate the distance between the Earth and the Sun.          Of course, his calculation was an approximation, but many scientists followed in his footsteps, including the Belgian astronomer Godfried, Johannes Kepler, the English astronomer Jeremiah Horrocks and his friend William Crabtree, Mikhail Lomonosov, and finally Edmond Halley. Each of these scientists conducted various experiments, each time arriving at a figure more accurate than the one before. This continued until we reached the current measurement: 159,597,870 kilometers. Now that we know the distance between the Earth and the Sun roughly 159 million kilometers—what do we do with it to calculate the Sun’s size and other properties? To determine the Sun’s size, we need to measure its diameter. And to do that, while staying right here on Earth, we don’t need to travel anywhere!

Scientists used the principles of proportionality between two similar triangles to calculate the Sun’s diameter. They directed sunlight through a small hole in an aluminum plate onto a white sheet of paper, creating an image of the Sun. By measuring the distance between the plate and the paper, the diameter of the reflected image, and using the known Earth-Sun distance, they solved for the Sun’s diameter, which was calculated to be 1,372,700 kilometers ( https://youtu.be/zioSpV2yq24).

3- The Sun’s composition

Let’s move on to the next point: the Sun’s composition. Now, even if we know the Sun’s size and diameter, how did we determine the elements and gases that make up the Sun? To understand this, we need to learn about the physical phenomenon known as spectroscopy.

When any element burns or emits light, its light, when passed through a prism, reveals a unique “fingerprint”—a set of spectral lines unique to that element. This is akin to human fingerprints, as no two elements have the same spectral signature. For example, if we take the element sodium and heat it up, then analyze the emitted light, we observe its spectral fingerprint: two bright yellow lines close together, with faint surrounding lines. This unique pattern only appears with sodium. Similarly, if we heat hydrogen gas and analyze its light, its spectral fingerprint shows as one bright red line accompanied by two faint blue lines. Oxygen’s spectral fingerprint has its own distinct pattern, and iron’s fingerprint has another unique appearance.

These spectral fingerprints enable us to identify the elements and gases present in the Sun by analyzing the light it emits.

Scientists, through experiments and research, were able to gather the spectral fingerprints of all the elements and gases present and compile them into a single table. This was the starting point that facilitated the identification of the elements that make up the sun. All the scientists did was use a spectroscope to analyze the white light coming from the sun. Then they began studying the patterns found in each spectrum individually. After that, they started comparing these patterns with the tables they had.

The result was that they observed a very large presence of hydrogen gas. When they compared its quantity with the other spectral lines, they found that hydrogen accounts for approximately 24.9%. They also identified helium gas, hydrogen gas, and the spectral lines of some other elements such as carbon, neon, and iron. In this way, they were able to determine the composition of the sun in detail.

4- The Sun’s temperature 

Interestingly, the spectroscopic method is not only used by scientists to identify the materials that make up distant stars, but it also allows them to determine the stars’ temperatures. For instance, if a star emits red light, it means the star’s temperature is not very high. If the light is yellowish, the temperature is higher. And if the light is blue or violet, the temperature is even higher. When they want to calculate the temperature accurately, they use Wien’s Displacement Law. This law states that the temperature of any object equals 2.898×10−32.898 \times divided by the wavelength of the spectrum or light emitted by the object. Therefore, once we know the wavelength of the spectrum or light emitted by a star, we can determine its temperature from Earth without needing to approach the star. Applying this principle to the sun, we find that the wavelength of the white light coming from the sun is, on average, approximately 493 nanometers.

When we plug this value into the equation, we find that the temperature of the sun is approximately 5,605 degrees Celsius. However, this is the temperature of the sun’s surface. The temperature of the core is estimated based on nuclear physics.

5- The mass of The Sun

Now, let’s move on to the final point, which is the mass of the sun. But before discussing how scientists calculated the sun’s mass, it’s important to clarify something: weight is a relative concept. Weight depends on the gravity in a given location. For instance, if an object weighs 100 kilograms on Earth, it would weigh about 16 kilograms on the moon because the moon’s gravity is one-sixth that of Earth’s. That’s why we can’t measure the weight of an object in space. However, we can determine the amount of matter it contains, or more precisely, its mass.

To determine the sun’s mass, scientists calculated its volume, identified the elements and components it is made of, and determined the proportion of each element. Since we already have prior knowledge of the density of these elements, using certain laws—such as Kepler’s laws of orbital mechanics and the laws of relativity—we can calculate the mass of the gases and elements that make up the sun’s volume. Thus, we can determine the total mass of the sun, which is estimated to be 1.9999 × 10³⁰ kilograms. The entire process is based on mathematical equations.