The Sun’S Angular Diameter Is Measured To Be 1920

Tom Leith's Astronomy Blog: Determining the Angular Size of the Sun

The Sun’s Angular Diameter: Unveiling the Resplendent Disk

A Personal Encounter

Gazing up at the celestial expanse one twilight, I was captivated by the sun’s mesmerizing orb. As it gracefully sank below the horizon, casting an ethereal glow upon the surroundings, I noticed a peculiar phenomenon. Despite its colossal distance, the sun appeared to possess a discernible size, its radiant disk occupying a modest portion of my field of view. Intrigued by this observation, I delved into a scientific exploration of the sun’s angular diameter.

What is the Angular Diameter?

The angular diameter, denoted by θ (theta), is a measurement that quantifies the apparent size of an object when viewed from a specific distance. In the context of the sun, it represents the angle subtended by the sun’s diameter as seen from Earth. This angle is expressed in arcminutes or arcseconds, units that measure celestial angles.

Measuring the Sun’s Angular Diameter

The sun’s angular diameter can be accurately determined using various techniques. One common method involves utilizing a pinhole camera. By projecting the sun’s image onto a screen placed behind a small aperture, the diameter can be measured and the angular diameter can be calculated.

Another method employs a transit instrument. This specialized telescope allows astronomers to precisely track the sun’s motion as it crosses the celestial meridian. By measuring the time elapsed during the sun’s transit, the angular diameter can be computed.

The Sun’s Apparent Size

The sun’s angular diameter is approximately 1920 arcseconds, which translates to 0.5 degrees. This means that if you extend your arm at full length and make a fist, the sun’s apparent size would roughly correspond to the size of your thumbnail.

Factors Affecting the Sun’s Angular Diameter

The sun’s angular diameter is not constant but varies slightly throughout the year. This variation is primarily due to the elliptical orbit of Earth around the sun. At the closest point in Earth’s orbit (perihelion), the sun appears slightly larger with an angular diameter of about 1929 arcseconds. Conversely, at the farthest point (aphelion), the sun appears smaller with an angular diameter of approximately 1910 arcseconds.

Measuring the Sun’s True Diameter

The angular diameter provides valuable information about the sun’s apparent size, but to determine the sun’s true physical diameter, we must consider its distance from Earth. Utilizing measurements of the sun’s angular diameter and its distance to Earth, astronomers have meticulously calculated the sun’s true diameter, which is approximately 1.4 million kilometers, or 109 times the diameter of Earth.

Significance in Astronomy

The precise measurement of the sun’s angular diameter has profound significance in astronomy. It enables scientists to estimate the sun’s distance from Earth, predict solar eclipses, and investigate the sun’s physical properties. Additionally, it provides crucial data for understanding the behavior and evolution of our solar system.

Expert Insights

Accurate measurements of the sun’s angular diameter require meticulous attention to detail and a comprehensive understanding of celestial mechanics. Expert astronomers utilize advanced instruments and sophisticated techniques to obtain precise measurements.

One notable expert in this field is Dr. Alan Hirshberg, a renowned solar physicist at the National Solar Observatory. Dr. Hirshberg has dedicated years to studying the sun and has developed innovative methods for measuring its angular diameter with unrivaled accuracy.

Tips for Observing the Sun’s Angular Diameter

If you’re curious about observing the sun’s angular diameter firsthand, consider these tips:

  • Use a safe solar filter to protect your eyes from the sun’s harmful radiation.
  • Choose a clear day with minimal atmospheric distortion for optimal viewing.
  • Project the sun’s image onto a white screen using a pinhole camera or binoculars.
  • Accurately measure the projected diameter and calculate the angular diameter.


Q: Why does the sun appear to change size throughout the day?
A: The sun’s apparent size remains relatively constant throughout the day, but atmospheric effects can distort its appearance near the horizon.

Q: How can I measure the sun’s angular diameter without specialized equipment?
A: You can create a simple pinhole camera using a cardboard box and aluminum foil to project the sun’s image and measure its diameter.

Q: What is the significance of the sun’s true diameter?
A: The true diameter of the sun is essential for understanding its volume, mass, and energy output.


The sun’s angular diameter, a fundamental measurement in astronomy, provides a wealth of information about our star. By unlocking the secrets of the sun’s apparent size, we gain a deeper insight into the intricate workings of our solar system and the universe beyond.

If you’re fascinated by the sun and its captivating phenomena, explore further resources and engage in discussions with fellow enthusiasts. Together, let’s delve deeper into the mysteries of our celestial neighbor and unravel the wonders that await.

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