On a clear day when the sun glares brightly, people “cover” the sun with their hands to protect their eyes. From a great distance, some 150 million kilometres, the sun appears to be no bigger than a human hand. But it’s a known fact that the sun is definitely bigger than that. In a nutshell, this is what astronomers call the apparent diameter of the sun.
The apparent sizes of heavenly bodies like the sun can be approximately measured in various ways. One way of approximating the apparent diameter of the sun is by reflection. Using a ball covered with pieces of cut mirror and exposed to direct sunlight, the reflection of the sun on a perpendicular wall from a right distance can be used for this purpose.
Angular Diameter Formula
Another way is by an angular diameter formula. The angular diameter is derived by getting half of the shorter distance divided by the longer distance and then multiplied by 2 arc tan. Or, the apparent diameter of the sun = 2 arc tan (1/2 d/D). The answer is only around 32 arc minutes or 1,920 arc seconds. No wonder it’s easily “covered” with one hand. The apparent sizes of heavenly bodies make extra-terrestrial objects thousands of times smaller than they actually are. However, this is just a rough calculation. In reality, the sun is 1,391, 980 kilometers in diameter, or something like 1,300,000 earths could be fitted into it.
When the atmosphere allows for a direct view of the sunset, people see the golden yellow sun gradually disappearing in the horizon. It seems to hide behind mountains or buildings. Thus, children sometimes think it is smaller than mountains or buildings, or that the sea can easily swallow it. But in reality, Jupiter is more than 11 times bigger than the earth and the sun is more than 9 times bigger than Jupiter in diameter.
Hidden Behind a Thumb
During sunset, if a thumb is placed just in front of an eye (the other eye closed), with the sun behind it, it appears that this giant fireball can be hidden behind the thumb. It’s a result of optical illusion. It’s when the eyes are tricked into seeing things unrealistically. It’s also what makes the apparent diameter of the sun possible.
The apparent sizes of heavenly bodies are the effects of visual angles which are measured through the illusionary perspective projected by the object on the viewer from a great distance.