Buzz
It's simply an odd coincidence. The Sun's diameter is around 400 times larger than the Moon's, but it's also about 400 times farther from Earth. These two factors nearly balance each other out, making the Sun appear about the same size or slightly larger than the Moon from our perspective.
How can we determine the size and distance of the Sun and Moon?
Since we can't just wrap a massive tape measure around these celestial bodies, we need to think outside the box.
Imagine looking up at the sky and picturing it as a vast dome surrounding the Earth, with the Sun, Moon, and stars projected onto its surface. Astronomers refer to this concept as the celestial sphere. If we treat the sphere as a 360-degree circle, we can discuss the objects we see in terms of angular size (the 'visual diameter' measured as an angle) and angular distance (the separation between two objects measured as an angle).
Thanks to space exploration and advancements in laser and radar technology, we've gained a clearer understanding of the actual distances. For the Moon, Apollo astronauts left retro-reflectors on its surface, which we used to bounce lasers off from 1969 to 2009. By measuring how long it took for the laser beam to travel to the Moon and back, astronomers were able to calculate the Moon's distance.
For the Sun, a similar approach was tried by bouncing radio waves off it, but the Sun’s outer atmosphere scattered the waves too much to yield accurate measurements. Astronomers then turned to a more reliable method based on German astronomer Johannes Kepler’s Third Law of Planetary Motion, observations of other planets' movements, and radar measurements of their distances, allowing them to determine the Sun's distance indirectly but precisely.
Once we know an object's angular size in the sky and its actual distance, we can use these measurements to calculate the object’s true physical diameter.
But don’t get too comfortable with the way they appear now.
The Sun and Moon haven’t always appeared to be the same size from Earth, and they won’t continue to do so in the future. The Moon is drifting away from Earth at a rate of about 4 centimeters per year. In a few billion years, the sky will look very different to our descendants, and the size difference between the Sun and Moon will be far more apparent.
