What is the primary geometrical reason the Sun appears massive compared to other stars?

Answer

Its close proximity to Earth.

The Sun's colossal appearance is due entirely to perspective and distance. It resides at 1 astronomical unit (AU), while other visible stars are hundreds or thousands of light-years away, causing their angular diameter to drop off rapidly.

What is the primary geometrical reason the Sun appears massive compared to other stars?

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Why The Sun is Bigger Than You Think - YouTube

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