It is one of the most interesting coincidence in the solar system that enables full Solar Eclipse.
The ratio of distance between the Sun and Earth, & Moon and Earth is almost exactly proportionate to the ratio of size of Sun to Moon.
In astronomy the sizes of objects in the sky are often given in terms of their angular diameter as seen from Earth, rather than their actual sizes.
The angular diameter of Earth's orbit around the Sun, from a distance of one parsec, is 2" (two arcseconds).
The angular diameter of the Sun, from a distance of one light-year, is 0.03", and that of the Earth 0.0003". The angular diameter 0.03" of the Sun given above is approximately the same as that of a person at a distance of the diameter of the Earth
This table shows the angular sizes of noteworthy celestial bodies as seen from the Earth:
Sun : 31.6' – 32.7'
Moon : 29.3′ – 34.1'
Venus : 10″ – 66″
Jupiter : 30″ – 49″
Saturn : 15″ – 20″
Mars : 4″ – 25″
Mercury : 5″ – 13″
Uranus : 3″ – 4″
Neptune : 2″
Ceres : 0.8″
Pluto : 0.1″
Eris : 40 mas
As can be seen the angular diameter of the Sun is about the same as that of the Moon (the diameter is 400 times as large and the distance also; the Sun is 200,000-500,000 times as bright as the full Moon (figures vary), corresponding to an angular diameter ratio of 450-700, so a celestial body with a diameter of 2.5-4" and the same brightness per unit solid angle would have the same brightness as the full Moon).
The angular diameter of an object as seen from a given position is the "visual diameter" of the object measured as an angle. In the vision sciences it is called the visual angle. The visual diameter is the diameter of the perspective projection of the object on a plane through its centre that is perpendicular to the viewing direction. Because of foreshortening, it may be quite different from the actual physical diameter for an object that is seen under an angle. For a disk-shaped object at a large distance, the visual and actual diameters are the same.