Final answer:
An antumbra appears to an observer as a bright ring around an opaque object, which is visible during an annular solar eclipse when the Moon does not fully cover the Sun.
Step-by-step explanation:
To an observer, an antumbra appears as a bright ring around an opaque object. This is typically seen during an annular solar eclipse, when the Moon is farther away from the Earth and cannot completely cover the Sun. This results in a bright ring of sunlight surrounding the dark silhouette of the Moon, which is known as an 'annulus' or ring of fire.
The antumbra is different from the umbra, which is the part of the Moon's shadow where the Sun is completely hidden and a total eclipse is observed. It is also distinct from the penumbra, where a partial eclipse is seen, and only part of the Sun is blocked by the Moon. Therefore, an antumbra gives the unique visual effect of a complete ring of light encircling the darkness, unlike any other type of shadow.