Complex sentence: The Earth's shadow or Earth shadow (also sometimes known as the dark segment) are names for the shadow that the Earth itself casts on its atmosphere; a shadow which is often visible near the horizon from the surface of the Earth.
Split into simpler sentences: The Earth's shadow or Earth shadow (also sometimes known as the dark segment) are names for the shadow that the Earth itself casts on its atmosphere. That shadow is often visible near the horizon from the surface of the Earth.