That fine fillet of light, now hardly visible on her eastern border, will disappear altogether as soon as the Moon is full.

Then, lying as she will be between the Sun and the Moon, her illuminated face will be turned away from us altogether, and for several days she will be involved in impenetrable darkness." "And that's the Earth!" repeated Ardan, hardly able to believe his eyes, as he continued to gaze on the slight thread of silvery white light, somewhat resembling the appearance of the "Young May Moon" a few hours after sunset.
Barbican's explanation was quite correct.

The Earth, in reference to the Moon or the Projectile, was in her last phase, or octant as it is called, and showed a sharp-horned, attenuated, but brilliant crescent strongly relieved by the black background of the sky.

Its light, rendered a little bluish by the density of the atmospheric envelopes, was not quite as brilliant as the Moon's.

But the Earth's crescent, compared to the Lunar, was of dimensions much greater, being fully 4 times larger.