Why is the portion of the Moon opposite the direction of the Sun not apparent?
The portion of the Moon that is opposite the direction of the Sun is not apparent because of a phenomenon called "lunar phases." The Moon does not emit light of its own, but instead reflects the light from the Sun. As the Moon orbits the Earth, different portions of its surface are illuminated by the Sun, creating different phases.
When the Moon is directly between the Earth and the Sun, the side facing us is completely in shadow, and this is known as the "new moon" phase. During this phase, the portion of the Moon opposite the direction of the Sun is not visible from Earth because it is facing away from us and is not illuminated.
As the Moon continues its orbit, it gradually moves away from the Sun's direction, causing a crescent shape to become visible. Eventually, we see a "first quarter" moon when the right half of the Moon is visible from Earth.
The portion of the Moon that is opposite the direction of the Sun becomes visible when the Moon reaches the "full moon" phase. This is when the entire face of the Moon that is facing us is fully illuminated by the Sun. At this point, the side opposite the Sun is completely visible and appears as a full circle.
After the full moon phase, the illuminated portion gradually starts to decrease until we reach the "third quarter" moon, when the left half of the Moon is visible from Earth. Finally, the Moon returns to the new moon phase, and the cycle repeats.
So, the reason why the portion of the Moon opposite the direction of the Sun is not apparent is due to the different phases of the Moon caused by its position relative to the Earth and the Sun.