Here's why:

* The Moon is tidally locked: The Moon rotates at the same rate as it orbits Earth. This means the same side of the Moon always faces us.

* Lunar phases are an illusion of perspective: We see different amounts of the illuminated side of the Moon as it orbits the Earth. This doesn't change the features on the Moon's surface, including the Sea of Tranquility.

However, the Sea of Tranquility might be more easily visible during specific lunar phases.

* Full Moon: The entire illuminated hemisphere is facing Earth, making features like the Sea of Tranquility more prominent.

* Waxing Gibbous or Waning Gibbous: The illuminated portion of the Moon is large enough to show the Sea of Tranquility clearly, but there might be less glare than during a full moon.

Ultimately, the Sea of Tranquility is always there on the Moon's surface. The visibility depends on factors like atmospheric conditions and light pollution, but the lunar phases themselves don't affect whether the Sea of Tranquility is visible from Earth.