Here's why:
* The Earth is a sphere: This means that an observer on the Earth's surface is only able to see the hemisphere of the sky above them.
* The horizon: The observer's view is limited by the horizon, which is the line where the sky appears to meet the Earth. This horizon is determined by the observer's location and the curvature of the Earth.
However, it's important to note that this is just a rough estimate. The actual amount of the celestial sphere visible can vary due to:
* Atmospheric conditions: Clouds, haze, and other atmospheric phenomena can obstruct the view of certain parts of the sky.
* Light pollution: Artificial light from cities can make it difficult to see fainter stars.
* The observer's location: Observers at higher altitudes have a wider view of the sky than those at sea level.
Therefore, while an observer can theoretically see half of the celestial sphere, the actual amount visible will vary based on these factors.
