* Earth's Tilt: The Earth is tilted on its axis. This tilt means that people in the Northern Hemisphere see a different set of stars in the night sky than people in the Southern Hemisphere.
* Perspective: Imagine looking at a globe. If you are on the top half (Northern Hemisphere), you can only see the stars that are above the top half of the globe. The same is true for the Southern Hemisphere.
* Circumpolar Stars: Some stars are visible from only one hemisphere. Stars near the celestial poles are called circumpolar stars. In the Northern Hemisphere, Ursa Major (the Big Dipper) is a circumpolar constellation. In the Southern Hemisphere, Crux (the Southern Cross) is a circumpolar constellation.
Think of it this way:
* If you are standing in the Northern Hemisphere and look straight up, you are looking at the northern sky.
* If you are standing in the Southern Hemisphere and look straight up, you are looking at the southern sky.
There are some constellations visible from both hemispheres, but their position in the sky will be different. For example, Orion is visible from both hemispheres, but it appears upside down in the Southern Hemisphere.
