When is the North Star visible and what other constellations are visible at the same time?
Polaris, or the North Star, is always visible in the Northern Hemisphere. The entire sky rotates around it during the night. The Big and Little Dipper aka Big and Little Bear are visible, as Polaris is a part of the Little Dipper. Draco, Casseopia, and Capheus are also formations normally visible around the North Star. The other constellations vary with the seasons, see the star chart referenced.
Why do some constellations such as ursa minor are visble in the sky year round other constellations appear for only part of the year?
What constellations are circumpolar (visible all year, at any time of the night) depends on your latitude. If you are (for example) at a latitude of 50Â° north, then any start that is 50Â° or less from the sky's north pole will never go below the horizon. For a more detailed explanation, read the Wikipedia article on "Circumpolar star".
Some of them, yes. Constellations along the Zodiac can be seen from either hemisphere, but constellations well away from the celesial equator cannot be seen very far in the other hemisphere. For example, Ursa Major is not visible in Australia, and the Southern Cross is not visible in Europe or North America. Most of the "official" constellations were named by northern hemisphere astronomers, or European navigators sailing in southern waters. Look at how many southern…