天象仪是安放在建筑物剧场内的一种仪器，主要用于展示天文和夜空有关的教育与娱乐节目，或用于天文导航的训练。大多数天象仪的主要特征是有巨大的圆顶投影屏幕，可以在上面呈现恒星、行星和其他的天体，也可以演出和仿真它们在天球上复杂的运动和移动的现象。可以使用多种技术创建天体的场景，例如结合光学和机电技术等精密工程的恒星球，幻灯片投影机、放映机、全天投影系统和雷射。无论使用那些技术，目的都是将天空中的目标链接在一起，提供它们精确的位置和相对运动。典型的系统可以依照地球上的纬度任意的设置一个时间点，无论是过去或未来，呈现出世界任一地点夜晚的天空。

A planetarium (plural planetaria or planetariums) is a theatre built primarily for presenting educational and entertaining shows about astronomy and the night sky, or for training in celestial navigation.

A dominant feature of most planetaria is the large dome-shaped projection screen onto which scenes of stars, planets and other celestial objects can be made to appear and move realistically to simulate the complex 'motions of the heavens'. The celestial scenes can be created using a wide variety of technologies, for example precision-engineered 'star balls' that combine optical and electro-mechanical technology, slide projector, video and fulldome projector systems, and lasers. Whatever technologies are used, the objective is normally to link them together to provide an accurate relative motion of the sky. Typical systems can be set to display the sky at any point in time, past or present, and often to show the night sky as it would appear from any point of latitude on Earth.