This is a planetarium, a building where people see images of the stars projected on the ceiling. The roof is a hemisphere, from the Greek words "hemi" (half) and sphere (ball shape). We also talk about the western hemisphere (North and South America) and the eastern hemisphere (Europe, Africa and Asia) or the northern hemisphere (the countries above the equator) and the southern hemisphere (where it is summer when the north has winter), like Australia and Brazil.
