sfero

The base that atmosfero was waiting for. When the atmosphere arrived at rank 2,002 this course flagged that sfero was a real Esperanto word further down the list, at 2,635, and warned you not to conclude anything from that. Here it is - and the warning still stands. Atmosfero is not atmo plus sfero, because there is no root atmo in Esperanto: the word came in whole from scientific Latin. Greek atmos is genuinely in there historically, but history is not morphology. Two real words can share a syllable and still have no seam between them.

What sfero does on its own. Both of the English senses, and they divide the way English divides them. Geometrically it is the perfectly round solid - la surfaco de sfero, duonsfero a hemisphere, sfera the adjective. Figuratively it is a domain, a field, the territory somebody's work or authority covers: la sfero de la scienco, ekster mia sfero. The metaphor is inherited rather than invented, which is why it will feel familiar.

Against the neighbours. Globo at rank 2,018 is the everyday round object, and a globe of the world specifically; rondo is a circle, which is flat, and also a circle of people; cirklo, arriving at rank 2,649, is the mathematician's circle. Reach for sfero when you want either the solid or the abstract domain, and for globo when you want a ball you could pick up.