making observations without any optical aid such as a telescope.

Using only the human eye for observations. That is with no artificial help, eg binoculars or a telescope.

The naked eye is a figure of speech referring to human visual perception that is unaided by enhancing equipment, such as a telescope or binoculars. (It, therefore, does not refer to smaller scale aids such as glasses.) The term is often used in astronomy when referring to events that can be viewed by the general public, such as astronomical conjunctions, passage of comets or meteor showers.