Stellarium

Stellarium is the closest thing on this list to placing a transparent dome over your desk. Set a location and time, and it draws the sky as it should appear from that viewpoint, including the horizon, atmosphere, planets, stars and deep-sky objects. Time can run forward or backward, constellation artwork can be switched among many sky cultures, and the view can be changed from a naked-eye panorama to binocular or telescope-like framing. The browser edition is convenient for a quick look; the desktop program adds the deeper catalogues, plug-ins and control options that make the project unusually broad.

That breadth does not turn a simulation into a guarantee of visibility. A galaxy plotted above the horizon may still disappear in haze, moonlight or urban skyglow, and a telescope view depends on the observer's actual optics. Stellarium works best as the visual layer in a planning stack: learn where an object will be and how the sky turns, then consult a local forecast and darkness map before travelling. Beginners can use it to recognize constellations, while experienced observers can prepare charts, test an eyepiece view or drive supported telescope mounts.