In science fiction, a time viewer, temporal viewer, or chronoscope is a device that allows another point in time to be observed. The concept has appeared since the late 19th century, constituting a significant yet relatively obscure subgenre of time travel fiction and appearing in various media including literature, cinema, and television. Stories usually explain the technology by referencing cutting-edge science, though sometimes invoking the supernatural instead. Most commonly only the past can be observed, though occasionally time viewers capable of showing the future appear; these devices are sometimes limited in terms of what information about the future can be obtained. Other variations on the concept include being able to listen to the past but not view it.

One reason authors may choose to write about time viewers rather than time machines is to circumvent the issue of temporal paradoxes. Recurring applications include studying history, solving crimes, and entertainment in the form of displaying historic events to an audience. Because the past includes events as recently as the previous second, privacy may be compromised by such devices; several stories explore the implications thereof. Other stories examine the effects of being observed by onlookers further into the future. An unanticipated influence on past events is a common motif in stories about time viewers, and exploiting this side-effect appears in some stories.

In its most basic form, a time viewer is a device that only allows the observation of the past. Unlike with a time machine, the user is not transported from one moment in time to another. Under the strictest definition it cannot alter the past; however, the unexpected discovery that the device does indeed affect the past is a common motif. Variations on the concept where the future rather than the past is observed are more uncommon but nevertheless appear in multiple works. Another variation involves listening to the past rather than viewing it.

In-universe justifications for the ability to observe the past vary, typically corresponding to contemporary scientific developments; time viewers exploit impressions on the aether in the 1926 novel The Vicarion by Gardner Hunting, exotic neutrino properties in the 1956 short story "The Dead Past" by Isaac Asimov, and wormholes in the 2000 novel The Light of Other Days by Stephen Baxter and Arthur C. Clarke. A common explanation involves the finite speed of light and astronomical distances; this method appears in the 1935 short story "The Space Lens" by Donald A. Wollheim, among others. A variation that appears in the 1966 short story "Light of Other Days" by Bob Shaw (later included in the 1972 fix-up novel Other Days, Other Eyes) is using slow glass whose high refractive index means light takes years to pass through it. Supernatural explanations also occur in works like the 1925 short story "A View From a Hill" by M. R. James, where a pair of binoculars are enchanted to show the past, and the 1976 short story "Balsamo's Mirror" by L. Sprague de Camp, where the titular mirror allows a present-day person to view the world through the eyes of one from the past.

The earliest known example of a fully fledged time viewer in fiction appears in the 1882 short story "L'historioscope" by Eugène Mouton in the form of an electrical telescope, though it was prefigured by a couple of proto-variations on the concept; in the 1872 work Recits de l'infini (which later turned into the 1887 novel Lumen) by Camille Flammarion a spirit accomplishes the same effect by travelling faster than light, and the titular device in the 1873 short story "The Automaton Ear" by Florence McLandburgh enables listening to the past.

In film, the first time viewer appeared in the 1918 film The Ghost of Slumber Mountain. The concept has appeared regularly in works of fiction ever since, creating a sub-genre within time travel fiction, but remained comparatively obscure.

Science fiction author Stephen Baxter identifies several different ways time viewers are used in fiction. The most basic premise is of the time viewer as simply a "neat gadget", with a common variation being something going wrong, typically the past being unintentionally altered. Changing the past on purpose is another recurring application. According to Baxter, the wider implications of the existence of time viewers are sometimes explored in hard science fiction with a PEST (political, economic, social, and technical) analysis.

Several authors consider time viewers to be inherently more plausible than time machines. Science fiction author Damien Broderick says that "using a time viewer is in essence no more absurd than watching a movie made 50 years ago" since the past cannot be affected by it. Baxter similarly says that time viewers are more extrapolation than fantasy, comparing them to archaeological research. For this reason, science writer Paul J. Nahin and physicist Stephen Webb say that a benefit for writers is being able to write time travel stories without needing to consider the possibility of time paradoxes; Nahin nevertheless notes that interacting with the past via a time machine, or even affecting it, does not necessarily cause paradoxes.