Sunrise shown in time lapse. The motions of Sun and Moon have demonstrated and symbolized time throughout humanity's existence

The flow of sand in an hourglass can be used to keep track of elapsed time. It also concretely represents the present as being between the past and the future. Time is a fundamental component of our measuring system and has long been a major subject of art, philosophy, and science. Whether it is possible to formulate a noncontroversial definition, applicable to all fields, remains an issue for scholars. In physics and other sciences, time is considered a fundamental quantity, i.e. one that cannot be defined in terms of other quantities because those other quantities – such as velocity, force, energy – are already defined in terms of that fundamental quantity (in these cases, both time and another fundamental quantity, space). Within science, the only definition needed or possible is an operational one, in which time is defined by the process of measurement and by the units chosen. Among philosophers, there are two distinct viewpoints on time. One view is that time is part of the fundamental structure of the universe, a dimension in which events occur in sequence. This is the realist's view, to which Sir Isaac Newton subscribed, and hence is sometimes referred to as Newtonian time.[1] The opposing view is that time is part of the fundamental intellectual structure (together with space and number) within which humans sequence events, quantify the duration of events and the intervals between them, and compare the motions of objects. According to this second view, in the tradition of Gottfried Leibniz[2] and Immanuel Kant,[3][4] time does not refer to any kind of entity that "flows", that objects "move through", or that is a "container" for events, and it cannot itself be measured. Temporal measurement has occupied scientists and technologists, and was a prime motivation in astronomy. Periodic events and periodic motion have long served as standards for units of time. Examples include the apparent motion of the sun across the sky, the phases of the moon, the swing of a pendulum, and the beat of a heart. Currently, the international unit of time, the second, is defined as a certain number of hyperfine transitions in caesium atoms (see below). Time is also of significant social importance, having economic value ("time is money") as well as personal value, due to an awareness of the limited time in each day and in human lifespans.