  Scintillation noise due to the Earth's turbulent atmosphere can be a dominant
noise source in high-precision astronomical photometry when observing bright
targets from the ground. Here we describe the phenomenon of scintillation from
its physical origins to its effect on photometry. We show that Young's (1967)
scintillation-noise approximation used by many astronomers tends to
underestimate the median scintillation noise at several major observatories
around the world. We show that using median atmospheric optical turbulence
profiles, which are now available for most sites, provides a better estimate of
the expected scintillation noise and that real-time turbulence profiles can be
used to precisely characterise the scintillation noise component of
contemporaneous photometric measurements. This will enable a better
understanding and calibration of photometric noise sources and the
effectiveness of scintillation correction techniques. We also provide new
equations for calculating scintillation noise, including for extremely large
telescopes where the scintillation noise will actually be lower than previously
thought. These equations highlight the fact that scintillation noise and shot
noise have the same dependence on exposure time and so if an observation is
scintillation limited, it will be scintillation limited for all exposure times.
The ratio of scintillation noise to shot noise is also only weakly dependent on
telescope diameter and so a bigger telescope may not yield a reduction in
fractional scintillation noise.
