In geosciences, an annual cycle is the part of a measured quantity's fluctuation that is attributed to Earth's changing position in orbit over the course of the year. Such quantities might be influenced directly (e.g. incoming solar radiation at a point at the surface) or indirectly (e.g. stratospheric westerlies and easterlies over the winter and summer hemispheres, resp.) by orbital position.
The annual cycle is commonly estimated from observational data or model output by taking the average of all Januaries, all Februaries, and so forth. If the observational record is long enough and conditions are stationary (i.e. there is no significant long-term trend), a meaningful annual cycle will result that can be used to calculate an anomaly time series.
(Note that grouping data according to the months in the Gregorian calendar makes an implicit assumption that the annual cycle in question is the vernal equinox tropical year followed by that calendar. However, the annual cycle for some phenomena may be a different kind of year; see the seasonal year article for discussion.)