| Sun Cycle Calculator
Sun cycle time estimates are approximate for several reasons:
NOTE: The U.S. Naval Observatory hosts a web page (USNO Sun and Moon Data) that permits very accurate calculations of solar and lunar data. It incorporates ‘fixes’ for many of the issues described above, and — if you are connected to the internet — can be accessed with the ‘Naval Observatory Website’ button.
- Sunrise and sunset times are based on local solar noon, defined as the time when the sun is at its zenith (highest above the horizon) as seen from the observer’s position. Times based on solar noon are likely to be somewhat different from local time. As defined by people, time zones are arbitrary, and since they are roughly 15 degrees of longitude wide and often do not run strictly north and south, sunrise and sunset times can vary by an hour or more within a single time zone. Due to the arbitrary positioning of time zones, local times of sunrise and sunset can also be an hour different (sometimes more) from results of this calculator; these differences are typically greater a high latitudes.
- The calculator has no allowance for an observer's elevation above sea level (which makes rise times earlier and set times later, if the horizon is at sea level). It also has no allowance for local topography — for example, if the observer is in a deep valley, the visual horizon is elevated above the ‘real’ horizon, and rise times will be later and set times will be earlier than shown here.
- The precise times of sunrise and sunset are of concern to astronomers and the like, but for most purposes they are rather ‘fuzzy’ due to the angular diameter of the solar disc (about 32 minutes of arc or 0.53 degrees), atmospheric refraction, etc. Also, there are differing concepts of sunrise and sunset (e.g., the first or last glimpse of the solar disk above the horizon, vs. the time when the sun's equator at the horizon, and so forth).
• The 'Annual Plot' button will compute and display an entire year's day length cycle, based on latitude-longitude position. The 'Print Data to Spreadsheet' button makes a tab-delineated .xls spreadsheet containing the annual cycle (date, sunrise time, sunset time, and day length). This example shows a Polar-region cycle, with complete darkness in winter and 24-hour sunlight in summer: