What the calculation actually does
The tool implements the NOAA solar position equations, the same low-precision model used in the agency's own sunrise spreadsheet. From the day of the year it derives the equation of time (how far the sun's transit drifts from clock noon) and the solar declination (how far overhead the sun sits). Those, plus your latitude, set the hour angle at which the sun's upper edge — corrected for atmospheric refraction — crosses the horizon, and the longitude fixes when in the day that crossing happens. Results are typically within a couple of minutes of almanac values.
Reading sunrise times sensibly
The output assumes an unobstructed sea-level horizon, so a coastal sunrise is punctual while a valley or high-rise one runs late. Displayed times use the fixed UTC offset you pick — helpful for planning travel or comparing places, but it means a location inside a daylight-saving zone will disagree with the local newspaper around the switch dates. Photographers chasing golden light should also note solar noon: the halfway point of usable light, when shadows are shortest.
Frequently Asked Questions
Why is the longest day not the day of the earliest sunrise?
The equation of time tilts the whole schedule: near the June solstice, sunrise keeps getting earlier for several days after the solstice while sunset has already turned. Clock time and solar time drift against each other through the year, so the extremes of sunrise, sunset, and total daylight land on different dates.
What are polar day and polar night?
Above the polar circles, parts of the year have no sunrise or sunset at all — the midnight sun in summer and polar night in winter. When the geometry gives no horizon crossing for your date and latitude, the calculator says so instead of inventing times.
Why do my local almanac times differ by a minute or two?
Small differences come from a rounded horizon (the convention here uses the sun 90.833° from the zenith, which accounts for refraction and the solar disk), your exact position versus a city reference point, and time-zone history. Within a couple of minutes, everything agrees.