Daylight Hours Calculator

How long a day is depends on two things and nothing else: how far north or south you are, and where the earth is in its orbit. The tilt of the axis is what makes the second one matter. Through the year the sun's declination, which is how far it sits above or below the plane of the equator, swings from about 23.4 degrees south to 23.4 degrees north and back. Put that together with your latitude and you get the hour angle, which is how far the earth has to turn between the sun crossing your horizon and the sun standing due south. Twice that angle is the length of the day, at four minutes to the degree.

The arithmetic in between is the algorithm the US National Oceanic and Atmospheric Administration publishes for its own solar calculator, taken from Jean Meeus'sAstronomical Algorithms. It is accurate to about a minute, and this page reproduces NOAA's own published figures to fourteen digits, which is the only reason to trust it.

Two details that matter more than they look. The sun is not a point, and the atmosphere bends its light up over the horizon, so the convention is to call it sunrise when the centre of the sun is 0.833 degrees below the horizon rather than level with it. And the clock drifts away from the sun through the year, by as much as a quarter of an hour either way, because the earth's orbit is an ellipse and its axis is tilted. That is the equation of time, and it is why the earliest sunset is not on the shortest day.

You can run a mower, a combine or a nail gun before the sun is up and after it has gone down. The usual bound on that is civil twilight, the stretch while the sun is within six degrees below the horizon, and it is the default here because it is the honest answer for outdoor work. At 39.1 degrees north it is worth 350 hours a year, or about 58 minutes a day, over counting sunrise to sunset. Over a season that is real money.

The third setting, good light, asks for the sun at least six degrees up. That is roughly where a low sun stops fighting fine finish work and colour photography. It is a convention rather than a standard, and it is here because the honest answer to "when can I actually see what I am doing" is not the same as "when is it not dark".

Type a ZIP code, or set the latitude directly if you know it. Put in the shift you actually work. The summary gives the year: hours of light, the longest and shortest days, and how many of your working hours fall outside the light. The chart switches between hours of light and the clock times, and the clock view is the one to look at when you are planning, because the question in December is not how many hours there are but what time it gets light.

The month table is the planning tool. A 10 hour day, 5 days a week, loses 23 hours to the dark in December here and 0 in June. That difference is the one that decides whether you need lights, a different start time, or a smaller crew for three months.

The further from the equator, the harder the seasons bite. Between the longest and shortest day there is 3.3 hours of difference at 25.8 degrees north, around Miami, 5.6 hours at 39.1 degrees, and 7.7 hours at 47.6 degrees, around Seattle. Inside the polar circles the curve goes flat at both ends, with weeks where the sun does not set and weeks where it does not rise, and the figures here will say so rather than giving you a number.

One thing this does not know is your ground. A valley, a treeline or a building to the east takes light off the start of the day and off the end, sometimes an hour of it. The figures here are for a flat horizon, which is the best case.

• The sun is taken as rising and setting when its centre is 0.833 degrees below the horizon, which allows for its own width and for the atmosphere bending its light over the horizon.
• Civil twilight is the sun within six degrees below the horizon, which is the usual bound on working outdoors without lights.
• Hills, trees and buildings are not in this. A valley loses light at both ends of the day and this does not know your valley.
• A ZIP code is resolved to the middle of its area, which puts the day length within five minutes of the figure for the ZIP itself for 99 of every 100 US ZIP codes.
• Clock times follow the United States summer time rule where it is switched on, from the second Sunday in March to the first Sunday in November.

• Solar Calculation Details — NOAA Global Monitoring Laboratory
• Astronomical Algorithms, 2nd edition — Jean Meeus, Willmann-Bell
• Rise, Set, and Twilight Definitions — US Naval Observatory
• ZIP Code Tabulation Area Gazetteer, 2024 — US Census Bureau