A superintendent who keeps a 7:00 a.m. start through December puts the first minutes of the shift in the dark in four of the seven cities below. The amount depends on the date, the latitude and where the site sits inside its time zone. The U.S. Naval Observatory (USNO) publishes the sunrise and twilight times to compute it for any site, and OSHA’s construction standards set how much light the work needs while it is in progress.
What OSHA requires
OSHA 1926.26 says construction areas, aisles, stairs, ramps, runways, corridors, offices, shops and storage areas where work is in progress must be lighted with natural or artificial illumination. The minimums sit in Table D-3 of 1926.56, stated in foot-candles, a measure of how much light reaches a surface. The table lists these minimums:
- 5 foot-candles for general construction area lighting.
- 3 foot-candles for general construction areas, concrete placement, excavation and waste areas, access ways, active storage areas, loading platforms, refueling and field maintenance areas.
- 5 foot-candles for indoor warehouses, corridors, hallways and exitways, and for tunnels, shafts and general underground work, with 10 at a tunnel or shaft heading during drilling, mucking and scaling.
- 10 foot-candles for general construction plant and shops, such as batch plants, mechanical and electrical equipment rooms and carpenter shops.
- 30 foot-candles for first aid stations, infirmaries and offices.
For areas the table does not cover, 1926.56(b) refers to ANSI A11.1-1965, R1970, Practice for Industrial Lighting.
Read the twilight times
USNO defines sunrise and sunset as the moments the upper edge of the Sun’s disk sits on the horizon, assuming average atmospheric conditions and level ground. Civil twilight begins in the morning and ends in the evening when the center of the Sun is 6 degrees below the horizon. The observatory’s definition page says that before morning civil twilight and after evening civil twilight, “artificial illumination is normally required to carry on ordinary outdoor activities.”
That gives a schedule three bands. Before civil twilight, the work runs on temporary light. Between civil twilight and sunrise, natural light is building, and USNO notes that the amount at any moment depends on cloud cover, local atmospheric conditions and the view of the horizon. After sunrise, a cleared outdoor site has daylight, though shaded floors and interiors still need their own lighting under Table D-3.
USNO’s Complete Sun and Moon Data for One Day service returns these times for any date from 1700 to 2100 from a latitude, longitude and time zone. A companion service builds a one-page table for a full year at one location, and the observatory’s API returns the same data as JSON. USNO says computed rise and set times can be off by a minute or more, and that hills on the horizon and the observer’s height can move them further.
Seven cities on December 21
The first table pulls December 21, 2026, from the USNO API for seven cities, in local standard time. It assumes a 7:00 a.m. start and counts the minutes of that start that fall before civil twilight and before sunrise.
| City | Time zone | Civil twilight begins | Sunrise | Sunset | Civil twilight ends | Twilight-to-twilight window (h:mm) | Minutes of a 7:00 start before civil twilight | Minutes of a 7:00 start before sunrise |
|---|---|---|---|---|---|---|---|---|
| Detroit | Eastern | 7:26 a.m. | 7:58 a.m. | 5:03 p.m. | 5:35 p.m. | 10:09 | 26 | 58 |
| Seattle | Pacific | 7:19 a.m. | 7:55 a.m. | 4:20 p.m. | 4:56 p.m. | 9:37 | 19 | 55 |
| Minneapolis | Central | 7:14 a.m. | 7:48 a.m. | 4:34 p.m. | 5:08 p.m. | 9:54 | 14 | 48 |
| Atlanta | Eastern | 7:11 a.m. | 7:38 a.m. | 5:33 p.m. | 6:01 p.m. | 10:50 | 11 | 38 |
| Dallas | Central | 6:58 a.m. | 7:26 a.m. | 5:25 p.m. | 5:52 p.m. | 10:54 | 0 | 26 |
| Chicago | Central | 6:43 a.m. | 7:15 a.m. | 4:23 p.m. | 4:54 p.m. | 10:11 | 0 | 15 |
| Boston | Eastern | 6:38 a.m. | 7:10 a.m. | 4:15 p.m. | 4:47 p.m. | 10:09 | 0 | 10 |
Where a city sits inside its time zone shows up in these numbers alongside latitude. Atlanta is far south of Boston, yet a 7:00 start in Atlanta has 11 minutes before civil twilight and Boston has none. Atlanta sits toward the western side of the Eastern time zone, and its sunrise comes at 7:38 a.m. against 7:10 in Boston. Detroit and Boston have the same window from morning to evening civil twilight, 10 hours 9 minutes, but Detroit’s opens 48 minutes later on the clock.
Seattle has the shortest window of the seven at 9 hours 37 minutes. An 8.5-hour shift with a 30-minute lunch still fits inside it if the start falls between 7:19 and 8:26 a.m.
A worked example
A job in Detroit runs a 7:00 a.m. to 3:30 p.m. shift with a 30-minute lunch. The second table compares that start with a 7:30 a.m. start, ending at 4:00 p.m., on the 21st of each month from November through February. The times come from the USNO API in Eastern Standard Time.
| Date | Civil twilight begins | Sunrise | Sunset | 7:00 start: minutes before civil twilight | 7:00 start: minutes before sunrise | 7:30 start: minutes before civil twilight | 7:30 start: minutes before sunrise | 7:30 start: minutes from 4:00 p.m. end to sunset |
|---|---|---|---|---|---|---|---|---|
| Nov 21 2026 | 7:00 a.m. | 7:30 a.m. | 5:05 p.m. | 0 | 30 | 0 | 0 | 65 |
| Dec 21 2026 | 7:26 a.m. | 7:58 a.m. | 5:03 p.m. | 26 | 58 | 0 | 28 | 63 |
| Jan 21 2027 | 7:25 a.m. | 7:55 a.m. | 5:32 p.m. | 25 | 55 | 0 | 25 | 92 |
| Feb 21 2027 | 6:52 a.m. | 7:20 a.m. | 6:12 p.m. | 0 | 20 | 0 | 0 | 132 |
December and January carry nearly the same dark time at a 7:00 start, 26 and 25 minutes, even though sunset on January 21 comes 29 minutes later than on December 21. Sunrise moves only 3 minutes earlier between those dates. Moving the start to 7:30 removes the minutes before civil twilight in all four months and still ends the shift at least 63 minutes before sunset.
The 7:30 start leaves 28 minutes of partial light before sunrise on December 21 and 25 on January 21. Those minutes fall in the band where USNO says the light depends on cloud cover, so a light meter reading at the work surface decides whether the general area meets 5 foot-candles or the towers stay on.
A round-number example shows what the dark minutes add up to. A 10-person crew that keeps the 7:00 start for 20 working days with 25 minutes before civil twilight each day works 5,000 crew-minutes, about 83 crew-hours, entirely under temporary light, with towers or task lights set and fueled for each of those mornings.
Daylight saving time moves every clock time one hour later. USNO’s service adds that hour when the option is selected and the date falls in the daylight saving period, so turn it on for any dates after the March clock change.
How to use it on your jobs
Put the twilight times next to the lookahead schedule. The USNO one-year table gives every date for the site’s coordinates. In Excel, with the shift start in A2 and the civil twilight time in B2, both formatted as times, =MAX(0,(B2-A2)*1440) returns the dark minutes for that day.
Then price the lighting in general conditions the same way the temporary heat post prices heat from degree days: run hours per day, times working days, times the cost per tower hour. The wind chill post covers the warm-up breaks that come out of the same winter shifts, and the weather delay days post covers the days lost outright.
Record what happened. The superintendent logs the start time, the time the towers went off and a foot-candle reading in the daily log. After a winter of entries, the method in the labor productivity post can compare units installed in the first hour under temporary light with the same hour later in the season.
To start, pick one job with outdoor work planned for December through February. Pull the USNO one-year table for the site’s coordinates and compute the dark minutes for each week at the current start time.