Day Length: How Latitude and the Date Set the Hours of Daylight
Day length is set by two things, where you are and what day it is. Earth's tilt links them, and the Orbit Dial lets you test that link.
Try it in the instrument
Set the sky to the latitude, in the Northern Hemisphere, where the June solstice day lasts 16 hours.
Also called
duration of daylight, length of day, hours of daylight
A simplified model
The drawings are computed from the numbers in the sources. They simplify the real sky. For exact times and dates, use official tables.
Anyone who lives far from the equator knows the pattern. Summer evenings seem to go on forever, and winter afternoons are over almost before they start. Near the equator, the day hardly changes at all. The hours of daylight depend on where you stand on the globe and on the date, and the two effects work together.
This guide explains what counts as daylight, why Earth's tilt stretches and shrinks it, and how the U.S. Naval Observatory's tables show the pattern at different latitudes. It ends with a task in the Orbit Dial: finding the latitude where the June solstice day lasts 16 hours.
What counts as daylight
The Naval Observatory's duration-of-daylight table counts the time that any part of the Sun is above the horizon. In low and middle latitudes, that is simply the time from sunrise to sunset.
Sunrise is the instant when the leading edge of the Sun's disk shows on the horizon. At that moment the Sun's center is still about 50 minutes of arc below the horizon, a little less than one degree. The Naval Observatory breaks the 50 into two parts: 34 minutes of arc for the bending of light by the air, and 16 for the Sun's radius. Both parts add a few minutes of light at each end of the day.
Why the tilt changes the length of the day
Earth's axis is tilted by 23.4 degrees, according to the Naval Observatory. The National Weather Service rounds it to 23.5. The direction of the tilt stays fixed in space while Earth goes around the Sun, so in June the Northern Hemisphere leans toward the Sun and in December it leans away.
NASA describes the June solstice this way: the north pole is tipped more directly toward the Sun, every place north of the equator has days longer than 12 hours, and every place south of it has days shorter than 12 hours. The seasons are reversed in the south, as the Naval Observatory notes.
Distance from the Sun is not the cause. Earth is closest to the Sun in early January, in the middle of the shortest days in the north. The guide on perihelion and aphelion covers that surprise.
How latitude changes the hours
On the equator, daylight stays within a few minutes of 12 hours all year, according to the National Weather Service. The farther you go from the equator, the larger the swing between summer and winter.
The Naval Observatory publishes the longest and shortest days for several northern latitudes. Use this table as a check on any guess.
| Latitude | Longest day | Shortest day |
|---|---|---|
| 25° N | 13 h 42 min | 10 h 35 min |
| 35° N | 14 h 31 min | 9 h 48 min |
| 40° N | 15 h 02 min | 9 h 20 min |
| 45° N | 15 h 38 min | 8 h 46 min |
| 50° N | 16 h 23 min | 8 h 04 min |
| 60° N | 18 h 53 min | 5 h 52 min |
The National Weather Service gives a simple reason for the stretch: at higher latitudes it takes the Sun longer to rise and set. The Naval Observatory adds that the Sun's path meets the horizon at a shallower angle there, so the Sun needs more time to cover the same vertical distance.
Go far enough north and the stretch becomes total. At 66.5° N, the Sun never sets from early June to early July, according to the National Weather Service. The guide on the midnight Sun explains that limit. NASA also notes that the gap between summer and winter solstice grows as you go farther north.
How the date changes the hours
Latitude is only half of the answer. The same place has a different day length every week. Here is the Naval Observatory's table for 2026 at 49° N.
| Date | Daylight at 49° N |
|---|---|
| March 1 | 11 h 01 min |
| March 21 | 12 h 13 min |
| June 21 | 16 h 12 min |
| September 21 | 12 h 15 min |
| December 21 | 8 h 14 min |
The change is not steady. Around the equinoxes the day grows or shrinks quickly: in March the table climbs from 11 h 01 min on the 1st to 12 h 49 min on the 31st. Around the solstices it nearly stops. Every day from June 17 to June 24 lists the same 16 h 12 min.
The longest day is also not the day with the earliest sunrise or the latest sunset. At 40° N, the Naval Observatory puts the earliest sunrise near June 14 and the latest sunset near June 28. In winter, the earliest sunset comes near December 8 and the latest sunrise near January 5. Two effects mix here: the clock does not keep exact Sun time, and the length of the Sun's daily track changes with the season.
Day and night at the equinox
On the equinox, day and night are close to equal but not exact. The National Weather Service lists about 12 hours 6.5 minutes at the equator, 12 hours 8 minutes at 30 degrees and 12 hours 16 minutes at 60 degrees. The Naval Observatory says the days of truly equal day and night fall a few days away from the equinox. At 40° N they come near March 17 and September 26.
Here two official sources differ. NOAA's education page says that, except at the equator, the equinoxes are the only dates with equal daylight and dark. The Naval Observatory's more careful definition of sunrise and sunset shows why that is only roughly true.
The relation in one line
The Naval Observatory gives an approximate formula for the hours of Sun above the horizon: 2/15 times arccos of (minus tan of latitude times tan of declination). The declination is the angle between the Sun's center and the plane of Earth's equator. That angle is the part that changes with the date.
You do not need to do the math to use the idea. Day length depends on only two inputs, latitude and date. When the declination is zero, as at the equinox, the formula returns exactly 12 hours at any latitude. The formula leaves out the bending of light, so the real day is longer by at least a few minutes. It also works only between the Arctic and Antarctic Circles.
Common mistakes
- Blaming distance from the Sun. The tilt of 23.4 degrees causes the seasons, and Earth is nearest the Sun in January.
- Expecting the longest day to have the earliest sunrise and latest sunset. Those dates sit a week or two away from the solstice.
- Expecting exactly 12 hours at the equinox. The real figure is a few minutes more, and it grows with latitude.
- Thinking day length is the same everywhere on the same date. On June 21 it ranges from about 12 hours near the equator to 24 hours north of the Arctic Circle.
- Reading high-latitude numbers as exact. The Naval Observatory warns that unusual bending of light can shift sunrise and sunset a lot there.
- Staring at the Sun to time it. NASA's eclipse viewing safety page says it is not safe to look directly at the Sun without specialized eye protection for solar viewing. To check sunrise and sunset, read a clock and an almanac table instead of watching the Sun.
Try it in the instrument
Open the Orbit Dial and set the sky to the latitude where the June solstice day lasts 16 hours. The dial takes a date in 2026 and a latitude, then shows the Sun's height at noon and the length of the day.
Set the date to the June solstice, June 21, 2026, which the Naval Observatory lists at 08:24 Universal Time. Then move the latitude until the day-length readout shows 16 hours. The target is about 49° N.
Check it against the official tables. At 45° N the longest day is 15 h 38 min, and at 50° N it is 16 h 23 min, so 16 hours must sit between them. The 2026 table for 49° N lists 16 h 12 min on June 21. Put 49 degrees and 23.4 degrees into the formula above and you get about 16 hours.
The dial is a simplified model. The official tables include the bending of light and the Sun's radius, so they add a few minutes. Expect small differences, and treat the relation as the lesson: more latitude means a bigger swing, and the date decides which way it goes.
Questions people ask
Why are summer days longer than winter days?
Earth's axis is tilted and keeps pointing the same way as Earth orbits. In summer your hemisphere leans toward the Sun, so the Sun's daily path is longer and more of it is above the horizon.
How long is the longest day at 40° N?
The Naval Observatory lists 15 hours 2 minutes for the longest day and 9 hours 20 minutes for the shortest at 40° N.
Are day and night exactly equal on the equinox?
No. They are close, but sunrise and sunset are defined by the Sun's upper edge, which adds a few minutes of daylight. Exactly equal day and night comes a few days off the equinox.
Where does the Sun stay up all day?
North of about 66.5° N around the June solstice. The Arctic Circle is the line where the Sun does not set on that day, and the Sun stays up for longer stretches farther north.
Does the equator have changing day length?
Hardly. It stays within a few minutes of 12 hours all year, according to the National Weather Service.
Sources
- Comparative Lengths of Days and Nights, U.S. Naval Observatory. https://aa.usno.navy.mil/faq/longest_day. Opened November 10, 2026.
- Length of Day and Night at the Equinoxes, U.S. Naval Observatory. https://aa.usno.navy.mil/faq/equinoxes. Opened November 10, 2026.
- Duration of Daylight/Darkness Table for One Year, U.S. Naval Observatory. https://aa.usno.navy.mil/data/Dur_OneYear. Opened November 10, 2026.
- Sunrise and Sunset Times Near the Solstices, U.S. Naval Observatory. https://aa.usno.navy.mil/faq/rs_solstices. Opened November 10, 2026.
- The Seasons and the Earth's Orbit, U.S. Naval Observatory. https://aa.usno.navy.mil/faq/seasons_orbit. Opened November 10, 2026.
- The Astronomical Seasons Page, National Weather Service, Tucson. https://www.weather.gov/twc/Astronomical_Seasons. Opened November 10, 2026.
- What Causes the Seasons?, National Weather Service, Sioux Falls. https://www.weather.gov/fsd/season. Opened November 10, 2026.
- What Is a Solstice?, NOAA NESDIS. https://www.nesdis.noaa.gov/about/k-12-education/optical-phenomena/what-solstice. Opened November 10, 2026.
- June Solstice Brings Summer, Winter Seasons, NASA. https://www.nasa.gov/blogs/watch-the-skies/2021/06/17/june-solstice-brings-summer-winter-seasons/. Opened November 10, 2026.
- Helio and You: Learning About the Sun in the Far North, NASA Science. https://science.nasa.gov/helio-and-you-learning-about-the-sun-in-the-far-north/. Opened November 10, 2026.
- Eclipse Viewing Safety, NASA Science. https://science.nasa.gov/eclipses/safety/. Opened November 10, 2026.
We opened each source ourselves on the date shown. The drawings are our own and were checked against the numbers in these sources.
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