The length of nighttime varies significantly depending on location and time of year. To truly understand how long night lasts, we must first define what constitutes night. For most practical purposes, nighttime is considered to be the period between sunset and sunrise where natural light is minimal or absent. Using astronomical data on sunrise and sunset times, we can precisely calculate the duration of nighttime around the world.
Defining Nighttime
Night is commonly defined as the period between sunset and sunrise. Sunset refers to when the upper edge of the sun’s disk disappears below the horizon in the evening. Sunrise conversely marks when the upper edge of the sun’s disk appears over the horizon in the morning. Civil twilight occurs right before sunrise and again right after sunset. During civil twilight there is still enough natural light from the sun to carry out most outdoor activities without artificial lighting. Astronomical twilight is defined when the center of the sun’s disk is 18 degrees below the horizon. At this point, residual illumination after sunset or before sunrise is very faint and typically insufficient for most activity. For the purposes of calculating nighttime duration, sunset to sunrise offers the most practical definition.
Factors Affecting Nighttime Duration
There are two primary factors that determine the length of night at any given location: geographic latitude and time of year. The further away from the equator, the longer nights tend to be on average. This is because at higher latitudes, the sun takes a shorter path across the sky even at solar noon. The low angle of the sun around the winter solstice also extends nighttime at higher latitudes. The longest nights occur at the winter solstice in December. Night duration then progressively shortens as summer approaches. The summer solstice in June brings the shortest nights of the year. The effect is most pronounced the closer you get to the poles.
Latitude
Latitude affects night length because it determines your angular distance from the equator. At the equator, the sun passes directly overhead at noon and night is consistently around 12 hours long year-round. But as you increase latitude north or south of the equator, the sun follows a lower arc across the sky. Places farther from the equator experience longer nights, especially around the winter solstice. While the equator has a constant 12 hours of night, a latitude of 60° might see nights last 16 hours during the winter. This effect intensifies nearer the poles.
Seasonal Variation
The second factor impacting night length is seasonal date. As the Earth orbits the sun tilted on its axis, exposure to direct sunlight changes. Around the winter solstice in December, the northern hemisphere is tilted away from the sun. This brings the longest nights of the year. The further from the equator, the more pronounced this seasonal swing in night length. Closer to the poles, the variation can be extreme. While June nights might last only a few hours, December nights can extend for 20 hours or more of darkness. The cycle is reversed in the southern hemisphere with the longest nights occurring around the June solstice.
Night Duration by Latitude
To demonstrate how night length changes with latitude, here are some examples from locations around the world. Values are given for both the June and December solstices when seasonal variation is most extreme:
| Latitude | June Night Length | December Night Length |
|---|---|---|
| Equator (0°) | 12 hours | 12 hours |
| London, England (51°N) | 6 hours | 16 hours |
| Moscow, Russia (55°N) | 5 hours | 19 hours |
| Juneau, Alaska (58°N) | 4 hours | 18 hours |
| Nuuk, Greenland (64°N) | 3 hours | 21 hours |
| Longyearbyen, Svalbard (78°N) | 0 hours | 0 hours |
This table illustrates how at low latitudes near the equator, the length of night remains quite consistent throughout the seasons. But as you increase latitude, seasonal variability becomes more extreme. Within the Arctic Circle above 66°N, the sun does not rise at all for a period around the winter solstice resulting in 24 hours of continuous nighttime.
Night Duration by Season
We can also look at how nighttime duration changes over the course of the year at different latitudes:
| Date | Night Length at Equator | Night Length at 45°N | Night Length at 60°N |
|---|---|---|---|
| June Solstice | 12 hours | 8 hours | 4 hours |
| September Equinox | 12 hours | 12 hours | 12 hours |
| December Solstice | 12 hours | 16 hours | 20 hours |
| March Equinox | 12 hours | 12 hours | 12 hours |
At the equator, night consistently lasts 12 hours regardless of season. But at higher latitudes, the variation through the seasons is very noticeable. Locations above the Arctic Circle experience at least one 24-hour polar night around the winter solstice.
Longest and Shortest Nights
The longest night of the year occurs around the winter solstice in December. It reaches its maximum length at the poles where the sun does not rise at all for up to 6 months. The Antarctic continent experiences a continuous polar night for about 3 months centered around the June solstice. Conversely, the North Pole has civil polar night from mid-November to late January. Nighttime exceeds 20 hours for several months at latitudes above 60°N.
The shortest night of the year conversely occurs around the summer solstice in June. At latitudes above 66°N, the sun does not set at all around the summer solstice producing a midnight sun effect. Even at more temperate latitudes, the shortest nights last only 4-6 hours in June. The further south you are, the less variation there is between the longest and shortest nights of the year.
Nighttime Duration in Major Cities
Here are some examples of how long night lasts in major cities around the world on the solstices:
| City | Latitude | June Night Length | December Night Length |
|---|---|---|---|
| Singapore | 1°N | 12 hours | 12 hours |
| Miami | 26°N | 9 hours | 13 hours |
| Cairo | 30°N | 8 hours | 14 hours |
| Beijing | 40°N | 7 hours | 15 hours |
| New York | 41°N | 7 hours | 15 hours |
| London | 51°N | 6 hours | 16 hours |
| Moscow | 55°N | 5 hours | 19 hours |
| Edmonton | 53°N | 6 hours | 17 hours |
| Reykjavik | 64°N | 3 hours | 21 hours |
Cities at low latitudes near the equator experience very little seasonal variation in night length. But cities farther from the equator can see extreme shifts in night duration from summer to winter. Higher latitude cities above 50°N may have less than 6 hours between sunset and sunrise in June, but 19 hours or more of nighttime in December.
Conclusion
In summary, the duration of nighttime is variable across different latitudes and seasons. Night consistently lasts around 12 hours on the equator regardless of season. But as latitude increases, seasonal variability becomes more pronounced. The longest nights occur around the winter solstice in December. Above the Arctic Circle, the winter solstice brings 24 hours of polar night with no sunrise at all. The shortest nights of the year are in June at the summer solstice. Understanding nighttime duration requires looking at both latitude and time of year. The closer you are to the poles and the closer to the winter solstice, the longer nights become.
