Day and Night
Explore how Earth's rotation creates day and night — watch the terminator sweep across continents live, see city lights glow on the night side, and inspect daylight hours at any latitude.
What Causes Day and Night?
Day and night are caused by Earth's rotation on its own axis. Earth completes one full rotation approximately every 24 hours, spinning from west to east (counterclockwise when viewed from above the North Pole). As it rotates, one hemisphere faces the Sun — experiencing daytime — while the opposite hemisphere faces away — experiencing nighttime.
The boundary between the day and night sides is called the terminator line or twilight zone. It is not a sharp line but a gradual transition, because Earth's atmosphere scatters sunlight for some time after the geometric horizon.
Key Facts & Formulas
The Terminator
The terminator is a great circle (a circle whose plane passes through Earth's centre) that is always perpendicular to the Sun-Earth line. It divides Earth into exactly two halves at the equinoxes (when it passes through both poles), but is tilted up to 23.5° away from the pole-to-pole axis at the solstices, due to Earth's axial tilt.
Daylight Hours Formula
cos(H₀) = −tan(φ) × tan(δ) φ = observer latitude, δ = solar declination (−23.5° to +23.5°) Hours of daylight = 2H₀ / 15°/hr H₀ = half-day angle in degrees Time Zones
Earth's 360° longitude is divided into 24 standard time zones, each covering approximately 15°. The prime meridian (0° longitude) passes through Greenwich, UK, defining Coordinated Universal Time (UTC). Each 15° east adds 1 hour; each 15° west subtracts 1 hour.
Local Solar Time = UTC + (Longitude / 15°) Positive for East longitudes, negative for West longitudes Twilight Types
Sun 0° – 6° below horizon
Bright sky, no artificial light needed
Sun 6° – 12° below horizon
Horizon visible, stars appear
Sun 12° – 18° below horizon
Sky darkens enough for observations
Sun > 18° below horizon
Full darkness for stargazing
Solved Examples
At 40°N latitude, sunrise occurs when the Sun crosses the horizon. How many hours of daylight are there near the summer solstice?
- At summer solstice, the subsolar point is at 23.5°N (axial tilt)
- Hour angle of sunrise: cos(H₀) = −tan(φ)·tan(δ) = −tan(40°)·tan(23.5°)
- cos(H₀) = −(0.839)(0.4348) = −0.3648
- H₀ = arccos(−0.3648) = 111.4° = 7.43 hours from noon
Daylight ≈ 2 × 7.43 = 14.86 hours ≈ 14 h 52 min
Earth's rotation is 15°/hour. A location is at longitude 75°E. What is its local solar time when UTC is 12:00?
- Longitude offset = 75°E
- Time offset = 75° ÷ 15°/hr = +5 hours ahead of UTC
- Local Solar Time = 12:00 UTC + 5:00
Local Solar Time = 17:00 (5:00 PM)
Calculate the width of Earth's twilight zone (civil twilight) at the equator.
- Civil twilight ends when Sun is 6° below the horizon
- Earth rotates 360° in 24h → 0.25°/min
- Time for Sun to drop 6°: 6° ÷ 0.25°/min = 24 minutes
- Arc distance: v_equator = 465 m/s × 60 × 24 min
Twilight zone width ≈ ≈669 km along equator
Practice Questions
Why does the Sun appear to rise in the east and set in the west?
Earth rotates from west to east (counterclockwise when viewed from above the North Pole). As Earth spins, the Sun appears to move in the opposite direction relative to the surface — rising in the east and setting in the west. It is Earth's rotation, not the Sun's movement, that causes this apparent motion.
What is the terminator line and why does it curve?
The terminator is the boundary circle dividing the illuminated (day) and dark (night) halves of Earth. It appears curved in satellite images because Earth is a sphere. The terminator always passes through the poles only at the equinoxes; at other times it is tilted by up to 23.5° due to Earth's axial tilt, causing different day lengths at different latitudes.
A city is at 60°N latitude. Estimate its daylight hours at winter solstice.
cos(H₀) = −tan(60°)·tan(−23.5°) = −(1.732)(−0.4348) = +0.753. H₀ = arccos(0.753) = 41.1° = 2.74 hours from noon. Daylight ≈ 2 × 2.74 ≈ 5.5 hours.
What is the difference between a solar day and a sidereal day?
A sidereal day (23h 56m 4s) is the time for Earth to complete exactly 360° of rotation relative to distant stars. A solar day (24h) is the time for the Sun to appear in the same position in the sky, which is slightly longer because Earth has also moved along its orbit during that time, requiring an additional ~1° of rotation.
At what latitudes do polar day and polar night occur?
At latitudes above 66.5° (the Arctic/Antarctic circles), the Sun can remain above or below the horizon for 24 hours at a time. At the poles (90°N/S), there is 6 months of continuous day and 6 months of continuous night per year. This is a direct consequence of Earth's 23.5° axial tilt combined with its revolution around the Sun.
Frequently Asked Questions
What causes day and night?
Day and night are caused by Earth's rotation on its own axis once every 24 hours. As Earth rotates, one side of the globe faces the Sun (experiencing daytime) while the opposite side faces away (experiencing nighttime). The boundary between these two regions is called the terminator.
How long does it take for Earth to complete one rotation?
Earth completes one full rotation in approximately 23 hours 56 minutes 4 seconds (a sidereal day relative to stars), or exactly 24 hours (a solar day relative to the Sun). The difference exists because Earth also moves along its orbit, requiring slightly more than one full rotation for the Sun to return to the same apparent position in the sky.
Why do different places on Earth have different amounts of daylight?
Due to Earth's axial tilt of 23.5°, the amount of daylight varies with latitude and season. Near the equator, day and night are roughly equal year-round. At higher latitudes, the difference is more extreme — from nearly 24 hours of daylight in summer to very short winter days. Near the poles, there are periods of continuous day or continuous night.
What is the terminator?
The terminator (or twilight zone) is the great-circle boundary on Earth that separates the illuminated day side from the dark night side. It moves westward at about 1,670 km/h at the equator (matching Earth's rotation speed). The terminator is not a sharp line in reality but a gradual transition through civil, nautical, and astronomical twilight.
What is twilight?
Twilight is the period of scattered sunlight that illuminates the sky after sunset or before sunrise, when the Sun is below the horizon but its light is still refracted by the atmosphere. Civil twilight occurs when the Sun is 0–6° below the horizon; nautical twilight at 6–12°; astronomical twilight at 12–18°. Earth's atmosphere extends daylight beyond the geometric terminator.
Why is daytime warmer than nighttime?
During daytime, the Sun continuously delivers solar radiation (≈1361 W/m² outside the atmosphere) to Earth's surface, warming it. During nighttime, this energy input stops but the surface continues to radiate infrared heat into space, causing temperatures to drop. The ground radiates away energy at night without replenishment from the Sun.
How do time zones relate to Earth's rotation?
Earth rotates 360° in 24 hours, which is 15° per hour. The world is divided into 24 standard time zones, each roughly 15° of longitude wide, so that noon (when the Sun is highest) corresponds approximately to 12:00 local time. International Date Line (180° meridian) marks where one calendar day transitions to the next.
What happens at the equinoxes regarding day and night?
At the vernal (March 20) and autumnal (September 22) equinoxes, Earth's axial tilt is perpendicular to the Sun's direction, so the terminator passes exactly through both poles. This means every location on Earth (from pole to pole) experiences exactly 12 hours of daylight and 12 hours of night — the word 'equinox' means 'equal night.'
Can the Sun be seen from space at night?
From space, there is no 'night' in the traditional sense, because there is no atmosphere to scatter light. The ISS orbits Earth every 90 minutes, experiencing about 16 sunrises and sunsets per day. Astronauts on the day side of orbit see brilliant sunlight, and on the night side see the dark of space punctuated by stars and city lights below.
What is polar day (midnight sun) and polar night?
Polar day (midnight sun) occurs above the Arctic/Antarctic Circle when Earth's axial tilt causes the Sun to remain above the horizon for 24 continuous hours — typically around the summer solstice. Conversely, polar night (polar darkness) occurs in winter when the Sun stays below the horizon for a full day. At the poles themselves, these periods each last about 6 months.