SolarWatch Glossary

Explore our comprehensive collection of terms related to sun, daylight, astronomy, and solar phenomena. This glossary helps explain key concepts about when it gets dark and how sunlight affects our daily lives.

A

Albedo

Albedo is the measure of how much sunlight a surface reflects back into space. It is expressed as a ratio or percentage, with higher values indicating more reflective surfaces.

Also known as: surface reflectivity, reflection coefficient, reflectance

Astronomical Dawn

Astronomical Dawn is the moment when the center of the Sun is 18 degrees below the horizon in the morning, marking the beginning of astronomical twilight and the end of night.

Also known as: First light, Beginning of astronomical twilight, End of night
Related terms: Astronomical Twilight

Astronomical Dusk

Astronomical dusk is the moment when the sun is 18 degrees below the horizon in the evening, marking the end of astronomical twilight and the beginning of complete darkness.

Also known as: End of astronomical twilight, Beginning of night, Complete darkness, True night
Related terms: Astronomical Twilight

Astronomical Twilight

Astronomical twilight is the darkest stage of twilight, occurring when the Sun is between 12 and 18 degrees below the horizon. During this period, the sky is dark enough for most stars to be visible, but there's still a faint glow from the Sun in the sky.

Also known as: Dark twilight, Astronomical dusk, Astronomical dawn, Stargazing twilight

Autumnal Equinox

The Autumnal Equinox is the moment when the Sun crosses the celestial equator from north to south, marking the beginning of autumn in the Northern Hemisphere and spring in the Southern Hemisphere. It occurs around September 22 or 23 each year, when day and night are nearly equal in length.

Also known as: Fall Equinox, September Equinox, Autumn Equinox

Azimuth

Azimuth is the horizontal angle measured clockwise from true north to the direction of the sun's position in the sky. It is an important concept in understanding the sun's movement and its impact on daylight.

Also known as: Solar azimuth, Sun angle, Solar angle, Sun direction

B

Blue Hour

Blue Hour is the brief period of twilight around dawn and dusk when the sky takes on a deep blue color, occurring just before sunrise or just after sunset.

Also known as: l'heure bleue, twilight blue, dusk blue, dawn blue

C

Circadian Rhythm

Circadian rhythm is the natural, internal process that regulates the sleep-wake cycle and repeats roughly every 24 hours. It's influenced by environmental cues, primarily light and darkness.

Also known as: biological clock, sleep-wake cycle, body clock, diurnal rhythm

Civil Twilight

Civil twilight is the period of time just after sunset or just before sunrise when the sun is below the horizon, but there's still enough natural light for most outdoor activities to be carried out without artificial illumination.

Also known as: Civil dusk, Civil dawn, Twilight period, Gloaming

D

Dark Twilight

Dark twilight, also known as astronomical twilight, is the darkest stage of twilight that occurs when the sun is between 12 and 18 degrees below the horizon. During this period, the sky appears almost completely dark to the naked eye, but some sunlight still illuminates the upper atmosphere.

Also known as: Astronomical twilight, Night twilight, Dusk's end, Twilight's darkest phase

Daybreak

Daybreak, also known as dawn, is the time at the beginning of the day when light first appears in the sky, just before the sun rises above the horizon.

Also known as: Dawn, First light, Crack of dawn, Sunrise, Break of day

Daylight Hours

Daylight hours refer to the period of time between sunrise and sunset when natural sunlight is present and visible.

Also known as: Sunlight hours, Hours of daylight, Daytime length, Sunshine duration
Related terms: Daytime Length

Daytime Length

Daytime length is the duration of time between sunrise and sunset when the sun is visible above the horizon, varying throughout the year and by location on Earth.

Also known as: Day length, Length of day, Sunlight duration, Daylight period, Solar day length
Related terms: Daylight Hours

E

Eclipse Of The Sun

An eclipse of the sun, also known as a solar eclipse, is a celestial event where the moon passes between the Earth and the sun, temporarily blocking some or all of the sun's light from reaching Earth.

Also known as: Solar eclipse, Sun eclipse, Eclipse of the sun, Solar occultation

Elevation

Solar elevation is the angle between the horizon and the center of the sun's disc in the sky, measured in degrees. It indicates how high the sun appears in the sky at a specific time and location on Earth.

Also known as: Sun angle, Solar altitude, Sun elevation angle, Solar height

Equinox

An equinox is a moment in time when the Sun crosses the Earth's equator, making day and night nearly equal in length all around the world.

Also known as: Vernal Equinox, Autumnal Equinox, Spring Equinox, Fall Equinox

F

Fall Equinox

The Fall Equinox, also known as the Autumnal Equinox, is the moment when the Sun crosses the celestial equator from north to south, marking the beginning of autumn in the Northern Hemisphere and spring in the Southern Hemisphere. It occurs around September 22 or 23 in the Northern Hemisphere, when day and night are nearly equal in length.

Also known as: Autumnal Equinox, September Equinox, Autumn Equinox

First Light

First Light refers to the moment when sunlight first becomes visible on the horizon at the beginning of a new day, marking the initial appearance of the sun before it fully rises.

Also known as: Dawn's early light, Daybreak, First rays of sun, Morning's first light

G

Golden Hour

The Golden Hour, also known as the "magic hour," is the period shortly after sunrise or before sunset when daylight is redder and softer compared to when the sun is higher in the sky. This time is particularly valued by photographers and cinematographers for its warm, diffused light that creates stunning visual effects.

Also known as: Magic Hour, Sweet Light, Golden Light

H

Hours Of Daylight

Hours of daylight refer to the period of time between sunrise and sunset when natural sunlight is visible and illuminates the Earth's surface.

Also known as: Daylight duration, Sunlight hours, Length of day, Daytime length

L

L'heure Bleue

L'heure Bleue, French for "The Blue Hour," is a brief period during twilight, occurring just before sunrise or after sunset, when the sky takes on a deep blue color and indirect sunlight creates a soft, ethereal atmosphere.

Also known as: Blue Hour, Twilight, Golden Blue Hour, Magic Hour

Local Apparent Noon

Local Apparent Noon is the moment when the Sun appears highest in the sky from a specific location on Earth, marking the midpoint of the Sun's visible path across the sky for that day.

Also known as: Apparent Noon, True Solar Noon, Local Noon, Midday Sun

Longest Day

The Longest Day refers to the day of the year with the most daylight hours, typically occurring on the summer solstice in the Northern Hemisphere (around June 20-22) and on the winter solstice in the Southern Hemisphere (around December 21-23).

Also known as: Summer Solstice, Midsummer, Litha, Longest Day of the Year, Peak Daylight

M

March Equinox

The March Equinox, also known as the Spring Equinox in the Northern Hemisphere, is the moment when the Sun crosses the celestial equator from south to north, marking the beginning of spring in the Northern Hemisphere and autumn in the Southern Hemisphere.

Also known as: Spring Equinox, Vernal Equinox, Northward Equinox, March Equinox

Midsummer

Midsummer is the period of time centered around the summer solstice, typically occurring on June 20th or 21st in the Northern Hemisphere. It marks the longest day and shortest night of the year, when the Sun reaches its highest point in the sky.

Also known as: Summer Solstice, Litha, St. John's Day, Midsummer's Day, Longest Day

Midwinter

Midwinter is the period of time centered around the winter solstice, typically occurring on December 21 or 22 in the Northern Hemisphere. It marks the shortest day and longest night of the year, when the Sun reaches its lowest point in the sky at noon.

Also known as: Winter solstice, Shortest day, Longest night, Yule, Midwinter's Day

N

Nautical Twilight

Nautical twilight is the period of time when the center of the sun is between 6 and 12 degrees below the horizon, occurring just before sunrise or just after sunset. During this time, the horizon and some objects are still visible, but artificial light is generally required for outdoor activities.

Also known as: Blue hour, Navigational twilight, Mariner's twilight, Sailor's twilight
Related terms: Navigational Twilight

Navigational Twilight

Navigational Twilight is the period of twilight when the sun is between 6 and 12 degrees below the horizon, during which celestial navigation using the horizon and stars becomes possible.

Also known as: Nautical Twilight, Maritime Twilight, Seafarer's Twilight
Related terms: Nautical Twilight

Nightfall

Nightfall is the onset of darkness at the end of the day, marking the transition from twilight to night when the sun has completely set below the horizon.

Also known as: Dusk, Sundown, Darkness, Evenfall, Twilight's end

S

Solar Altitude

Solar altitude is the angle between the horizon and the center of the sun's disc, measured in degrees. It indicates how high the sun appears in the sky from a specific location on Earth at a given time.

Also known as: Sun angle, Solar elevation angle, Sun height

Solar Azimuth

Solar azimuth is the horizontal angle between the sun's position in the sky and true north, measured clockwise around the observer's horizon.

Also known as: Sun angle, Solar angle, Sun position, Solar direction
Related terms: Solar Altitude, Sun Angle

Solar Eclipse

A solar eclipse is a celestial event where the Moon passes between the Sun and Earth, temporarily blocking some or all of the Sun's light from reaching Earth's surface.

Also known as: Sun eclipse, Eclipse of the Sun, Solar occultation

Solar Noon

Solar noon is the moment when the Sun reaches its highest point in the sky during a given day, marking the midpoint between sunrise and sunset.

Also known as: Midday sun, True noon, Local apparent noon, Sun transit time
Related terms: Sun Transit Time

Solstice

A solstice is an astronomical event that occurs twice a year when the Sun reaches its highest or lowest point in the sky at noon, marking the longest and shortest days of the year.

Also known as: Longest day, Shortest day, Summer solstice, Winter solstice, Midsummer, Midwinter
Related terms: Summer Solstice

Spring Equinox

The Spring Equinox, also known as the Vernal Equinox, is an astronomical event that occurs when the Sun crosses the celestial equator, marking the beginning of spring in the Northern Hemisphere and autumn in the Southern Hemisphere. During this time, day and night are approximately equal in length.

Also known as: Vernal Equinox, March Equinox, Northward Equinox

Summer Solstice

The Summer Solstice is the day when the Sun reaches its highest point in the sky, marking the longest day and shortest night of the year in the Northern Hemisphere.

Also known as: Midsummer, Longest day of the year, June Solstice, Litha, Estival solstice
Related terms: Solstice

Sun Angle

Sun angle refers to the height of the sun in the sky relative to the horizon, measured in degrees. It changes throughout the day and year, affecting the intensity and duration of sunlight a location receives.

Also known as: Solar elevation, Sun elevation, Solar altitude, Sun height

Sundial

A sundial is a device that tells time using the position of the sun's shadow cast by a vertical stick or plate called a gnomon onto a flat surface marked with hour lines.

Also known as: Sun clock, Solar timepiece, Shadow clock, Gnomon

Sun Direction

Sun direction refers to the apparent position of the sun in the sky relative to an observer on Earth, typically described by its azimuth (horizontal angle) and elevation (vertical angle). It changes throughout the day and varies with seasons and geographical location.

Also known as: solar position, sun angle, solar direction, sun path
Related terms: Sun Angle, Sun Path

Sundown

Sundown, also known as sunset, is the daily disappearance of the Sun below the western horizon as a result of Earth's rotation.

Also known as: Sunset, Dusk, Twilight, Nightfall, Eventide
Related terms: Sunset

Sun Elevation Angle

Sun elevation angle is the vertical angle between the horizon and the center of the sun's disc in the sky. It measures how high the sun appears above the horizon at any given time and location on Earth.

Also known as: Solar Altitude, Sun Angle, Solar Elevation
Related terms: Solar Altitude, Sun Angle

Sunlight Duration

Sunlight duration, also known as sunshine duration, is the length of time the sun's direct rays are visible at a specific location on Earth's surface during a given day or period.

Also known as: Sunshine duration, Sunshine hours, Sun hours, Insolation period
Related terms: Sunshine Duration

Sun Path

A sun path is the apparent daily and seasonal movement of the sun across the sky as observed from a specific location on Earth. It describes the sun's changing position throughout the day and year, which affects daylight hours, shadows, and solar energy potential.

Also known as: Solar path, Sun trajectory, Solar trajectory, Sun movement, Solar movement
Related terms: Sun Direction

Sunrise

Sunrise is the daily phenomenon when the sun first appears on the horizon, marking the beginning of daylight in a specific location on Earth.

Also known as: Daybreak, Dawn, First light, Sunup, Cockcrow

Sunset

A sunset is the daily phenomenon when the Sun appears to sink below the horizon in the western sky, marking the end of daylight and the transition from day to night.

Also known as: Sundown, Dusk, Twilight, Nightfall
Related terms: Sundown

Sunshine Duration

Sunshine duration is the total amount of time in a day when direct sunlight reaches the Earth's surface, typically measured in hours.

Also known as: Sunlight hours, Hours of sunshine, Sun hours, Insolation duration
Related terms: Sunlight Duration

Sun Transit Time

Sun Transit Time is the moment when the sun reaches its highest point in the sky during a given day, crossing the local meridian. This occurs at solar noon and marks the midpoint of the sun's visible path across the sky.

Also known as: Solar noon, Midday sun, Sun culmination, Solar meridian passage, Local apparent noon
Related terms: Solar Noon

T

True Noon

True Noon, also known as Local Apparent Noon, is the moment when the Sun reaches its highest point in the sky at a specific location, marking the middle of the day based on the Sun's position rather than clock time.

Also known as: Local Apparent Noon, Solar Noon, Midday, High Noon

U

Ultraviolet Light

Ultraviolet (UV) light is a type of electromagnetic radiation emitted by the sun that has shorter wavelengths than visible light but longer wavelengths than X-rays. It is invisible to the human eye and can have both beneficial and harmful effects on living organisms and materials.

Also known as: UV light, UV radiation, Ultraviolet radiation

V

Vernal Equinox

The Vernal Equinox, also known as the Spring Equinox, is the moment when the Sun crosses the celestial equator from south to north, marking the beginning of spring in the Northern Hemisphere and autumn in the Southern Hemisphere. It occurs around March 20 or 21 each year, when day and night are approximately equal in length.

Also known as: Spring Equinox, March Equinox, Northward Equinox, First Point of Aries

Vitamin D

Vitamin D is a fat-soluble vitamin that is essential for human health, primarily produced in the skin when exposed to sunlight. It plays a crucial role in calcium absorption, bone health, and immune function.

Also known as: Sunshine vitamin, Calciferol, Ergocalciferol, Cholecalciferol

W

Winter Solstice

The Winter Solstice is the day when the Northern Hemisphere experiences the shortest period of daylight and the longest night of the year, typically occurring around December 21 or 22 in the Northern Hemisphere.

Also known as: Midwinter, Hibernal Solstice, December Solstice, Shortest Day of the Year