Glossary of Sunrise, Twilight and Moon Terms

This glossary explains the terms used on SunriseAlmanac. Where a term depends on a specific sun or moon altitude, the value given is the one our calculations use; see the methodology page for details.

A

Altitude (solar elevation)

Altitude is the angle of the sun or moon above the horizon, measured in degrees. An altitude of 0° means the body's centre is on the geometric horizon, 90° means it is directly overhead, and negative values mean it is below the horizon. Sunrise, sunset, twilight, golden hour and blue hour are all defined by specific solar altitudes.

Astronomical twilight

Astronomical twilight is the period when the centre of the sun is between 12° and 18° below the horizon. In the evening it ends when the sun reaches −18°, after which the sky is fully dark and the faintest stars are visible. In summer at latitudes above about 48.5°, the sun may never sink to −18°, so astronomical twilight continues all night.

Azimuth

Azimuth is the compass direction of the sun or moon, measured in degrees clockwise from true north: 90° is east, 180° is south and 270° is west. The sun rises exactly due east and sets due west only around the equinoxes. In summer it rises and sets toward the poleward side of east and west, and in winter toward the equatorward side.

B

Blue hour

Blue hour is the period of twilight when the sun is between 4° and 6° below the horizon and the sky takes on a deep blue color. It occurs just before morning golden hour and just after evening golden hour.

C

Civil twilight

Civil twilight is the period when the centre of the sun is between the horizon (at sunrise or sunset) and 6° below it. During civil twilight there is usually enough natural light for most outdoor activities without artificial lighting. Morning civil twilight begins at first light and evening civil twilight ends at last light.

D

Day length

Day length is the time between sunrise and sunset on a given date. Because sunrise and sunset are defined by the upper edge of the sun and include refraction, day length at the equinoxes is slightly longer than 12 hours. Day length varies most through the year at high latitudes and least near the equator.

Daylight saving time

Daylight saving time (DST) is the practice of setting clocks one hour ahead of standard time during the warmer months, shifting an hour of daylight from the morning to the evening. In 2026 the US and most of Canada observe DST from 8 March to 1 November, and the UK and Ireland from 29 March to 25 October. See the daylight saving time dates for every country we cover.

E

Equation of time

The equation of time is the difference between apparent solar time (shown by a sundial) and mean solar time (shown by a clock). It ranges from about 14 minutes negative in mid-February to about 16 minutes positive in early November. It is why the earliest sunset does not fall on the winter solstice.

Equilux

An equilux is a date on which day and night are as close as possible to equal length, 12 hours each. Because refraction and the sun's apparent size make the day slightly longer than the night at the equinox, the equilux occurs a few days before the spring equinox and a few days after the autumn equinox. The exact date depends on latitude.

Equinox

An equinox is the moment when the sun crosses the celestial equator, around 20 March and 22 or 23 September. The March equinox marks the start of astronomical spring in the northern hemisphere and autumn in the southern hemisphere. See solstices and equinoxes for exact times.

F

First light

First light is the moment morning civil twilight begins, when the centre of the sun rises to 6° below the horizon. Some military and aviation sources use a different threshold, so definitions can vary.

Full moon

A full moon is the instant when the moon is opposite the sun in ecliptic longitude, so the side facing Earth is fully lit. It occurs about every 29.5 days. Our moon phase calendar lists every full moon with its traditional name.

G

Golden hour

Golden hour is the period when the sun is low in the sky, between 6° above and 4° below the horizon, giving warm, soft, directional light. It occurs shortly after sunrise and shortly before sunset. Despite the name, its length varies with latitude and season, from well under an hour in the tropics to several hours near the poles in summer.

H

Harvest moon

The Harvest Moon is the full moon closest to the September equinox. Around this time in the northern hemisphere, the moon rises only a little later on successive evenings, providing bright light soon after sunset for several days.

I

Illumination (moon)

Illumination is the fraction of the moon's visible disk that is lit by the sun, as seen from Earth, expressed as a percentage. It is 0% at new moon, 50% at first and last quarter and 100% at full moon.

L

Last light

Last light is the moment evening civil twilight ends, when the centre of the sun sinks to 6° below the horizon. As with first light, some sources use a different threshold.

M

Moonrise

Moonrise is the moment the upper edge of the moon appears above a sea-level horizon. On SunriseAlmanac it is calculated with the moon's parallax and standard atmospheric refraction taken into account. Because the moon rises about 50 minutes later each day on average, there is roughly one day each month with no moonrise.

Moonset

Moonset is the moment the upper edge of the moon disappears below a sea-level horizon, calculated with parallax and refraction. Like moonrise, moonset skips one calendar day roughly once a month.

N

Nautical twilight

Nautical twilight is the period when the centre of the sun is between 6° and 12° below the horizon. Historically it was the time when sailors could see both the horizon and bright stars for navigation.

New moon

A new moon is the instant when the moon and the sun share the same ecliptic longitude, so the side of the moon facing Earth is unlit. The new moon is usually invisible because it is close to the sun in the sky.

P

Perigee and apogee

Perigee is the point in the moon's elliptical orbit where it is closest to Earth, and apogee is where it is farthest. The moon's distance varies from roughly 356,500 km at the closest perigees to about 406,700 km at the farthest apogees. A full moon near perigee looks slightly larger and brighter than one near apogee.

Polar day (midnight sun)

Polar day is a period when the sun stays above the horizon for 24 hours or more. It occurs in summer north of the Arctic Circle and south of the Antarctic Circle, for example in northern Alaska and Nunavut. On such days a city page states that the sun does not set.

Polar night

Polar night is a period when the sun stays below the horizon for 24 hours or more. It occurs in winter at high polar latitudes, such as Utqiagvik, Alaska. Polar night is not necessarily total darkness, because twilight can still brighten the sky around midday.

R

Refraction

Atmospheric refraction is the bending of light as it passes through the atmosphere, which makes the sun and moon appear higher than they really are. At the horizon the effect is about 34 arcminutes, slightly more than the width of the sun. Refraction is why the sun is visible for a few minutes before its geometric rise and after its geometric set.

S

Solar noon

Solar noon is the moment the sun crosses the local meridian and reaches its highest altitude for the day. It rarely falls at exactly 12:00 on the clock, because of the observer's position within the time zone, daylight saving time and the equation of time. Sunrise and sunset are roughly symmetric around solar noon.

Solstice

A solstice is the moment when the sun reaches its northernmost or southernmost position relative to the celestial equator, around 21 June and 21 or 22 December. The June solstice brings the longest day in the northern hemisphere and the shortest in the southern hemisphere; the December solstice does the reverse. See solstices and equinoxes for exact times.

Sunrise

Sunrise is the moment the upper edge of the sun appears above the horizon in the morning. SunriseAlmanac calculates it for a sea-level horizon with standard refraction, which corresponds to the sun's centre being 0.833° below the horizon.

Sunset

Sunset is the moment the upper edge of the sun disappears below the horizon in the evening. It uses the same definition as sunrise: the sun's centre at an altitude of −0.833°, on a sea-level horizon with standard refraction. Hills or buildings to the west will make the sun disappear earlier than the listed time.

Supermoon

A supermoon is a full moon that occurs when the moon is near perigee and therefore unusually close to Earth. SunriseAlmanac labels a full moon a supermoon when the moon is closer than about 361,900 km. A supermoon can appear up to about 14% larger and 30% brighter than a full moon at apogee.

Synodic month

The synodic month is the average time between two successive identical moon phases, such as from one new moon to the next. It averages about 29.53 days, but individual lunations vary by several hours because of the shape of the moon's orbit.

T

Twilight

Twilight is the period before sunrise and after sunset when the sky is partly lit by sunlight scattered in the upper atmosphere. It is divided into three stages by the sun's depth below the horizon: civil (to −6°), nautical (to −12°) and astronomical (to −18°).

W

Waxing and waning

The moon is waxing when its illuminated portion is growing, from new moon to full moon, and waning when it is shrinking, from full moon back to new moon. A waxing moon is visible in the evening sky, and a waning moon is best seen late at night and in the morning. The phase names crescent and gibbous describe whether less or more than half of the disk is lit.