Astronomical Definitions: A Comprehensive Guide

Most of the terms below have a precise technical definition and a looser everyday one. Where those two disagree, the disagreement is noted, because that is usually where the confusion starts.

Sunrise and Sunset

Sunrise is the instant the upper edge of the sun’s disk first appears above the horizon. Sunset is the instant the upper edge disappears below it. Neither refers to the center of the disk.

Both definitions include a correction for atmospheric refraction, which bends light near the horizon and lifts the sun’s apparent position by about 34 arcminutes. Add the sun’s 16 arcminute radius and you get the standard: sunrise and sunset happen when the sun’s geometric center is 50 arcminutes, or 0.833 degrees, below the horizon. That is why calculators use a zenith angle of 90.833 degrees rather than 90. The full derivation is in how sunrise is calculated.

A detail that surprises people: the earliest sunset of the year does not fall on the winter solstice. At 40 degrees north the earliest sunset is around December 9, the shortest day is December 21, and the latest sunrise is around January 5. The three dates are spread over almost a month, because the equation of time, described in the next section, is shifting solar noon later each day through that period. The same effect puts the earliest sunrise a week or so before the June solstice. The full picture, including how the spread widens as you move toward the equator, is in the shortest day of the year.

Solar Noon

Solar noon is the instant the sun crosses your local meridian, reaching its highest point of the day. It marks the midpoint between sunrise and sunset.

It rarely lands on 12:00 on your clock, for four reasons that stack:

• Time zone width. Your zone runs on one reference meridian, and you are probably not standing on it. Every degree of longitude away from it shifts solar noon by 4 minutes.
• Daylight saving time. An hour of pure bookkeeping offset.
• The equation of time. Earth’s orbit is elliptical and its axis is tilted, so the sun runs ahead of or behind clock time by anywhere from about 14 minutes slow to 16 minutes fast over the year.
• Zone boundaries drawn for politics. Some zones are enormous. Solar noon in western China arrives close to 3 pm on the official clock.

Twilight, Dawn, and Dusk

Twilight is the period when the sun is below the horizon but the sky is still lit by sunlight scattering off the upper atmosphere. It happens twice a day and comes in three phases, defined by how far the sun has sunk:

• Civil twilight: sun 0 to 6 degrees below the horizon. Bright enough to work and move around outside without lights.
• Nautical twilight: 6 to 12 degrees. Stars are visible and the sea horizon is still faintly distinguishable, which is the combination that allows sextant navigation.
• Astronomical twilight: 12 to 18 degrees. Dark enough for most stargazing, but a faint glow still washes out the dimmest objects.

Night begins when the sun passes 18 degrees below the horizon. Astronomical twilight is the last phase before full darkness, not full darkness itself, and that distinction gets mangled constantly.

Dawn and dusk are the boundary instants rather than periods. Each twilight phase has one of each: civil dawn and civil dusk at 6 degrees, nautical at 12, astronomical at 18. A calculator showing a single time labeled “dusk” means civil dusk. See sunset vs dusk for where the everyday usage diverges.

How long all of this takes depends heavily on latitude, from about 70 minutes near the equator to never on a June night in London. The numbers by latitude are worth a look before planning around them.

Golden Hour and Blue Hour

Neither is an hour, and neither is an official astronomical term.

Golden hour is the stretch before sunset (or after sunrise) when the sun is low enough that its light is warm and shadows run long, usually taken as the sun between 6 degrees above the horizon and 4 degrees below it, which is the range this site’s API reports. Nearer the poles it can run past two hours; in the tropics it is closer to 40 minutes. Golden hour and blue hour has the measured durations by latitude and season.

Blue hour is the part of civil twilight after the sun is down, when the sky is lit mostly by short wavelengths scattered from high above and turns a deep even blue. It runs from 4 degrees below the horizon to 6 degrees below, which in the mid-latitudes is ten to fifteen minutes rather than an hour.

Twilight Phenomena

Five things around sunset that have proper names, and get called the wrong ones often enough to be worth listing.

Green flash. A green rim or spot on the top edge of the sun for a second or two as the last of the disk goes under. The atmosphere disperses sunlight like a weak prism, so the green image of the sun sets fractionally after the red one. It needs a sharp, distant, unobstructed horizon, which usually means the ocean, and air clear enough that the sun stays bright right down to the end.

Earth’s shadow. A dark blue-gray band that rises in the eastern sky starting a few minutes after sunset. It is the planet’s own shadow projected onto the atmosphere, and it climbs as the sun sinks further. The reverse happens before sunrise, when it sinks in the west.

Belt of Venus. The pink to salmon arch sitting directly on top of Earth’s shadow, roughly 10 to 20 degrees above the eastern horizon. Also called the anti-twilight arch. It is sunlight that has already been reddened by a long path through the atmosphere and is then scattered back toward you. Clean dry air makes it obvious and haze erases it.

Purple light. A violet glow well above the western horizon, strongest around 15 to 25 minutes after sunset. It comes from sunlight scattering off fine aerosol in the stratosphere rather than off the lower atmosphere, and it is much stronger in the years after a large volcanic eruption.

Alpenglow. The reddish light on mountain faces after the sun has already set for the observer. Strictly it refers to the indirect glow from light backscattered by the atmosphere once the sun is below the horizon, though the word gets used loosely for any warm light on a summit.

Moonrise and Moonset

Moonrise and moonset are the moments the moon’s upper edge crosses the horizon. The moon rises in the east and sets in the west for the same reason the sun does, which is Earth’s rotation rather than the moon’s own motion.

The moon also orbits Earth, moving about 13 degrees east against the stars each day, so it rises an average of about 50 minutes later each day. That lag is why some calendar days have no moonrise at all: the moon’s rising slides past midnight and skips a date.

Lunar Phases

As the moon orbits, the fraction of its lit half facing us changes. The cycle runs 29.5 days from new moon to new moon.

• New moon: moon between Earth and sun. The lit side faces away and the disk is dark.
• Waxing crescent: a growing sliver, visible in the west after sunset.
• First quarter: half the visible disk lit, one quarter of the way through the cycle.
• Waxing gibbous: more than half lit and still growing.
• Full moon: Earth between the moon and sun, the visible disk fully lit.
• Waning gibbous: past full and shrinking.
• Last quarter: half lit again, on the opposite side from first quarter.
• Waning crescent: a thinning sliver, visible in the east before sunrise.

“Quarter” refers to the position in the cycle, not the amount lit. A quarter moon looks half full, which is a naming choice that has confused people for centuries.

Eclipses

Moment of totality during the 2024 total eclipse in USA

A solar eclipse happens at new moon, when the moon passes between Earth and the sun and casts a shadow on Earth. A lunar eclipse happens at full moon, when Earth passes between the sun and moon and its shadow falls on the moon.

They do not occur every month because the moon’s orbit is tilted about 5 degrees from Earth’s orbital plane, so the alignment usually misses.

Partial lunar eclipse, 2024. The curved bite out of the upper limb is the edge of Earth’s umbra. Aristotle used the fact that this shadow is always round as an argument that Earth is a sphere.

• Total eclipse: the sun is completely covered, or the moon passes fully into Earth’s umbra.
• Annular eclipse: the moon is near the far point of its orbit and looks too small to cover the sun, leaving a bright ring.
• Partial eclipse: only part of the disk is covered.
• Hybrid eclipse: shifts between annular and total along the path, because Earth’s curvature puts some observers slightly closer to the moon than others. Rare, and only a handful occur per century.

Equinoxes and Solstices

These four moments are instants in time, not whole days, and they happen simultaneously worldwide.

• Equinox: the moment the sun crosses the celestial equator, so its declination is zero. Around March 20 and September 22. Earth’s axis is tilted neither toward nor away from the sun along the Earth-sun line.
• Solstice: the moment the sun reaches its greatest declination north or south, 23.4 degrees, and its apparent north-south motion pauses before reversing. The name comes from Latin sol plus sistere, the sun standing still.
• June solstice: longest day in the Northern Hemisphere, shortest in the Southern. June 20 or 21. See the summer solstice for dates and details.
• December solstice: the reverse. December 21 or 22.

The equinox is not the day with exactly 12 hours of daylight. Because sunrise and sunset are defined by the sun’s upper edge and corrected for refraction, the day runs several minutes longer than 12 hours on the equinox itself: about 12h 07m at 40 degrees north. The date with a true 12-hour day, sometimes called the equilux, falls a few days before the March equinox and a few days after the September one. At 40 degrees north it lands between March 17 and 18, and again between September 26 and 27. The same refraction offset is why the sun rises due east a day either side of the equinox rather than on the equinox itself.

That two-to-three day gap holds across the mid and high latitudes. It stretches in the tropics: at 10 degrees north the equilux falls around March 9, eleven days early. At the equator there is no equilux at all, because the day never gets as short as 12 hours.