Solar time

This time calendar contains solar time units, where the boundary of each day is at apparent solar midnight. Solar events define the ampm (midnight and noon) and illumination (dawn, sunrise, sunset, dusk) units.

Usage

cal_time_solar

Format

A location-based calendar containing solar time units.

Details

The following time units are available in the solar calendar systems.

• day(): Day unit

• ampm(): Half-day units (AM = before solar noon, PM = after solar noon)

• hour(): Hour units within the solar day

• minute(): Minute units within the solar hour

• second(): Second units within the solar minute

• degree(): Solar angle units within the day

• arcminute(): Arcminute units within the solar degree

• arcsecond(): Arcsecond units within the solar arcminute

• illumination(): Illumination phases (night, astronomical/nautical/civil dawn, day, civil/nautical/astronomical dusk)

AM/PM half-days

The ampm unit divides each solar day into two halves between solar noon and solar midnight:

Half	Period	Description
AM	Solar midnight to noon	Morning half; before solar transit
PM	Solar noon to midnight	Afternoon half; after solar transit

Solar illumination phases

Phases describe the illumination state of the sky and correspond to standard twilight definitions used in astronomy and navigation. Each phase is bounded by a pair of solar altitude thresholds:

Phase	Solar altitude range	Description
Night	< -18°	Sky fully dark; from last dusk to first dawn (spans noon)
Astronomical dawn	-18° to -12°	Astronomical twilight before sunrise; faint objects obscured
Nautical dawn	-12° to -6°	Nautical twilight before sunrise; horizon visible at sea
Civil dawn	-6° to -0.833°	Civil twilight before sunrise; sky brightening in the east
Day	> -0.833°	Sun above the horizon; spans solar noon
Civil dusk	-0.833° to -6°	Civil twilight after sunset; sky fading in the west
Nautical dusk	-6° to -12°	Nautical twilight after sunset; horizon visible at sea
Astronomical dusk	-12° to -18°	Astronomical twilight after sunset; faint objects obscured

The -0.833° threshold for sunrise and sunset accounts for the mean angular radius of the solar disc (0.267°) plus the standard atmospheric refraction at the horizon (0.566°). Noon and midnight are derived from the equation of time rather than a fixed altitude. Locations that experience polar day or polar night (civil days where sunrise does not occur) are not currently supported, it is recommended to use an alternative reference location.

See also

cal_time_civil

Examples

# Find the current solar time in Melbourne
datetime(Sys.time(), calendar = cal_time_solar, lat = -37.8136, lon = 144.9631)
#> <mixtime[1]>
#> [1] 2026-05-08 01:53:57 [37.81S 144.96E]