Search for Sun longitude — astro_search_sun

Searches for the time when the Sun reaches a specified apparent ecliptic longitude as seen from the center of the Earth.

Usage

astro_search_sun_longitude(target_lon, start_time, limit_days)

Arguments

target_lon: Numeric. The desired ecliptic longitude in degrees, relative to the true equinox of date. Must be in the range [0, 360). Conventional values: 0 = March equinox, 90 = June solstice, 180 = September equinox, 270 = December solstice.
start_time: POSIXct. The date and time for starting the search.
limit_days: Numeric. The number of days to search forward from start_time. Recommended range: 1 to 10 days.

Value

A list with element:

time: POSIXct value indicating when the Sun reaches the target longitude.

Details

This function can be used to determine equinoxes and solstices. However, for calculating all equinoxes and solstices for a calendar year, astro_seasons() is usually more convenient and efficient.

The search is performed within the time window from start_time to start_time + limit_days. It is recommended to keep the search window smaller than 10 days when possible.

Examples

# Find the March equinox in 2025
start <- as.POSIXct("2025-03-15", tz = "UTC")
astro_search_sun_longitude(0, start, 10)
#> [1] "2025-03-20 09:01:26 UTC"