User Guide"

In SunCalcMeeus: Sun Position and Daylight Calculations

Radiation, astronomy and atmosphere

The functions described here used to be part of package 'photobiology', but as of version 0.11.4 have been moved to this package. To ensure backwards compatibility, package 'photobiology' will depend on 'SunCalcMeeus' once these functions are removed.

knitr::opts_chunk$set(fig.width=8, fig.height=4)

# library(tibble)
options(tibble.print_max = 6, tibble.print_min = 4)

Getting started

We load two packages, our 'SunCalcMeeus' and 'lubridate', as they will be used in the examples.

library(SunCalcMeeus)
library(lubridate)
# if installed, we use 'lutz' to lookup time zones from geocodes
eval_lutz <- requireNamespace("lutz", quietly = TRUE)
if (eval_lutz) {library(lutz)}

Introduction

Most organisms, including plants and animals, have circadian internal clocks. These clocks are entrained to the day-night cycle through perception of light. For example, night length informs plants about seasons of the year. This information allows the synchronization of developmental events like flowering to take place at the "right" time of the year.

From the point of view of interactions between light and vegetation, the direction of the direct light beam is of interest. Hence, the position of the sun in the sky is also important for photobiology. This is the reason for the inclusion of astronomical calculations about the sun in this package. On the other hand, such calculations are also highly relevant to other fields including solar energy.

The functions and methods described in this volume return either values that represent angles or times. In the current version angles are always expressed in degrees. In the case of times, the unit of expression, can be changed through parameter unit.out which accepts the following arguments "datetime", "hours", "minutes", "seconds". For backwards compatibility "date" is also accepted as equivalent to "datetime" but deprecated.

All astronomical computations rely on the algorithms of Meuss (1998), and consequently returned values are very precise. However, these algorithms are computationally rather costly. Contrary to other faster algorithms, they give reliable estimates even for latitudes near the poles.

However, at high latitudes due to the almost tangential path of the sun to the horizon, atmospheric effects that slightly alter the apparent elevation of the sun have much larger effects on the apparent timing of events given that the solar elevation angle changes at a slower rate than at lower latitudes.

Position of the sun

The position of the sun at an arbitrary geographic locations and time instant can be described with two angles: elevation above the horizon and azimuth angle relative to the geographic North. If expressed in degrees, solar elevation ($h$) varies between -90 and 90 degrees, while being visible when angles are positive and otherwise occluded below the horizon. Azimuth angle ($\alpha$) varies clockwise from North between 0 and 360 degrees. Zenith angle ($z$), provides the same information as the elevation angle but using the zenith as starting point, consequently taking values between 0 and 180 degrees, such that $z = 90 - h$. Declination angle describes the angle between the plane of the Equator and the plane of the Earth's orbit around the sun, and varies with the seasons of the year.

The function sun_angles returns location, civil time, local solar time, the azimuth in degrees eastwards, elevation in degrees above the horizon, declination, the equation of time and the hour angle.

For calculation of the position of the sun we need to supply geographic coordinates and a time instant. The time instant passed as argument should be a POSIXct vector, possibly of length one. The easiest way create date and time constant values is to use package 'lubridate'.

The object to be supplied as argument for geocode is a data frame with variables lon and lat giving the location on Earth. This matches the return value of functions tidygeocoder::geo_osm(), tidygeocoder::geo_google() and ggmap::geocode(), functions that can be used to find the coordinates using an address entered as a character string understood by the OSM or Google maps APIs (Google requires an API key and registration, while OSM is open). We use the "geocode" for Helsinki, defined explicitly rather than searched for.

my.geocode <- data.frame(lat = 60.16, lon = 24.93, address = "Helsinki")

Be aware that to obtain correct computed values the time zone must be correctly set for the argument passed to time. To obtain results based on local time, this time zone needs to be set in the POSIXct object or passed as a argument to tz. In the examples we use functions from package 'lubridate' for working with times and dates. The argument passed to parameter time can be a "vector" of POSIXct values. The returned value is a data.frame with one row per time instant or per geographic location.

The position of the sun at Helsinki, at the given instant in time for time zone "Europe/Helsinki", which matches Eastern European Time.

sun_angles(time = ymd_hms("2017-06-20 08:00:00", tz = "Europe/Helsinki"), 
           geocode = my.geocode)

Functions have defaults for their arguments, Greenwhich in U.K. and the corresponding time zone "UTC". In most cases Greenwich will not be the location you are interested in. Current UTC time is more likely to be a useful default as it avoids the difficulty of time shifts in local time coordinates.

sun_angles()

A vector of times is accepted as argument, and as performance is optimized for this case, vectorization will markedly improve performance compared to multiple calls to the function. The vector of times can be created on the fly, or stored beforehand.

sun_angles(time = ymd_hms("2014-01-01 0:0:0", tz = "Europe/Helsinki") + hours(c(0, 6, 12)), 
           geocode = my.geocode)

my.times <- ymd_hms("2014-01-01 0:0:0", tz = "Europe/Helsinki") + hours(c(0, 6, 12))
sun_angles(time = my.times, geocode = my.geocode)

Geocodes for several locations can be stored in a data frame with multiple rows.

two.geocodes <- data.frame(lat = c(60.16, 65.02), 
                           lon = c(24.93, 25.47),
                           address = c("Helsinki", "Oulu"))
sun_angles(time = my.times, geocode = two.geocodes)

If what is needed is only one of the solar angles, functions returning vectors instead of data frames can be useful. In their current implementation these functions do not have improved performance compared to sun_angles(). Thus if more than one angle is needed, it is more efficient to compute all angles with function sun_angles() and later extract the vectors from the returned data frame.

sun_elevation(time = my.times, geocode = my.geocode)

sun_zenith_angle(time = my.times, geocode = my.geocode)

sun_azimuth(time = my.times, geocode = my.geocode)

Times of sunrise, solar noon and sunset

Convenience functions sunrise_time(), sunset_time(), noon_time(), day_length() and night_length() have all the same parameter signature and are wrappers on function day_night(). Function day_night returns a data frame containing all the quantities returned by these other functions.

These functions are vectorized for their date and geocode parameters. They use as default location Greenwich in the U.K., and corresponding default time zone "UTC". The date is given by default by the current date based on "UTC". Universal Time Coordinate ("UTC") is the reference used to describe differences among time zones and is never modified by daylight saving time or summer time. The difference between "Europe/Helsinki" (matching Eastern European Time) and UTC is +2 hours in winter and (matching Eastern European Summer Time) +3 hours in summer.

Latitude and longitude are passed through a geocode (a data frame). If the returned value is desired in the local time coordinates of the argument passed to geocode, the time zone should match the geographic coordinates. If geocodes contain a variable "address" it will be copied to the output.

In some of the examples below we reuse the geocode data frames created above, and we here create a vector of datetime objects differing in their date. The default time zone of function ymd() is NULL, so we override it with Europe/Helsinki to match the geocodes for Finnish cities.

dates <- ymd("2015-03-01", tz = "Europe/Helsinki") + months(0:5)
dates

As first example we compute the sunrise time for the current day in Helsinki, with the result returned either in UTC or local time coordinates. Time-zone names based on continent and city ("Europe/Helsinki") or continent, country and city ("America/Argentina/Buenos Aires") are supported, while the names "EET" and "CET" and their summer-time versions are no longer supported by R (>= 4.5.0). They have been long deprecated as they do not describe true time zones, and different are within these regions have been in different time zones in the past, making it impossible some computations. Dates and the relationship between time zones and locations have been affected by changes in country boundaries and in national laws.

Use of the Olson time zone names like "Europe/Helsinki" is recommended. The list is available in R and can be searched.

grep("Argentina", OlsonNames(), value = TRUE)

The time zone in use by the computer on which R is running can be found out with the following code.

Sys.timezone()

At least in R (< 4.5.0) the "EET", "CET", etc. are still used when printing or formatting the output. When not needed, the time zone abbreviation can be disabled in printing and formatting.

# defaults to current UTC date and Greenwich, UK as location
sunrise_time()
sunrise_time(date = now(), tz = "Europe/Helsinki", geocode = my.geocode)
sunrise_time(date = now(tzone = "Europe/Helsinki"), geocode = my.geocode)
# time zone abbreviation not shown
print(sunrise_time(date = now("Europe/Helsinki"), 
                   geocode = my.geocode),
      usetz = FALSE)

---

Be aware of the behaviour of functions ymd(), dmy(), ym() and my() from package 'lubridate'. A function call like ymd("2015-03-01", tz = "UTC") returns a POSIXct object while a call like ymd("2015-03-01") is equivalent to ymd("2015-03-01", tz = NULL) and returns a Date object. Date objects do not carry time zone information in the way POSIXct objects do, and consequently Dates do not uniquely match a period between two absolute instants in time, but rather between two instants in local time. Given these features, it is safer to use POSIXct objects as arguments to the functions from package 'SunCalcMeeus'. Function today() always returns a Date while function now() always returns a POSIXct, independently of the argument passed to their tzone parameter. Consequently it is preferable to use now(), but if you do use today() make sure to path the same value as argument to parameter tzone of today() and to parameter tz of the functions from package 'SunCalcMeeus'. An instant in time and time zone strictly define a corresponding date at any location on Earth, even though the date is not the same at all these locations.

The time zone used by default for the returned value is that of the POSIXct value passed as argument to parameter date. This behaviour can be overridden by an argument passed to tz. However, to obtain a correct value expressed in local time We must make sure that the time zone matches that at the geocode location.

## Using date times as POSIXct
sunrise_time(geocode = my.geocode)
sunrise_time(date = now("UTC"), geocode = my.geocode)
sunrise_time(date = now("UTC"), tz = "UTC", geocode = my.geocode)
sunrise_time(date = now("Europe/Helsinki"), geocode = my.geocode)
sunrise_time(date = now(""), tz = "Europe/Helsinki", geocode = my.geocode)
## Using Date
# correct always as time zones match
sunrise_time(today("Europe/Helsinki"), tz = "Europe/Helsinki", geocode = my.geocode)
# sometimes the value returned will be correct and sometimes off by 1 d at Helsinki
sunrise_time(today("Australia/Canberra"), tz = "Europe/Helsinki", geocode = my.geocode)

---

We can also compute the time at solar noon and at sunset.

noon_time(now("UTC"), geocode = my.geocode)
noon_time(now("Europe/Helsinki"), geocode = my.geocode)

By default, sunset and sunrise are defined as the time when the upper rim of the solar disk is at the horizon. How to override this default to account for twilight and or obstacles that occlude the sun will be shown later.

sunset_time(now("UTC"), geocode = my.geocode)
sunset_time(now("Europe/Helsinki"), geocode = my.geocode)
sunrise_time(now("Europe/Helsinki"), geocode = my.geocode)

Functions day_length() and night_length() return the length of time between sunrise and sunset and between sunset and sunrise, respectively, by default expressed in hours and fraction.

day_length(dates, geocode = my.geocode)
night_length(dates, geocode = my.geocode)
day_length(dates, geocode = my.geocode, unit.out = "day")

Southern hemisphere latitudes as well as longitudes to the West of the Greenwich meridian should be supplied as negative numbers. In this case the time zone is abbreviated as a time difference from UTC.

sunrise_time(dates, tz = "America/Argentina/Buenos_Aires",
             geocode = data.frame(lat = -34.6, lon = -58.3))
noon_time(dates, tz = "America/Argentina/Buenos_Aires",
             geocode = data.frame(lat = -34.6, lon = -58.3))

The angle used in the twilight calculation can be supplied, either as the name of a standard definition, or as an angle in degrees (negative for sun positions below the horizon). Positive angles can be used when the time of sun occlusion behind a building, mountain, or other obstacle needs to be calculated. The default for twilight is "none" meaning that times correspond to the occlusion of the upper rim of the sun disk below the theoretical horizon.

sunrise_time(ymd("2017-03-21", tz = "Europe/Helsinki"), 
             tz = "Europe/Helsinki", 
             geocode = my.geocode,
             twilight = "none") # center of the sun disk
sunrise_time(ymd("2017-03-21", tz = "Europe/Helsinki"), 
             tz = "Europe/Helsinki", 
             geocode = my.geocode,
             twilight = "sunlight") # upper rim of the sun disk
sunrise_time(ymd("2017-03-21", tz = "Europe/Helsinki"), 
             tz = "Europe/Helsinki", 
             geocode = my.geocode,
             twilight = "rim") # lower rim of the sun disk
sunrise_time(ymd("2017-03-21", tz = "Europe/Helsinki"), 
             tz = "Europe/Helsinki", 
             geocode = my.geocode,
             twilight = "civil") # civil twilight = -6 degrees
sunrise_time(ymd("2017-03-21", tz = "Europe/Helsinki"), 
             tz = "Europe/Helsinki", 
             geocode = my.geocode,
             twilight = -10) # 10 degrees below the horizon
sunrise_time(ymd("2017-03-21", tz = "Europe/Helsinki"), 
             tz = "Europe/Helsinki", 
             geocode = my.geocode,
             twilight = +12) # 12 degrees above the horizon

Twilight is also relevant to the computation of day length and night length. The default is to use the centre of the sun disk, but this can be changed. For the values returned by day_length() and night_length() to add to 24 h they must be computed using the same twilight definition.

day_length(ymd("2017-03-21", tz = "Europe/Helsinki"), 
             tz = "Europe/Helsinki", 
             geocode = my.geocode)
day_length(ymd("2017-03-21", tz = "Europe/Helsinki"), 
             tz = "Europe/Helsinki", 
             geocode = my.geocode,
             twilight = 0)
day_length(ymd("2017-03-21", tz = "Europe/Helsinki"), 
             tz = "Europe/Helsinki", 
             geocode = my.geocode,
             twilight = "rim")
day_length(ymd("2017-03-21", tz = "Europe/Helsinki"), 
             tz = "Europe/Helsinki", 
             geocode = my.geocode,
             twilight = "civil")
day_length(ymd("2017-03-21", tz = "Europe/Helsinki"), 
             tz = "Europe/Helsinki", 
             geocode = my.geocode,
             twilight = -10)

Say if there is a mountain blocking the view above the Western horizon, we can set different twilight angles for sunrise and sunset.

day_length(ymd("2017-03-21", tz = "Europe/Helsinki"), 
           tz = "Europe/Helsinki", 
           geocode = my.geocode,
           twilight = c(0, 12))
night_length(ymd("2017-03-21", tz = "Europe/Helsinki"), 
             tz = "Europe/Helsinki", 
             geocode = my.geocode,
             twilight = c(0, 12))

Parameter unit.out can be used to obtain the returned value expressed as time-of-day in hours, minutes, or seconds since midnight, instead of the default datetime.

sunrise_time(ymd("2017-03-21", tz = "Europe/Helsinki"), 
             tz = "Europe/Helsinki", 
             geocode = my.geocode)
sunrise_time(ymd("2017-03-21", tz = "Europe/Helsinki"), 
             tz = "Europe/Helsinki", 
             geocode = my.geocode,
             unit.out = "hours")

Similarly, the units can also be selected for the values returned by day_length() and night_length().

day_length(dates, geocode = my.geocode, unit.out = "days")
night_length(dates, geocode = my.geocode, unit.out = "days")

---

The core function is called day_night() and returns a data frame containing both the input values and the results of the calculations. See above for convenience functions useful in the case one needs only one of the calculated variables. In other cases it is more efficient to compute the whole data frame and later select the columns of interest.

day_night(dates[1:3], 
          geocode = my.geocode)

The default for unit.out is "hours" with decimal fractions, as seen above. However other useful units like "seconds", "minutes", and "days" can be useful.

day_night(dates[1:3], 
          geocode = my.geocode, 
          unit.out = "days")

Finally it is also possible to have the timing of solar events returned as POSIXct time values, in which case lengths of time are still returned as fractional hours.

day_night(dates[1:3], 
          geocode = my.geocode, 
          unit.out = "datetime")

When multiple times and locations are supplied as arguments, the values returned are for all combinations of locations and times.

day_night(dates[1:3], 
          geocode = two.geocodes)

Solar time

In field research it is in many cases preferable to sample or measure, and present and plot data based on local solar time. A new class is defined in package 'SunCalcMeeus', with print() and format() method, a constructor, a conversion function and a class query function.

The constructor takes as arguments a POSIXct object describing and instant in time and a geocode describing the geographic coordinates.

Paris.geo <- data.frame(lon = 2.352222, lat = 48.85661, address = "Paris")
Paris.time <- ymd_hms("2016-09-30 06:00:00", tz = "UTC")
solar_time(Paris.time, geocode = Paris.geo)
solar_time(Paris.time, geocode = Paris.geo, unit.out = "datetime")

my.solar.t <- solar_time(Paris.time, geocode = Paris.geo)
is.solar_time(my.solar.t)
is.numeric(my.solar.t)

my.solar.d <- solar_time(Paris.time, geocode = Paris.geo, unit.out = "datetime")
is.solar_date(my.solar.d)
is.timepoint(my.solar.d)

Time of day

When analysing data as a time series the usual way to represent time is as a date plus time value, i.e., as an instant in time. In contrast, when data need to summarised or plotted as a function of time of day, the date portion of a data time representation of time becomes a nuisance.

Function as_tod() facilitates conversion of R's time date objects into values representing the time of day as numerical value giving the time elapsed since the most recent past midnight. This value can be represented as a numeric value using "day", "hour", "minute" or "second" as unit of expression. While solar time is based on the astronomical position of the sun, time of day is based on the time coordinates for a time zone.

times <- now(tzone = "UTC") + hours(0:6)
times
as_tod(times)
as_tod(times, unit.out = "minutes")

Relative air mass

Solar elevation determines the length of the path of the sun beam through the Earth's atmosphere. This affects the solar spectrum at ground level, specially in the UV-B region. Function relative_AM() can be used to calculate an empirical estimate of this quantity from elevation angles in degrees. This function is vectorised. As seem above the apparent position of the sun for an observer on Earth differs from the true astronomical position as a result of atmospheric refraction. The functions presented above, can apply a correction based on an estimate of atmospheric refraction. If the correction is applied we obtain apparent sun elevation angles suitable for estimating the relative air mass with Kasten and Youngs' (1989) equation as follows.

relative_AM(33)

relative_AM(c(90, 60, 40, 20, 10, 5, 2, 1, 0.5))

Young's (1994) equation can be used to estimate AM from the true sun elevation. At high solar elevations the impact of atmospheric refraction is very small but at low elevations is is large enough to make a clear difference in the AM estimates.

relative_AMt(33)

relative_AMt(c(90, 60, 40, 20, 10, 5, 2, 1, 0.5))

Two additional functions make it possible to compute the relative AM from time and geographic coordinates. They both apply a correction for atmospheric refraction, but using two different algorithms in the sun elevation computation or in Young's (1994) equation. The difference is small. These convenience functions are wrappers on relative_AM() and relative_AMt() that call function sun_elevation() to obtain the input to pass to the wrapped functions.

january.times <- ymd_h("2020-01-01 12", tz = "Europe/Helsinki") + hours(-2:+2)
relative_AM_geotime(january.times, my.geocode, tz = "Europe/Helsinki")

relative_AMt_geotime(january.times, my.geocode, tz = "Europe/Helsinki")

SunCalcMeeus documentation built on April 4, 2025, 1:43 a.m.