# library(tibble) options(tibble.print_max = 6, tibble.print_min = 4)
We load two packages, our 'photobiology' and 'lubridate', as they will be used in the examples.
The functions and methods described in this section 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
unit.out which accepts the following arguments
"seconds". For backwards compatibility
"date" is also
accepted as equivalent to
"datetime" but deprecated.
In photobiology research we sometimes need to calculate the position on the sun at arbitrary geographic locations and times. The function
sun_angles returns the azimuth in degrees eastwards, altitude in degrees above the horizon, solar disk diameter in degrees and sun to earth distance in astronomical units. The time should be a
POSIXct vector, possibly of length one. The easiest way create date and time constant values is to use package
geocodemost functions also had the redundant formal parameters
latwhich were removed in version 0.9.12. This change may require users' scripts to be revised.
For calculation of the position of the sun we need to supply geographic coordinates and a time instant. The object to be supplied as argument for
geocode is a data frame with variables
lat giving the location of Earth. This matches the return value of function
ggmap::geocode(), function that can be used to find the coordinates using any address entered as a character string understood by the Google maps API. We use the "geocode" for Helsinki.
Be aware that to obtain results based on local time, the correct time zone needs to be set. 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
my.geocode <- data.frame(lat = 60.16, lon = 24.93, address = "Helsinki")
The position of the sun at Helsinki, at the given time instant decoded for time zone Eastern European Time.
sun_angles(time = ymd_hms("2017-06-20 08:00:00", tz = "EET"), geocode = my.geocode)
Functions have defaults for their arguments, but rarely Greenwich will be the location you are interested in.
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 = "EET") + hours(c(0, 6, 12)), geocode = my.geocode)
my.times <- ymd_hms("2014-01-01 0:0:0", tz = "EET") + 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)
When spectra contain suitable metadata, the position of the sun for the spectral irradiance data measurement can be easily obtained.
sun_angles(time = getWhenMeasured(sun.spct), geocode = getWhereMeasured(sun.spct))
One what is needed is only one of the solar angles, functions returning vectors instead of data frames can be useful.
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)
night_length have all the same parameter signature. An additional function,
day_night returns a data frame containing all the quantities returned by the other functions. They are all vectorized for the
geocode parameters. As arguments are the same for all these functions, we use
sunrise_time in the examples below, but they apply to the other functions described in this section.
Both latitude and longitude should be supplied through a
geocode, but be aware that 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. We reuse the geocode data frames created above, and create a vector
of datetime objects differing in their date. The default time zone of function
ymd is UTC, so we override it with EET to match the geocodes for Finnish cities.
dates <- ymd("2015-03-01", tz = "EET") + months(0:5) dates
sunrise_time(now("UTC"), tz = "UTC", geocode = my.geocode) sunrise_time(now("EET"), tz = "EET", geocode = my.geocode)
Southern hemisphere latitudes as well as longitudes to the West of the Greenwich meridian should be supplied as negative numbers.
sunrise_time(dates, geocode = data.frame(lat = 60, lon = 0)) sunrise_time(dates, geocode = data.frame(lat = -60, lon = 0))
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
"none" meaning that times correspond to the occlusion of the upper rim of the sun below the theoretical horizon.
sunrise_time(ymd("2017-03-21", tz = "EET"), tz = "EET", geocode = my.geocode, twilight = "civil") sunrise_time(ymd("2017-03-21", tz = "EET"), tz = "EET", geocode = my.geocode, twilight = -10) sunrise_time(ymd("2017-03-21", tz = "EET"), tz = "EET", geocode = my.geocode, twilight = +12)
Default latitude is zero (the Equator), the default longitude is zero (Greenwich). Be also aware that for summer dates the times are formatted for printing accordingly. In the examples below this can be recognized by the time zone being reported as EEST instead of EET during the summer for Eastern Europe.
The main function is called
day_night and returns a data frame containing both the input values and the results of the calculations. See below for additional 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
"hours" with decimal fractions, as seen above. However other useful units like
"days" can be useful.
day_night(dates[1:2], 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[1:2], 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)
Different convenience functions return the calculated variables individually as vectors.
sunrise_time(date = dates, geocode = my.geocode)
As seen above the default for
tz is the time zone of the argument passed to
date. This can be overridden with an explicit value as argument. In this example, when interpreted in the UTC time zone, the time instants correspond to the previous calendar day compared to the EET time zone. We can also see that "summer time" applies to the EET time zone but not to UTC (universal time coordinates).
sunrise_time(date = dates, tz = "UTC", geocode = my.geocode)
noon_time(date = dates, geocode = my.geocode)
sunset_time(date = dates, geocode = my.geocode)
The default for
date is the current day, using the system time zone settings.
noon_time(geocode = my.geocode)
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
sunrise_time(ymd("2017-03-21", tz = "EET"), tz = "EET", geocode = my.geocode) sunrise_time(ymd("2017-03-21", tz = "EET"), tz = "EET", geocode = my.geocode, unit.out = "hours")
night_length return by default the length of time in hours.
day_length(dates, geocode = my.geocode) night_length(dates, geocode = my.geocode)
day_length(dates, geocode = my.geocode, unit.out = "days") night_length(dates, geocode = my.geocode, unit.out = "days")
In field research it is in many cases preferable to sample or measure, and present and plot data based on local solar time. Two functions are provided. They differ in the value returned, either a time of day in hours, or a datetime.
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)
as_tod() facilitates conversion of R's time date objects into a
numeric value representing the time of day in one of hour, minute or second
as unit of expression.
times <- now(tzone = "UTC") + hours(0:6) times as_tod(times) as_tod(times, unit.out = "minutes")
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 the UVB region. Function
relative_AM() can be used to calculate an empirical estimate of this quantity from elevation angles in degrees. This function is vectorised.
relative_AM(c(80, 60, 40, 20))
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