World map showing day and night regions

Question

I'm trying to add a daytime/nighttime line to a world map using ggplot in order to indicate day and night regions; something like this:

The plan is to animate my map over a 24 hour cycle like this:

The above animation is achieved using a sine wave, which I know is totally inaccurate. I'm aware that geosphere::gcIntermediate allows me to draw great-circle lines, like so:

library(ggplot2)
library(ggthemes)
library(geosphere)

sunPath1 <- data.frame(gcIntermediate(c(-179, -30), c(0, 30), n=100))
sunPath2 <- data.frame(gcIntermediate(c(0, 30), c(179, -30), n=100))
sunPath <- rbind(sunPath1, sunPath2)

ggplot(sunPath) +
  borders("world", colour = "gray95", fill = "gray90") +
  geom_ribbon(aes(lon, ymax = lat), ymin=-180, fill="black", alpha=0.2) +
  theme_map()

Although I'm not sure if it will be possible to draw the desired lines at different points during the year, e.g. in March it looks like so:

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I've had no luck finding a solution but am guessing I don't know the keywords to search for as this is way outside my sphere of knowledge. I think the answer may lie somewhere in the sunrise equation, but I've no idea how to apply these to find a solution, nor do I know how to vary these parameters over the course of the year. This website (used for the plot above) also seems useful but I'm not yet sure how!

Answer 1

I've solved this issue with the help of @jazzurro pointing me to the Leaflet R package. I've ported their javascript plugin, L.Terminator.js, to R in order to use outside of interactive leaflet maps.

Function is available here.

Here's an example of a 24 hr animation:

library(dplyr)
library(ggplot2)
library(ggthemes)
library(gganimate)
library(animation)

terminatorLatLon <- lapply(seq(0, 23, 1), function(x) {

  t0 <- as.POSIXct(Sys.Date()) + (60*60*x)

  terminator(t0, -180, 190, 0.5) %>%
    mutate(frame = x)
}) %>%
  plyr::rbind.fill()

chart <- ggplot(terminatorLatLon, aes(frame = frame)) +
  borders("world", colour = "gray90", fill = "gray85") +
  geom_ribbon(aes(lat, ymax = lon), ymin = 90, alpha = 0.2) +
  coord_equal(xlim = c(-180, 190), ylim = c(-58, 85), expand = 0) +
  theme_map()

gganimate(chart, interval = 0.1, ani.width=1000, ani.height=600, filename = "terminator-animation.gif")