Get started • ggsky

TL;DR: ggsky adds galactic and equatorial sky-map coordinates to ggplot2, so you can plot points, paths, and segments on a Hammer projection with readable sky-grid labels.

library(ggsky)
library(ggplot2)
theme_set(theme_light())

Coordinate Systems

Galactic Coordinate System

ggplot() +
  coord_galactic()

N <- 100
df_gal <- data.frame(
  l = runif(N, 0, 360), 
  b = runif(N, -90, 90)
)

ggplot(df_gal, aes(l, b)) +
  geom_point() +
  coord_galactic()

Galactic longitude vs latitude.

Equatorial coordinate system

ggplot() +
  coord_equatorial()

df1 <- data.frame(
  ra = runif(N, 0, 360),
  dec = runif(N, -90, 90)
)

ggplot(df1, aes(ra, dec)) +
  geom_point() +
  coord_equatorial()

Right ascension versus declination. Coordinates ra and dec must be in degrees.

Custom geoms

`geom_path`

It projects geom_path between points along Great circle, i.e., the shortest path on the sky map.

df_path_gal <- data.frame(
  l = c(110, 110),
  b = c(-4, 60),
  g = 1
)

ggplot(df_path_gal, aes(l, b, group = g)) +
  geom_path(colour = "blue", linewidth = 1) +
  coord_galactic()

`geom_segment`

df_seg_eq <- data.frame(
  x = 30, y = -10,
  xend = 120, yend = 40
)

ggplot(df_seg_eq) +
  geom_segment(
    aes(x = x, y = y, xend = xend, yend = yend),
    linewidth = 1, colour = "orange",
    arrow = arrow(type = "closed", length = unit(0.1, "inches"))
  ) +
  coord_equatorial()

Custom scales

scale_gal_lat(), scale_gal_lon(), scale_eq_ra(), scale_eq_dec()

df_path_eq <- data.frame(
  ra = c(0, 60),
  dec = c(30, 30),
  g = 1
)

ggplot(df_path_eq, aes(ra, dec, group = g)) +
  geom_path() +
  coord_equatorial() +
  scale_eq_ra(breaks = seq(0, 330, by = 30))

Plot celestial equator on galactic plane

Use built-in dataset equator.

ggplot(equator, aes(l, b)) +
  geom_path(linetype = "dotted", color = "red") +
  geom_text(
    data = subset(equator, ra %% 30 == 0),
    aes(label = sprintf("%d*degree", ra)),
    parse = TRUE,
    vjust = -0.5,
    size = 3,
    color = "red"
  ) +
  coord_galactic()