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4.2 The RadialStarChart Function

RadialStarChart is useful for displaying the relative layout of stars. You specify a direction and a field of view, and the appropriate graphic is returned.

Plotting stars near a central point.

An advantage of RadialStarChart over StarChart is that it can produce a plot with much less distortion. No matter which direction in the celestial sphere you look, RadialStarChart gives you a relatively undistorted representation, whereas StarChart has considerable distortion near the celestial poles.

As with the other star charts, green lines are used to highlight common constellations to aid identification. These lines can be switched off with the option setting Constellations -> False.

Many options are available with RadialStarChart.

A chart looking in the direction of the north celestial pole shows the pole star Polaris in the center and the Big Dipper UrsaMajor to the left. The field of view is 50 degrees in angular radius, so the graphic is 100 degrees across.

RadialAngle -> 50*Degree];

RadialStarChart[Scorpius] creates a chart of the Scorpius constellation. The two special nine-dot symbols to the left of Scorpius are open star clusters. The bigger cluster is M7 and the much smaller one is M6, which is sometimes known as the Butterfly cluster. To remove star clusters use the option setting Clusters -> False.


This is a chart looking toward Venus at sunset. Note the use of the option setting Horizon -> True as well as the other options. The brown-colored curve at the bottom is the line of the local horizon, into which the yellow Sun is setting. At the very bottom the compass direction is written. The Sun, which is shown sitting directly on the horizon, obviously sets into the west.

In[13]:=RadialStarChart[Venus, SunSet[{1994,10,13}],
RadialAngle -> 45*Degree,
Planets -> All,
PlanetLabels -> True,
ConstellationLabels -> True,
Horizon -> True,
MagnitudeRange -> {-Infinity, 3.5}];

You can project the line of the local horizon onto a star chart by using the Horizon -> True option. The Horizon option is applicable to StarChart and RadialStarChart, with the default being Horizon -> False. Since the horizon changes with time, you need to supply a date to the star chart function.

When the Horizon -> True option is used in a RadialStarChart, the graphic is rotated so that the horizon line is horizontal. This is not possible with the plain StarChart function, which has a more rigidly aligned coordinate system. In that function, the horizon line and the ground below it are colored brown.

Stars have generally fixed positions in the rotating sky and so are easy to display consistently in the various star charts. Planets, however, continually wander through the sky (although always near the ecliptic) and so are not normally displayed in the star charts. To render planets in a star chart you can set the Planets option to the value All, or to a list of planet names, or even to other objects. The default is Planets -> None.

When planets are displayed you might have trouble identifying which planet is which. To remedy this the option PlanetLabels -> True is available to place labels below the planets. The default is PlanetLabels -> False.

Sometimes it is useful to have constellations labeled as well, since not all constellations are easily identifiable by sight. This is where the ConstellationLabels -> True option is useful. The default value is ConstellationLabels -> False.

By default, the background sky is white unless the StarColor option is set to True, in which case the background sky is black. If neither of these colors is suitable to you, then you can use the Background option. Whatever background color you choose, contrasting colors are used for stars. The Background option can be a CMYKColor, GrayLevel, Hue, or RGBColor directive. The default is Automatic, which chooses between GrayLevel[0] and GrayLevel[1] depending on whether StarColor is True or False.

There are many other options available to the star charts. For example, Mesh -> True superimposes an equator coordinates mesh. Mesh lines units are 1 hour of right ascension, and 15 degrees of declination. Crosses are placed at the north and south celestial poles. To suppress the text labeling around the edges use the option Text -> False.

The Background and Mesh settings designate a blue background and a gray equator coordinates mesh in this star chart.

RadialAngle -> 40*Degree,
Background -> RGBColor[0.1,0.1,1],
Mesh -> True,
ConstellationLabels -> True];

When you request star colors, a black background appears by default.

RadialAngle -> 40*Degree,
Mesh -> True,
StarColors -> True,
ConstellationLabels -> True];

As normal, you can look at any named object, such as Mars, Sirius, Leo, or Zenith, but it is also possible to look at a fixed point in the sky relative to the local horizon by using horizon coordinates. For example, RadialStarChart[{Azimuth -> 180*Degree, Altitude -> 20*Degree}, {1993,11,17,3,20,0}]; generates a chart centered on the point directly south and 20 degrees up in the sky. Similarly, you can use equator coordinates.