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Stars and ConstellationsRadialStarChart

4.1 The StarChart Function

StarChart is used to show a rectangular section of the sky. Several other types of star charts are available, but this one is the fastest. Like all the other star chart functions in Scientific Astronomer, StarChart accepts a large number of options for producing a wide variety of output styles.

Plotting stars in a rectangular region.

Like most other functions, StarChart is typically applied to solar system objects such as Mars, Moon, and Io; stars such as Sirius and Alpha.Centaurus; constellations such as Leo and UrsaMajor; special objects such as SouthCelestialPole and Zenith; and so on.

The solid angle covered by this chart of the Scorpius constellation is about four times the area of your hand when held at arm's length. The vertical axis indicates declination and the horizontal axis shows right ascension.


In some of the star chart graphical output you may notice a blue line crossing the field of view. This represents the ecliptic, which is the precise line that the Sun moves along during the course of a year. The Moon and all the planets approximately move along this line, too, so the ecliptic is very important. All the 12 zodiac constellations are located near the ecliptic as well.

To switch off the line, you can use the Ecliptic -> False option. The default is Ecliptic -> True.

All the star charts use thin green lines to join together some of the brighter stars to form the constellations. By default, constellation outlines are drawn, but you can suppress them using the option setting Constellations -> False.

A feature of all the star charts is that you can, on appropriate front ends, hold down the CommandKey key, click inside a graphic, and copy the pair of numbers representing the point where you clicked. That pair of numbers can then be used in other functions as if they were an object.

In the case of StarChart you can also hold down CommandKey-OptionKey and select a rectangle, and then copy that pair of number pairs into another StarChart call. This allows you to interactively zoom into a smaller portion of the sky. (Note the CommandKey-OptionKey feature may not be available on all front ends.)

Suppose, for example, that you generate a large section of the sky that contains the constellation of Orion. By using the CommandKey-OptionKey-select-and-copy feature you can place a rectangle around Orion to get a bounding box that you can use as the argument in a subsequent StarChart call.

Bounding boxes are a pair of number pairs giving first the minimum and maximum x values, and then the minimum and maximum y values of the rectangle. An x range of {-108.818, -56.455} corresponds to a right ascension range of 7.25 hours (from +108.818/15) to 3.76 hours. Similarly, a y range of {-22.091, 22.091} corresponds to a declination range of -22.09 degrees to +22.09 degrees.

In a chart zoomed into a smaller part of the sky, the CommandKey-OptionKey-select-and-copy feature is used in a previous StarChart graphic to get the bounding box that is used as the argument in this call.

In[8]:=StarChart[{{-108.818, -56.455}, {-22.091, 22.091}}];

A chart of the daytime sky near the constellation of Libra shows the Sun and the Moon superimposed. These are their positions 2 hours before the new moon nearest 1993 November 17, which occurs on 1993 November 14. The brown area on the upper left is the horizon, with the ground below it.

Planets -> All, Horizon -> True];

Here is a chart of the constellation of Orion with constellation labeling and an equator coordinates mesh superimposed. Spectral colors of the stars are also shown. Betelgeuse is the orange-red star on the right shoulder, which is our left, of Orion. Of course, in the southern hemisphere it appears upside down. If you look at Orion in the actual sky you should have little trouble identifying the unusual color of Betelgeuse.

ConstellationLabels -> True,
Mesh -> True,
StarColors -> True];

Star charts use black stars on a white background, because that is how you would typically print a chart. Sometimes, however, a more realistic rendering of the night sky is required. You can use the option StarColors -> True to color the brighter stars according to their actual visual spectral color. In this case the background sky is rendered as black. The default is StarColors -> False.

Some of the options for StarChart, RadialStarChart, CompassStarChart, ZenithStarChart, and Planisphere.

There are many other options to StarChart and the other star chart functions. To display stars on a black sky you can use the option setting Background -> RGBColor[0,0,0], and to label constellations you can use ConstellationLabels -> True. Similarly, you can label planets, superimpose an equator coordinates mesh, and color stars according to their spectral colors.

By default, only the brightest 300 stars are used by StarChart and the other star chart functions, but you can load extra stars such as those in the Star3000.m file, which includes the brightest 3,000 stars. You can artificially brighten or dim the stars by using the option MagnitudeScale.

Stars are drawn as small dots with a radius in proportion to their magnitude. You can use the MagnitudeScale option to increase the size of the dots if you decide they are too small. For example, to double the radius of the dots, use the setting MagnitudeScale -> 2. Similarly, you can use the setting MagnitudeScale -> 0.3 to decrease the size of the dots. The default value is, of course, MagnitudeScale -> 1.

Sometimes, when you load extra stars from the Star3000.m or Star9000.m files, you may not want to see all of these stars. In such a case, you can use the MagnitudeRange option with any star chart function to select the range of star magnitudes to be displayed.

The syntax is MagnitudeRange -> {min, max} or MagnitudeRange -> max. The default is MagnitudeRange -> {-Infinity, Infinity}, which means display stars of all magnitudes. An option value of MagnitudeRange -> 4.0 would request that only stars down to magnitude 4.0 be displayed.

Stars and ConstellationsRadialStarChart