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4.3 The CompassStarChart Function

CompassStarChart displays a global section of the sky. You specify a compass direction, and a graphic showing all the stars in that direction is displayed.

Plotting stars in half the sky.

Two CompassStarChart calls can be used to cover the entire sky above the local horizon.

At the top of the graphic, a cross is used to represent the zenith, which is the point directly above your head. The field of view is 180 degrees along the horizon and is shown as the brown line along the bottom. The blue line is the ecliptic, which can be removed using the option setting Ecliptic -> False.

As with other star chart functions, numerous options are available. The options are identical to those for StarChart. For instance, to add an equator coordinates mesh use Mesh -> True. To show the Milky Way use MilkyWay -> True. Use the option Text -> False to suppress the text printed at the top left and right edges of the graphic.

Observe the southern aspect of the sky above the horizon at 03:20 on 1993 November 17 as seen from Melbourne, Australia.

In[16]:=CompassStarChart[South, {1993,11,17,3,20,0},
Mesh -> True,
StarColors -> True,
MagnitudeRange -> 4.0];

This chart shows the northern aspect of the same sky.

In[17]:=CompassStarChart[North, {1993,11,17,3,20,0},
Mesh -> True,
StarColors -> True,
MagnitudeRange -> 4.0];

The western aspect of the sky displays part of the Milky Way, which can be seen on both the right and the left. The blue line sweeping from the western horizon to a point above north is the ecliptic.

In[18]:=CompassStarChart[West, {1993,11,17,3,20,0},
Mesh -> True,
MilkyWay -> True,
MagnitudeRange -> 4.0];

Note the usefulness of the option Mesh. Stars effectively move along the equatorial mesh lines, and the intersecting curves are one hour apart. You can, therefore, easily estimate where a given star will be in relation to the horizon in one or more hours time by simply following an equatorial mesh line.

If you select the graphic returned by CompassStarChart and then press the CommandKey key, the horizon coordinates of the mouse position are displayed in the bottom left hand corner of your notebook window. The first number is degrees east of north (e.g., 270 is west), and the second is altitude in degrees, with 90 being the zenith point.

As with all the star chart functions, if you copy the pair of numbers obtained by holding down the CommandKey key and clicking in a star chart, you can paste that pair into any other function. The pair of numbers is treated as an object corresponding to the point selected in the last star chart graphic.

One further option available to all the star charts is Skyline, which lets you map a sky line along the local horizon. The default value is Skyline -> {}, which does not draw a sky line, but you can set it to a more complicated value.

The value required by the Skyline option must be built out of normal graphics primitives such as Point, Line, Rectangle, Disk, Circle, Polygon, and Text. You can also use attributes such as RGBColor, PointSize, and Thickness.

The coordinates supplied to the graphics primitives must be given as horizon coordinates; that is, the first number must be the azimuth or compass direction in degrees and the second number must be the altitude in degrees. For example, the primitive {RGBColor[1,0,0], Rectangle[{85, 0}, {95, 15}]} corresponds to a red rectangular "building" in the east that is 10 degrees wide and 15 degrees high. Depending on how creative you are, you can create trees, hills, and other features. You can also use the Text primitive to label the features along the horizon.

Consider the sky line consisting of a building 30 degrees high and 10 degrees wide in the direction 210 degrees around the compass. Add a tree located at 290 degrees with a height of 20 degrees, and a mountain in the east.

In[19]:=myskyline = {
{RGBColor[1,0,0], Rectangle[{205, 0}, {215,30}]},
{RGBColor[1,1,0], Rectangle[{288, 0}, {292,20}]},
{RGBColor[0,1,0], Disk[{290, 20}, 8]},
{RGBColor[0,0,1], Text["Tree", {290, 20}]},
{RGBColor[0,0,1], Polygon[{{ 20, 0}, { 35, 10},
{ 50, 20}, { 60, 15},
{ 80, 30}, {100, 15},
{120, 0}, {100, 0},
{ 80, 0}, { 60, 0},
{ 40, 0}, { 20, 0}}]}};

This is a plot of the elementary sky line.

In[20]:=Show[Graphics[myskyline, AspectRatio->Automatic],
PlotRange -> {{0,360}, {0,90}},
Frame -> True];

The Skyline option maps the given sky line onto the star chart. Note the distortion at the edge of the plot. The normally rectangular building is somewhat tilted in this projection.

In[21]:=CompassStarChart[West, {1993,11,17,3,20,0},
MilkyWay -> True,
MagnitudeRange -> 3.5,
Ecliptic -> False,
Skyline -> myskyline];