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StarNamesChartCoordinates, ChartPosition

4.6 The OrbitTrack and OrbitMark Functions

All the standard graphics options are available to the star charts. The Prolog and Epilog options in particular will add improvements to these charts. For instance, Prolog lets you add a more complicated background than the Background option alone would allow. Similarly, the Epilog option allows you to enhance the foreground more than the Skyline option alone.

One important use of the Epilog option, in relation to star charts, involves the ability it gives you to add orbit tracks of satellites or planets, as well as to add other coordinate-dependent information. The functions that aid you in this are OrbitTrack, OrbitMark, DeepSkyMark, and ChartCoordinates.

OrbitTrack allows you to easily display the track of an object onto a star chart.

Drawing orbit tracks and labels on a star chart.

You can project the trajectory of Mars, showing its occasional retrograde motion, onto a star chart using OrbitTrack. Retrograde motion always occurs around opposition.

Mars is at opposition on 1993 January 8.

In[28]:=BestView[Mars, {1993,11,17}]


The trajectory of Mars during 1992-1993 is projected onto a star chart here. The motion of Mars is mostly right to left, or equivalently west to east, but note the retrograde motion.

In[29]:=StarChart[{{60,150}, {-20,40}},
Ecliptic -> False,
ConstellationLabels -> True,
Epilog -> {RGBColor[1,0,0],
OrbitTrack[Mars, {1992,6,1}, {1993,6,1}]}];

You can superimpose orbit tracks and other items on all the different types of star charts. A related function is OrbitMark, which lets you annotate marks and text onto a star chart. For example, you can mark the position of the asteroid Vesta month by month onto a star chart. Asteroid Vesta is in opposition on 1996 May 9; hence it is at its closest and brightest as seen from the Earth. During opposition it also undergoes retrograde motion.

OrbitMark is used to annotate the track of the asteroid Vesta. Blue crosses and text are used to mark the position of Vesta for each month during 1996. During month 5 (May) it is at its brightest.

In[30]:=StarChart[{{210, 240}, {-30, 0}},
ConstellationLabels -> True,
Epilog -> {RGBColor[1,0,0],
OrbitTrack[Vesta, {1996,1,1}, {1996,9,1}],
Table[OrbitMark[Vesta, {1996,m,9},

Using OrbitMark in this chart allows you to observe the position of the north celestial pole for the years from 12000 B.C. to 12000 A.D. This illustrates the precession effect of the Earth's axis-every 25,000 years the axis rotates in a full circle of radius 23.5 degrees. Note that the view direction {0,0,1} represents the ecliptic north pole, that is, the point directly above the plane of the Earth's orbit.

RadialAngle -> 50*Degree,
MagnitudeRange -> 4.0,
StarLabels -> True,
MilkyWay -> True,
Epilog -> {RGBColor[1,0,0],
PlotLabel -> year/1000],
{year, -12000, 12000, 1000}]}];

At year 2000, the north celestial pole is near Polaris.

In[32]:=FindNearestObject[NorthCelestialPole, {2000,1,1},
MagnitudeRange->3.5] === Polaris


However, at year -12000, the north celestial pole was near Vega.

In[33]:=FindNearestObject[NorthCelestialPole, {-12000,1,1},
MagnitudeRange->3.5] === Vega


The DeepSkyMark Function

Faint deep sky objects are not normally shown on the star charts because there are so many of them. A function called DeepSkyMark is provided to let you easily annotate any star chart to place a mark where a deep sky object is located.

Labeling of deep sky objects.

Specific marks are used for different types of objects. For instance, circles are used for galaxies, squares for nebulae, and nine-dot symbols for star clusters. The size of the mark represents the true apparent angular size of the object.

With this function you can use the option Epilog -> DeepSkyMark[TriangulumGalaxy] to mark the location of the Triangulum Galaxy on any star chart. More complicated forms, such as Epilog -> {RGBColor[1,1,0], DeepSkyMark /@ {EagleNebula, OmegaNebula, TrifidNebula, LagoonNebula}} are also possible.

There is a PlotLabel option available to suppress or add a new label to the mark. Thus, Epilog -> DeepSkyMark[TriangulumGalaxy, PlotLabel -> ""] marks the location of the Triangulum Galaxy, but will not label it as such. The default labels the mark with the NGC or IC name of the object. See Section 8.5 for a discussion on NGC and IC numbers.

StarNamesChartCoordinates, ChartPosition