Warning: the performance of this simulation may vary from one system to another. Please see the description of the Extra Options Menu below for information on how to achieve the best performance on your computer.

Celestial Globe

General Description

The program simulates the motion of the true Sun which does not move through the stars at a uniform rate because of Earth's elliptical orbit. A menu option changes the simulation to use a Sun that moves uniformly along the ecliptic.

Celestial Globe Frame

• Display Options Menu
  • Show Sky View: show a second frame that displays a view of the sky overhead as seen by the observer whose horizon plane is shown in the Celestial Globe frame.
  • Show Equator: show the plane of the celestial equator. (Note: the equator is also shown if Show Celestial Grid is checked.)
  • Show Ecliptic: show the ecliptic plane.
  • Show Celestial Grid: show lines of constant RA and Dec (including the celestial equator and meridian).
  • Show Celestial Axis: show the celestial axis.
  • Show Local Grid: show lines of constant altitude and azimuth.
  • Show Horizon Plane: show the observer's horizon plane.
  • Show Trace of Sun's Path: show the trace of Sun's path.
  • Show Sun: show a ball representing Sun.
  • Show Stars: show points representing stars.
  • Show North Arrow: show an arrow pointing northward along the horizon plane.
  • Show Ecliptic Axis: show the axis or Earth's orbit around Sun.
  • Show Ecliptic Grid: show lines of constant ecliptic longitude and latitude.
• Select Day Menu
  • Vernal Equinox (Time of Year = 0)
  • Summer Solstice (Time of Year = 0.252)
  • Autumnal Equinox (Time of Year = 0.507)
  • Winter Solstice (Time of Year = 0.756)
  • Note that if the tilt of Earth's axis is changed to that its rotation is in the opposite sense of its orbit then the equinoxes and solstices are reversed (ie the Vernal Equinox is at Time of Year = 0.507, etc).
• Extra Options Menu
  • Display Additional Controls: displays controls for adjusting the number of days per year, the tilt of Earth's axis, the eccentricity of Earth's orbit, and the year (to see the effects of precession).
  • Use Uniform Sun: use a Sun that moves uniformly along the ecliptic.
  • Connect Day Trace: connects the dots in the cyan trace of Sun's path while dragging the Time of Year slider. You may need to use this option for this trace to be visible on some systems (ie Macs with Java 1.5). Note, however, that connecting the dots can sometimes cause spurious lines to be produced.
  • Use Transparency While Running (only available when the simulation is paused): using transparency while running or dragging a slider drastically slows performance of this simulation on many systems, so the program is set to use no transparency while the surfaces are moving. However, on some systems (ie Macs with Java 1.5) the program performs well even with transparency on at all times. Select this option to test the performance of the simulation on your system.
• Visual Elements
  • Orange sphere: Sun.
  • White dots: stars. The simulation shows the 300 brightest stars, with positions taken from the Bright Star Catalog.
  • Red circle: the ecliptic plane.
  • Magenta circle: the plane of the celestial equator.
  • Blue grid: the celestial (RA-Dec) coordinate system.
  • Cyan line: the celestial axis.
  • Gray grid lines: the local (altitude-azimuth) coordinate system.
  • Orange trace: trace of Sun's motion during time evolution.
  • Cyan trace (points): trace of Sun's motion at fixed time as the day is varied. Note: some versions of Java (such as 1.5.0_19) will not display this trace.
  • Green plane: the horizon plane of an observer on Earth. Objects above the horizon are visible to the observer, objects below the horizon are not.
  • Black arrow (and label): points northward along horizon plane.
  • Red line: the ecliptic axis.
  • Pink grid: the ecliptic (longitude-latitude) coordinate system.

Sky View Frame

• Sky View Options Menu
  • Show Local Grid: if checked, lines of constant altitude and azimuth will be shown.
  • Show Trace of Sun's Path: if checked, the path of the trace of Sun's path will be shown.
  • Show Celestial Equator: show a curve representing the celestial equator.
  • Show Celestial Poles: show dots representing the north and south celestial poles.
  • Show Ecliptic: show a set of points that designates the ecliptic.
  • Show Ecliptic Poles: show dots representing the north and south ecliptic poles.
  • Show Daylight (Blue Sky): if selected the sky will be blue (but slightly transparent, so the stars are barely visible) whenever Sun is above the horizon. The blue sky will fade in as the sun rises and fade out as it sets.
  • Show Sun: show disk representing Sun.
  • Show Stars: show points representing the stars.
• Visual Elements
  • Orange disk: Sun.
  • White dots: the fixed stars. The simulation shows the 300 brightest stars, with positions taken from the Bright Star Catalog.
  • Gray lines: the local (altitude-azimuth) coordinate system. Lines of 30 and 60 degrees altitude are shown. Lines of azimuth are shown in 45 degree increments from zero.
  • N, S, E, W: compass directions. Note that the view is that of an observer on Earth looking up at the sky, so the directions are inverted from the way they would appear on a map (which is the perspective of an observer looking down onto Earth).
  • Orange trace: trace of Sun's motion during time evolution.
  • Cyan trace (points): trace of Sun's motion at fixed time as the day is varied. Note: some versions of Java (such as 1.5.0_19) will not display this trace.
  • Blue line: the Celestial Equator.
  • Red points: the ecliptic.
  • Red/magenta dot: north/south ecliptic pole.
  • Blue/cyan dot: north/south celestial pole.

Controls

• Play/Pause: start and stop the simulation.
• Step: moves the animation forward by one time step.
• Reset: reset sim to original state.
• Clear Traces: clear the traces of the sun's motion.
• Time of Day: measured in days. Midnight is 0 and noon is 0.5.
• Time of Year: measured in years. The vernal equinox is 0, the summer solstice is about 0.252, the autumnal equinox is 0.507, and the winter solstice is 0.756.
• Latitude (deg N): sets the latitude of the observer's location. For southern latitudes use negative values.

Additional Controls

• Days Per Year: number of solar days in a solar year. Fewer days per year means the sun moves faster along the ecliptic (i.e. relative to the fixed stars), more days per year means the sun moves slower relative to the stars. The real value for the Earth/Sun system is 365.25 days per year. This value is restricted to >= 0.01, and >= 2 for counterrotating situations.
• Axial Tilt: controls the tilt of Earth's rotational axis relative to the axis of its orbit. The real value is 23.5 degrees. If the stars are fixed relative to the ecliptic (which is the default) then they will not move as the tilt is changed. If the stars are fixed relative to the Celestial Equator then they will move with the axis.
• Eccentricity: controls the eccentricity of Earth's orbit. The real value is 0.0167. Note that increasing the eccentricity may cause the Select Day options to not function properly (because, for example, the summer solstice may no longer be very close to Time of Year = 0.252).
• Year: sets the calendar year (Gregorian calendar). Change the year to demonstrate the effects of precession.

Version History

• Version 1.0: first version with basic controls including time of day, time of year, latitude, days per year, and tilt. Stars were fixed to celestial sphere.
• Version 1.1: added control for eccentricity, fixed stars relative to ecliptic plane (with option to fix stars relative to celestial sphere), addressed some situations that produced divisions by zero, reorganized controls, provided option to use uniformly moving sun, added option to adjust stars to 2nd Cen BC positions, added option to show equator in Sky View, added option to show ecliptic in Sky View, fixed blue sky so that it fades in as sun rises and out as sun sets and is always slightly transparent.
• Version 1.2: removed fixed stars to celestial grid option (which is unphysical and caused problems when combined with precession) and Ancient Greek star positions option (which was obviated by the new calendar year control), added controls to adjust calendar year to demonstrate precession, added markers for celestial and ecliptic poles in sky view, added option to show ecliptic axis in globe frame, added ecliptic grid in globe frame.