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In honor of the
International Year of Astronomy, I've created several animations that show how a planet spins and orbits.
More animations may be found here.
There are vast numbers of easily accessible online text references—Wikipedia, for example—that do a very good job of explaining the various phenomena of celestial mechanics. The animations here are, hopefully, a useful adjunct to those references.
These animations were created and rendered using the open source 3D animation tool Blender.
Special thanks go to Brian Vanderkolk, whose help saved me a great deal of time.
Differences Between Earth and the Model
There are important differences between Earth and the model planet shown in the animations. These differences will be explored in future essays and animations.
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Earth1,2 |
Model Planet |
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| Volumetric radius |
6,371 kilometers |
2.000 Blender Units |
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| Mean orbit radius (1 AU) |
23,480 planet radii |
18.000 planet radii |
| Equatorial obliquity to ecliptic |
23.4° |
23.4° |
| Orbit eccentricity |
0.0167 (slightly elliptical) |
0.000 (circular) |
| Mean orbital velocity |
403.9 planet radii per solar day |
1.098 planet radii per solar day |
| Orbit path followed by |
Earth-Moon barycenter |
Planet's geometric center |
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| Sun volumetric radius |
109.2 planet radii |
0.500 planet radii |
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| Solar days per tropical year |
365.2422 |
103.0000 |
| Avergage length of solar day |
86,400 seconds |
3.0291 seconds |
Earth solar days represented by
one animation solar day |
N/A |
3.5460 |
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| Length of sidereal day |
86,164 seconds |
3.0000 seconds |
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Ratio of solar day length to
sidereal day length |
1.0027 |
1.0097 |
Animation Details
• These animations were rendered using perspective geometry. They are not isometric views.
• These animations start on the winter solstice at sunrise over the prime meridian at the equator.
• The background stars are random patterns, they do not accurately represent the Earth's celestial sphere.
• The planet's orbit track is rendered in fractured stone; terminator ring in hammered cast iron.
• Indicators for planetary equator, meridians, and angular momentum vector are rendered in tarnished brass.
• The planet's prime meridian indicator is inlaid with copper segments.
Animations are rendered as Windows Media, QuickTime, and Xvid videos.
The open source Xvid codec may be downloaded here.
The open source Media Player Classic is an excellent utility for Windows that will play all kinds of audio and video files.
TerminatorCam
View from 7 planet radii from the planet's surface, 47° field of view looking normal to the orbital plane.
PolarCam
View from 7 planet radii from the planet's surface at its North rotational pole, 47° field of view looking normal to the equatorial plane.
SystemCam
View from 23° north of the ecliptic, 2.75 planetary AU from the Sun in the direction of the vernal equinox, 49° field of view.
MultiCam
Combined synchronized views.
References
1
NASA/GSFC - Planetary Fact Sheets
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2
US Naval Observatory Astronomical Almanac Online - Selected Astronomical Constants, 2010
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