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Here are several more animations that show how a planet spins and orbits, this time showing a planet with a large moon.
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.
Thanks go to Evelyn Kinzel at Ohio Dominican University for explaining some finer points of spherical trigonometry.
Differences Between Reality and the Model
There are important differences and similarities between reality and the model shown in these animations. The differences will be explored in future essays and animations.
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Reality1,2,3,4 |
Model |
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| Earth volumetric radius |
6,371 kilometers |
2.000 Blender Units (BU) |
| Moon volumetric radius |
1,737 kilometers |
0.5452 Blender Units |
| Ratio of Earth radius to Moon radius |
3.668:1 |
3.668:1 |
| Sun volumetric radius |
109.2 planet radii |
0.500 planet radii |
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| Earth mean solar orbit radius (1 AU) |
23,480 planet radii |
18.000 planet radii |
| Earth orbit eccentricity |
0.0167 (slightly elliptical) |
0.000 (circular) |
| Earth orbit anomalistic precession |
0.323°/century |
N/A (circular orbit) |
| Earth mean orbital velocity |
403.9 planet radii per solar day |
1.098 planet radii per solar day |
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| Earth equatorial obliquity to ecliptic |
23.4° |
23.4° |
| Earth rotation general precession |
1.397°/century |
0.000°/century |
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| Moon mean orbit radius |
60.336 planet radii |
5.0000 planet radii |
| Moon orbit eccentricity |
0.0549 (slightly elliptical) |
0.000 (circular) |
| Moon orbit obliquity to ecliptic |
5.145° |
13.46° |
| Moon orbit nodal precession |
19.34°/year |
36.00°/year |
| Moon orbit anomalistic precession |
40.68°/year |
N/A (circular orbit) |
| Moon mean orbital velocity |
50.88 moon radii per solar day |
2.087 moon radii per solar day |
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| Moon equatorial obliquity to orbit plane |
6.68° |
15.00° |
| Moon rotation general precession |
19.34°/year, 180° phase to nodal |
0.00°/year |
| Moon equatorial obliquity to ecliptic |
1.54° (constant) |
1.54° (constant + fixed orientation) |
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| Earth mean distance to barycenter |
4,671 km (73.3% of planet radius) |
1.466 BU (73.3% of planet radius) |
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| Solar days per tropical year |
365.2422 |
359.0000 |
| Avergage length of solar day |
86,400 seconds |
0.5014 seconds |
| Sidereal days per tropical year |
366.2422 |
360.0000 |
| Length of sidereal day |
86,164 seconds |
0.5000 seconds |
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| Sidereal month |
27.3217 solar days |
27.6154 solar days |
| Tropical month |
27.3216 solar days |
27.6154 solar days |
| Anomalistic month |
27.5546 solar days |
27.6154 solar days |
| Draconic month |
27.2122 solar days |
27.4030 solar days |
| Synodic month |
29.5306 solar days |
29.9167 solar days |
| Synodic month |
29.6115 sidereal days |
30.0000 sidereal days |
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 under a new moon.
• The background stars are random patterns, they do not accurately represent the Earth's celestial sphere.
• The planet orbit tracks are rendered in fractured stone; terminator ring in hammered cast iron.
• Indicators for orbital nodes are rendered in red stone.
• Indicators for planetary equator, prime meridian, 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.
SystemCam
View from 27° north of the ecliptic, 2.75 planetary AU from the Sun in the direction of the vernal equinox, 56° field of view.
MultiCam
Combined synchronized views.
References
1
NASA/GSFC - Planetary Fact Sheets
2
US Naval Observatory Astronomical Almanac Online - Selected Astronomical Constants, 2009
3
US Naval Observatory, The Astronomical Almanac For The Year 2009, p. D2
4
US Naval Observatory, Explanatory Supplement to the Astronomical Almanac, 2006, pp. 698, 701
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