Science Videos

1. Apparent retrograde motion videos (i.e., Apparent retrograde motion videos):
   1. Mars Retrograde Orbit Simulation | 0:23: What it looks like on the sky---except the lines joining the constellations are phoney. That thing whizzing by periodically is the Moon (see Richard Pogge, 2011: Mars Retrograde Motion during 1994/95) Short enough for the classroom.
   2. Retrograde Motion | 0:19: Not great, but it shows apparent retrograde motion from the two points of view in two panels at the same time. Short enough for the classroom.

2. See also Asteroid videos all below (local link / general link: asteroid_videos_all.html).

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4. Axial Precession Videos:

   Axial precession videos (i.e., Axial precession videos):

   1. Milankovitch Cycles | 0:10: The axial precession is the motion of the Earth relative to the fixed stars. It has a period of approximately 26000 years, but there is NO fixed period due to gravitational perturbations and other astrophysical perturbations. Short enough for the classroom.
   2. Bicycle Wheel with gyroscope precession | 0:26: What happens to the Earth also happens to a bicycle wheel. They are somewhat different cases of precession, but there are basic similarities. Both are hard to explain though precession is very common. In the video, note, the precession turns the spinning bicycle wheel axle (its axis) in the direction of the leaning of the bicycle wheel: this is most easily seen just after the demonstrator releases the bicycle wheel. In cycling, cyclists usually lean into a turn to let ground compression forces provide some of the centripetal force needed to effect the turn and prevent the bicycle from torquing outward. They do this usually without having studied physics. The precession caused by leaning probably helps turn the bicycle wheel to increase the turn amount. Note, when the bicycle wheel is not spinning, it just hangs at rest with its center of mass below the pivot point: this is just the ordinary stable equilibrium hanging arrangement. Good and short enough for the classroom.

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9. Chaos videos (i.e., Chaos videos):
   1. Chaotic 1,3 pendulum | 40: Here we have a rather complex dynamical system of 3 coupled double pendulums. Though the 3 lower pendulums are nearly identical and their initial conditions are nearly the same, we see that their dynamical evolutions quickly diverge. We have chaos: super-sensitivity to initial conditions combined with complex dynamicsl evolution. Actually, the dynamical evolutions of the 3 lower pendulums are coupled as seen in the video: i.e., they interact and are NOT independent. In this real case of double pendulums, dissipation of mechanical energy due mainly to friction eventually will cause the dynamical system to come to rest with everything just hanging down, and so the dynamical evolutions do eventually converge. This hasn't quite happened by the end of the video. Without friction and other dissipation forces, the dynamical evolutions would probably NEVER converge again, although they would approach convergence from time to time. Short enough for the classroom.
   2. N body simulation in Python with code (precision approach) | 1:01: Good. N-body simulations for N = 3, 4, 13, and 30 point particles calculated using a Python code. Full information is NOT given with the video, but yours truly assumes the particles have equal mass and are point particles (i.e., they have zero size, and so NEVER collide in a body-body sense NOR merge. The initial conditions must chosen in some good fashion for illustrative purposes. Yours truly believes that initial conditions are such the systems are gravitationally bound which is equivalent to saying kinetic energy plus gravitational potential energy sums to less than zero. Being gravitationally bound means that NOT all the point particles can go to infinity relative to each other. Due to complex gravitational interactions (including explicit gravity assists: strong two-body interactions that change orbital trjectories) can eject individual point particles to infinity, but the remaining ones are more tightly gravitationally bound. which is equivalent to saying kinetic energy plus gravitational potential energy sums to less than zero. Being gravitationally bound means that NOT all the point particles can go to infinity relative to each other. Due to complex gravitational interactions (including explicit gravity assists: strong two-body interactions that change orbital trjectories) can eject individual point particles to infinity, but the remaining ones are more tightly gravitationally bound. In some cases depending on initial conditions, so many ejections may occur that only 2 point particles, and they can NEVER escape from each other.
            In the N=4 N-body simulation at t=0:23, there is a gravity assist probably leading to one point particle becoming unbound and going on an escape trajectories to infinity. Of course it is possible the point particle is merely on a very long bound orbit and would return to the other point particles if the N=4 N-body simulation ran longer.
            Another thing to notice is that point particles speed up as they approach each other. This is because their gravitational potential energy (which is greater when they are remoter) turns into kinetic energy as they approach each which is understood by the principle of conservation of mechanical energy.
            Note open star clusters if they are formed bound (and they NOT be) often suffer considerable escape during their existence to internal and external gravitational interactions and disperse in typically a few hundreds of millions of years, a process often called "evaporation" (see Wikipedia: Open cluster: Eventual fate).
            Note also that there are NO exact analytic solutions for 3 or more bodies interacting via gravity. One must solve such systems by perturbation theory or numerical methods (e.g., N-body simulations) on the computer. There is an exact analytic solution for gravitationally bound 2-body system within Newtonian physics.
            Short enough for classroom.
   3. N-Body Chaotic orbit | 0:50: Yours truly will NOT attest to the accuracy of this N-body simulation and it's NOT clear what dynamical system is being simulated. Yours truly guesses the dynamical system is a 3-dimensional system (shown in 2-dimensional projection) of point masses interacting only through gravity and exhibiting chaos. One guess again that the system is a gravitationally-bound system, but due to the chaotic motion some point masses achieve escape velocity it seems.
10. See Comet videos below (local link / general link: videos_comets.html).

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11. But for some of the folklore, see the Stars And Constellations | 3:11 and Carl Sagan's Cosmos - Constellations | 3:57 videos in Constellation videos (see below (local link / general link: constellation_videos.html).

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14. Hellas videos:

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16. Lunar phase videos (i.e., Lunar phase videos):
    
    1. Phases of the Moon | 0:16: Pretty good. Similar to the now defunct NAAP applet: Lunar Phase Simulator (see naap_lunar_phase.html). Short enough for the classroom.
    2. Earth-Moon System | 2:23: About lunar phases. Pretty good and sort of fun. Too long for the classroom.
    3. The Universe: The Phases of the Moon | 3:15: The History Channel's lengthy, flashy historical take. Too long for the classroom.

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18. See also the Newton videos below (local link / general link: newton_videos.html).

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19. Newton's cannonball videos (i.e., Newton's cannonball videos):
    
    1. Projectiles launched horizontally into orbit (Newton's cannonball) | 0:16: With continuous variation of launch speed. Short enough for classroom.
    2. Newton's Cannon in action | 0:28: Good. Short enough for classroom.
    3. Newton's cannon animation | 0:30: With background music. Short enough for classroom.

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23. Star trail videos (AKA Star trail videos):
    
    1. The motion rotation of stars around Polaris star | 0:49: Polaris is the pole star in the northern sky and makes only a small circle around the north celestial pole (NCP). Note the diurnal rotation is (counterclockwise looking north.
    2. North Celestial Pole Star Rotation | 0:59: Same comments as for item 1.
    3. Southern Celestial Pole timelapse | 1:09: Note the south celestial pole (SCP) has NO pole star and the diurnal rotation is clockwise looking south.
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25. The orbital inclinations are further illustrated below in the video Simulation solar system | 0:36 in Solar System videos below (local link / general link: videos_solar_system.html).

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27. Terminator videos (i.e., Terminator videos):
    1. File:ISS flies over Africa, the Mideast, and the Terminator line.ogv | 0:20: A flight into darkness. Actually, this is how Superman would see things. The film is time lapsed. The flight is a bit slower for the astronauts on the International Space Station (ISS). But even for them it is pretty quick. The time period for low Earth orbit is only about 90 minutes---there the day is 90 minutes long. Short enough for the classroom.