Orbits, eccentricity and gravity
1 min 49 s. The words are spoken and shown in the picture; the player can also show them as subtitles.
For the Earth and Space Sciences Regents. Topic: Orbits, eccentricity and gravity, with the past questions on it.
What Dr. Mars says (the words of the video)
Why does a planet speed up and slow down?
Planet orbits are ellipses, or ovals.
Dr. Mars draws it with pins and string.
Real planet orbits are much closer to circles.
The Sun sits at one focus, not at the center.
The other focus is empty.
Eccentricity: how stretched an ellipse is.
Eccentricity equals distance between foci divided by major axis.
Here: seven divided by ten equals zero point seven.
Eccentricity zero is a perfect circle.
Earth's orbit: zero point zero one seven, almost a circle.
Dots mark the planet after equal amounts of time.
Dots far apart mean fast motion.
Gravity pulls the planet toward the Sun all the way around the orbit.
Moving toward the Sun, the planet speeds up: gravity pulls harder.
Dr. Mars holds on tight.
Closest to the Sun, the planet is fastest.
At the far end, the dots are close together.
Farthest away, gravity is weakest and the planet moves slowest.
Check yourself! Where is the planet fastest: A, B, or C?
Look at the dot spacing.
Answer: A! It is nearest the Sun.
Dr. Mars held on tight at A, where the dots are far apart.
Dr. Mars. Free practice for the New York Earth and Space Sciences Regents.