When Pluto was discovered in , there was no accepted definition of the term planet. It was only in that a formal demarcation was offered. A planet has to: a orbit the Sun; b be big enough that gravity overcomes the strength of the material making it up, causing the planet to collapse more or less into the shape of a sphere, and; c it has to clear its path of all other debris.
Pluto satisfies categories a and b, but not c. So, Pluto fails the planet test and we move on. The orbits of the planets look spectacularly circular, and they are, more or less. So, this brings us to the point where we re-examine some things that we once learned. The orbits of planets are, strictly speaking, ellipses. This is a transcript from the video series Understanding the Misconceptions of Science.
Watch it now, Wondrium. You probably remember what an ellipse is. A circle has a center and if you tie a string to the center and to a pencil, you can draw a circle. In contrast, an ellipse has 2 foci, which are 2 spots separated slightly from one another. If you take a string and tie it to both of the foci, you can draw the ellipse by kind of doing the same thing you did with the circle, and get a squashed circle.
Learn more about the myths of orbital motion. Now, it turns out that the orbits of the planets are pretty circular. But they are actually ellipses, and this was first worked out in the early s by Johannes Kepler. That honor is held by Aristarchus of Samos, a Greek philosopher who lived in the second century B. Recalling that the Earth moves in an elliptical path much exaggerated in Fig. The difference is about three million miles out of an average distance of ninety-three million miles.
The speed of the Earth in its orbit increases as it gets nearer to the Sun. Since the Earth is closest to the Sun in January and furthest in July, it follows that the Earth is moving more rapidly in its orbit in January than in July!
Thus, the Earth must rotate a little more each day from October to April to return to a chosen spot to face the Sun again. This small amount each day accumulates until it amounts to a difference of 7. Having to turn a little more each day means the sundial lags behind the standard clock and so the sundial time minus standard time on April 2 is Beginner How do you measure a planet's mass?
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