# Kepler third law is a relationship between

### Kepler's Third Law | Imaging the Universe

It turns out that this relationship will serve as the basis for our attempts to derive stellar masses from observations of binary stars but notice how the Third Law. He finally found a relationship that worked: the speed of the planets around their orbits versus their distance from the sun. Kepler's third law states that the. Kepler discovered that the size of a planet's orbit (the semi-major axis of the ellipse) is simply related to sidereal period of the orbit. If the size of the orbit (a) is .

In the next part of Lesson 4these principles will be investigated as we draw a connection between the circular motion principles discussed in Lesson 1 and the motion of a satellite.

Establishing gravity as the cause of the moon's orbit does not necessarily establish that gravity is the cause of the planet's orbits. How then did Newton provide credible evidence that the force of gravity is meets the centripetal force requirement for the elliptical motion of planets? Known data for the orbiting planets suggested the following average ratio: Here is the reasoning employed by Newton: Newton's universal law of gravitation predicts results that were consistent with known planetary data and provided a theoretical explanation for Kepler's Law of Harmonies.

Scientists know much more about the planets than they did in Kepler's days.

**Kepler's Third Law of Planetary Motion Explained, Physics Problems, Period & Orbital Radius**

Use The Planets widget bleow to explore what is known of the various planets. Check Your Understanding 1. Our understanding of the elliptical motion of planets about the Sun spanned several years and included contributions from many scientists.

### Kepler's Third Law expresses the relationship between the semi-major a : Sentence Correction (SC)

Which scientist is credited with the collection of the data necessary to support the planet's elliptical motion? Which scientist is credited with the long and difficult task of analyzing the data? I'll do the first example: Or is there something we're missing? Just what is that constant, really? It turns out that the constant in Kepler's Third Law depends on the total mass of the two bodies involved.

Kepler himself, studying the motion of the planets around the Sun, always dealt with the 2-body system of Sun-plus-planet. The Sun is so much more massive than any of the planets in the Solar System that the mass of Sun-plus-planet is almost the same as the mass of the Sun by itself. Thus, the constant in Kepler's application of his Third Law was, for practical purposes, always the same. But in the case of the Moon's orbit around the Earth, the total mass of the two bodies is much, much smaller than the mass of Sun-plus-planet; that means that the value of the constant of proportionality in Kepler's Third Law will also be different.

On the other hand, if we compared the period and semimajor axis of the orbit of the Moon around the Earth to the orbit of a communications satellite around the Earth, we would once again have almost the same total mass in each case; and thus we would end up with the same relationship between period-squared and semimajor-axis-cubed.

To make a long story short -- we'll tell the whole story later, including a derivation of the formula below from Newton's Law of Gravitation -- one can write Kepler's Third Law in the following way: If we set up a system of units with period P in days semimajor axis a in AU mass Mtot in solar masses then we can determine k very precisely and very simply: Then we can simply turn Kepler's Third Law around to solve for the value of k: What is the value of the Gaussian gravitational constant k?

## Kepler's Three Laws

The key point here is that the only measured quantity we need to find k is time: Now, it's not quite so easy as it sounds, but it can be done without too much trouble.

Moreover, because we can average over many, many, many years, we can determine the length of the year very accurately -- to many significant figures.

Therefore, we can also determine the value of k to many significant figures.