- #1
AstrophysicsX
- 61
- 0
Can anyone help explain Kepler's 3rd Law of Planetary Motion to me?
AstrophysicsX said:Can anyone help explain Kepler's 3rd Law of Planetary Motion to me?
AstrophysicsX said:Can anyone help explain Kepler's 3rd Law of Planetary Motion to me?
Creator said:which is exactly how Kepler "measured" the (approx.) mass of the sun in the early 1600's !
Fewmet said:It looks like you are saying the Kepler used a result from Newton's Law of Gravity to find the mass of the Sun, but Kepler died before Newton's birth. If I am misreading you and Kepler did do a rough calculation I'd really be interested in seeing details. Can you provide any?
Thanks.
Creator said:Opps , my bad; it should say: "that is how "Newton" determined the mass of sun in 1600's".
Sorry.
Creator said:Opps , my bad; it should say: "that is how "Newton" determined the mass of sun in 1600's".
Sorry.
starfish99 said:Kepler could have known the approximate distance from the Earth to the Sun( the ancient Greeks may have known this). However, I think he had no way of knowing the distance of the Sun to the other five known planets. So how could he have established his Law with only one point(Earth-Sun distance)?
nasu said:These are relative values of the distances. And is all that Kepler needed to deduce his 3rd law.
Absolute values of the distances were determined in late 18th century by observing the transits of Venus.
Some crude estimate was done during some 17th century transits (by Jeremiah Horrocks) but they were published much later, after Kepler's death.
AstrophysicsX said:And what is that equation describing?
The estimate of the Earth-Sun distance according to Aristarchus was about 20 times smaller than the actual value.Fewmet said:I wonder: would Kepler have had access to the ancient Greek estimate of the Earth-Sun distance? I recall that they were reasonable accurate on the Earth-Moon distance, and the diameters of the Earth and Moon, but less on-target with the Earth-Sun distance.
Creator said:As 'Fewmet" explained ... FOR ANY planet in the solar system, its ratio of orbital period squared to radius cubed is a constant.
T^2 / R^3 = K
...but he didn't tell you what that constant is.
It turns out that the constant is always 4(pi)^2 / GM...where G is the gravitational constant and M is the mass of the sun. (using standard units..kgs system)
And BTW, it matters not if the orbit is highly elliptical; simply substitute the 'semi-major" axis for radius and the formula is still valid.
Similarly ALL satellites in orbit about the Earth will have the same ratio T^2 / R^3 using M as the Earth's mass.
Knowing the constant in terms of its central mass allows you to use the formula for finding orbits about any central mass...or for that matter you can find the mass of anybody by measuring the orbital period and semi-major axis of any of its satellites...which is exactly how Kepler "measured" the (approx.) mass of the sun in the early 1600's !
...
Kepler's 3rd Law of Planetary Motion states that the square of a planet's orbital period (the time it takes to complete one orbit around the sun) is directly proportional to the cube of its average distance from the sun.
Johannes Kepler was a German mathematician and astronomer who lived in the 17th century. His 3rd Law is important because it helped to explain and predict the motion of planets in our solar system, and it also paved the way for Isaac Newton's law of universal gravitation.
Kepler's 3rd Law was discovered through his observations of the orbit of Mars. He noticed that the ratio of a planet's orbital period to its distance from the sun was the same for all planets, and he used this ratio to formulate his 3rd Law.
The units of measurement for Kepler's 3rd Law are typically years (for orbital period) and astronomical units (AU) for distance. However, it can also be expressed in any unit of time and distance as long as they are consistent.
Kepler's 3rd Law can be applied to any two objects in orbit around each other, as long as one object is significantly larger than the other. This includes moons orbiting planets, or artificial satellites orbiting Earth. The law still holds true, but the units of measurement may vary depending on the specific objects and their distances from each other.