Kepler's laws of planetary motion - Wikipedia, the free encyclopedia
Basically the 3rd law
The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit.
This is due to sum of masses of the two objects (applies to stars) and the inverse square law of gravity, Newton's second law
Newton's laws of motion - Wikipedia, the free encyclopedia
A record player disc, or any disc, all the parts rotate at same angular speed. So a dot on label is 78 rpm and a dot on outer edge is also 78 rpm (Think of it as an orbit 78 times a minute)
Sun's Galactic rotation period 240 Million years
Spiral pattern rotation period 220–360 Million years
Bar pattern rotation period 100–120 Million years
There is not enough difference! Only a 2:1 ratio on a structure over 150 million light years across. It's behaving more solidly that it should.
Milky Way - Wikipedia, the free encyclopedia
We can see that other disc or spiral galaxies have this same strange rotational property,
Cf. variation in Solar system
Mercury: 87.97 days (0.2 years)
Venus : 224.70 days (0.6 years)
Earth: 365.26 days(1 year)
Mars: 686.98 days(1.9 years)
Jupiter: 4,332.82 days (11.9 years)
Saturn: 10,755.70 days (29.5 years)
Uranus: 30,687.15 days (84 years)
Neptune: 60,190.03 days (164.8 years)
Note that a planet further from the centre MUST have a much longer orbital period:
Orbital period - Wikipedia, the free encyclopedia
Due to masses and gravity. Thus Mars can ONLY be 1.9 yr and Jupiter ONLY 11.9 yrs due to Sun's mass + planet's mass and the distance
In
celestial mechanics, when both orbiting bodies' masses have to be taken into account, the
orbital period T can be calculated as follows:
[3]
T = 2 π a 3 G ( M 1 + M 2 ) {\displaystyle T=2\pi {\sqrt {\frac {a^{3}}{G\left(M_{1}+M_{2}\right)}}}}
where:
- a is the sum of the semi-major axes of the ellipses in which the centers of the bodies move, or equivalently, the semi-major axis of the ellipse in which one body moves, in the frame of reference with the other body at the origin (which is equal to their constant separation for circular orbits),
- M1 + M2 is the sum of the masses of the two bodies,
- G is the gravitational constant.
Note that the orbital period is independent of size: for a scale model it would be the same, when densities are the same (see also
Orbit#Scaling in gravity).[
citation needed]
In a parabolic or hyperbolic trajectory, the motion is not periodic, and the duration of the full trajectory is infinite
The explanation for the "wrong" expansion rate of the Universe is Dark Energy.