Understanding orbits
To my understanding, an orbit is a balance of centrifugal force generated by the speed of an object and the gravitational pull of what ever it is the object is orbiting. This is why Mercury orbits the sun rapidly. It is very close to the sun where its gravity is very strong. In order to counter the force of gravity it has to move very quickly. Vice versa for pluto, sedna and some exoplanets. Heres where I get bamboozled. Elliptical orbits. Does Pluto (or what ever else with elliptical orbits) vary in orbital speed? When Pluto gets nears perihelion then surely the suns gravitational forces are slightly stronger, and therefore the orbital speed must temporarily increase or else it would spiral inward towards the sun given enough time.
Heres where I get really mystified. Watched a space documentary on galaxies. Apparently the stars on the outer part of the galaxy orbit at similar speeds to the stars in the centre of the galaxy. (stuff in galaxies tends to orbit the centre of it, usually a black hole). Given the nature of gravity, scientists reckon that galaxies should fly apart, but they dont. This is because most matter is undetectable. This is referred to as dark matter. Dark matter maybe invisible and undetectable but it does have gravitational affect. I still fail to understand the concept of dark matter holding galaxies together. Surely the dark matter itself would also orbit galactic centres in the same way as normal matter. That or it would also be flung out. Can someone help me understand the concept of dark matter in thicky terminology?
I was watching "journey through the milky way" on youtube in case your interested. Well over half way through and I was confused.
Heres where I get really mystified. Watched a space documentary on galaxies. Apparently the stars on the outer part of the galaxy orbit at similar speeds to the stars in the centre of the galaxy. (stuff in galaxies tends to orbit the centre of it, usually a black hole). Given the nature of gravity, scientists reckon that galaxies should fly apart, but they dont. This is because most matter is undetectable. This is referred to as dark matter. Dark matter maybe invisible and undetectable but it does have gravitational affect. I still fail to understand the concept of dark matter holding galaxies together. Surely the dark matter itself would also orbit galactic centres in the same way as normal matter. That or it would also be flung out. Can someone help me understand the concept of dark matter in thicky terminology?
I was watching "journey through the milky way" on youtube in case your interested. Well over half way through and I was confused.
I will watch that documentary thanks for the suggestion. I can help with the second part about dark matter. I just watched a different documentary on that. Basically scientists were observing objects in space, and based on observations they were calculating the mass they thought the object in question had, based on what they could see. But when calculating this, they found that the values didn't work. There must be extra mass that they weren't and couldn't observe. This extra mass had to be present, in order for everything to fit into place properly, but what they could see did not contain that mass. Therefore they came up with the idea of dark matter, which is mass that cannot be seen, but is still present. About 80% of the universe is thought to be this dark matter.
Scientists believe that dark matter is non-baryonic, which means that it isn't the regular protons, neutrons etc that we know. Instead they think dark matter could be weakly interacting massive particles (WIMPS), which have much more mass than protons and the likes.
Scientists believe that dark matter is non-baryonic, which means that it isn't the regular protons, neutrons etc that we know. Instead they think dark matter could be weakly interacting massive particles (WIMPS), which have much more mass than protons and the likes.
And regarding elliptical orbits..
Yes, when a planet in an elliptical gets nearer the sun its speed increases. Maximum speed at perihelion and minimum at aphelion.
Yes, when a planet in an elliptical gets nearer the sun its speed increases. Maximum speed at perihelion and minimum at aphelion.
Original post by Stonebridge
And regarding elliptical orbits..
Yes, when a planet in an elliptical gets nearer the sun its speed increases. Maximum speed at perihelion and minimum at aphelion.
Yes, when a planet in an elliptical gets nearer the sun its speed increases. Maximum speed at perihelion and minimum at aphelion.
I thought that was correct, but I wasn't sure so I thought I wouldn't post it in case I was wrong.
Dark matter is an unproven theory to explain the observations you stated.
However the properties of dark matter would be such that it would need to be spread out very thinly throughout the universe, and, it does not interact with objects exerting increasing gravitational fields. i.e. the opposite of normal matter.
As for the first question: yes, the velocity of the planet in an elliptical orbit at any position, will vary dependent on the distance from the star. The period of orbit is simply the time taken for one revolution and so is an average speed. Careful with the words velocity (vector quantity with both magnitude and direction) and speed (scalar quantity with magnitude only). i.e. both circular and elliptical orbits are described by the orbital period (Earth is 1 year) but only the change in velocity explains how the gravitational laws work.
However the properties of dark matter would be such that it would need to be spread out very thinly throughout the universe, and, it does not interact with objects exerting increasing gravitational fields. i.e. the opposite of normal matter.
As for the first question: yes, the velocity of the planet in an elliptical orbit at any position, will vary dependent on the distance from the star. The period of orbit is simply the time taken for one revolution and so is an average speed. Careful with the words velocity (vector quantity with both magnitude and direction) and speed (scalar quantity with magnitude only). i.e. both circular and elliptical orbits are described by the orbital period (Earth is 1 year) but only the change in velocity explains how the gravitational laws work.
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