Really Basic Stuff

[bobloes]

A new member and new to astronomy. I have been looking at the sky (with my naked eye) and thinking about some basic things which have led me to some assumptions which I hope you can confirm or otherwise.

First assumption is that all the planets in our solar system are moving at different speeds. If not we would see (for example) the transit of Venus (dodgy because I do not know what I mean here but I have heard about it) every day.

Second assumption is that the stars in our galaxy (maybe all galaxies) are moving at roughly the same speed. This assumption is because the stars appear in the sky at the same place every day (earth rotation and seasonal light notwithstanding). I did some calculations based on a star 1 light year away and travelling at a speed of 220km/s relative to our sun and found that there would be a 1 degree movement every ten days.

Please treat me gently if the is embarassingly wrong.

[WayBig]

If not we would see (for example) the transit of Venus (dodgy because I do not know what I mean here but I have heard about it) every day.

.

I believe the reason that we dont have a transit of venus every day is that we dont orbt the sun in exactly the same plane as venus...it's close but not exactly the same....so most of the time when venus lies between us and the sun it is either above or below the sun

[themos]

First assumption is that all the planets in our solar system are moving at different speeds. If not we would see (for example) the transit of Venus (dodgy because I do not know what I mean here but I have heard about it) every day.

Every day? Why?

a star 1 light year away and travelling at a speed of 220km/s

220km/sec = 733micro light seconds/sec

1 light year = 31.4 mega light seconds

10days @ 200km/sec = 864kiloseconds @200km/sec = 172.8 mega km = 576 light seconds

angle = travel/distance = (576/31.4) microradians = 18.3 micro-radians = 1.04 milli degrees = 3.78 arc-seconds

[Glen]

Welcome to SGL bobloes you will find all the info you need right here, have a great time.

Glen

[rowan46]

yes the orbital speeds of the planets are faster as they get closer to the sun eg mercury travels at aprox 48km per sec earth aprox 30 and jupiter 13aprox stars should follow the same pattern with the closer ones moving faster round galactic central point but they don't. leading some scientists to postulate dark mass.

[rowan46]

by the way welcome to sgl:icon_salut:

[bobloes]

Rowan46

Thanks for your welcome and confirmation of my first assumption.

Does the second part of your reply mean that the stars do move at the same speed (around the centre of the galaxy) even though that was not what was expected?

I have heard of dark matter (same as dark mass?) being postulated as the reason for differences between the predicted and the actual.

I will be careful in future not to use simple trigonometry to try and calculate bigger things, the dangers of being held up to ridicule are too great.

Thanks in anticipation.

[perrin6]

I have seen pictures of what the constellations will look like in thousands of years time and they are all over the place. I don't think stars are in fixed orbits but rather like a swarm of bees that as a whole move in one direction but weave up and down and side to side due to gravitational affect of the stars around them.

[perrin6]

I dont quite know how Venus could be in transit every day even if it was exactly in the same plane as Earth because Venus orbits more quickly than us and so overtakes us as though in the inside lane of a race. Interesting topic.

[perrin6]

BTW transit just means 'crossing the sun's disc as seen from Earth'. (Like a race observer standing at the centre of the sports arena.

[rowan46]

from what I understand (which isn't much) it was expected that objects closer to the galactic centre would move much faster than they do, either we don't understand the laws of gravity as well as we thought (possible) or there is a large mass that we cannot detect, hence dark matter . I am sorry I can't explain it better but every time I come across this subject it degenerates into a mass of (to me ) meaningless mathematics. I am sure somebody better read and less thick will come and answer this. (i hope so because its a fascinating question) in answer to the second part of your question the answer is as I understand it yes

[Mark-mck]

intresting discussion.

i'd vote for our lack of understanding of physics on the whole as to why stars don't rotate around the center of a galaxy faster the closer they are.

when you put everything into perspective i.e. the age of the cosmos and the ammount of time we've even had the most basic understanding of physics and the universe then it's no wonder there are unanswered questions.

i'm sure that at somepoint in the not too distant future someone will prove einstien wrong. admittadly the man's been right in all his theory's thu's far but as i said on the great scheme of things we know very little about the cosmos and the law's if any it obides by.

[themos]

Galaxy rotation curve - Wikipedia, the free encyclopedia

[rowan46]

Thankyou for the link thats a little clearer than some of the pieces I've read

[WayBig]

I believe the reason that we dont have a transit of venus every day is that we dont orbt the sun in exactly the same plane as venus...it's close but not exactly the same....so most of the time when venus lies between us and the sun it is either above or below the sun

...wrote "every day" - meant "every year"

[bobloes]

Chris

Thanks for trying to get me off the hook but I was just trying to use something as an example which, if we were travelling at the same speed as Venus, would either never occur or occur all the time. My ignorance has only confused matters. Do not forget a fool can ask more questions in a minute than a wise man can answer in a lifetime. No need to ask which of those two characters I play! Thanks for all the points and discussion. I think for the moment I will work on my assumptions being largely true. Cheers.

[Mark-mck]

The only foolish question is the one that's never asked.

so another question is would a fool ask any questions at all?

[Glen]

Barnard's star can be seen in the summer months in Ophiucus is racing towards us at about 140km/s relative and can be seen to move across the sky at 10 arc sec a year (proper motion), itll cover 1/4 of the moons witdh in an average liftime.

also check out RX J0822-4300 a nuetron star booted out of the center of our Galaxy by interaction with the supermassive black hole, that star is whooshing along at 4.8 Million kp/h!

Regards

Glen

[Glen]

With the motion of stars in a galaxy, i believe Einstein didn't include torque or the coriolis effect into his equations, when used negate the use of 'dark matter' to get the required form/result. Is this correct? Also i believe Einstein's field equation is flawed by the fact he based it in a prue vacuum. just my thoughts.

Glen