How are light-years measured?
- Thread starter Jacob
- Start date
Relatively near stars can be measured by parallax. It means that look at a star at a certain exact time of year and measure the angle it is from earth.
Then you wait 6 months until the earth has moved round to the opposite side of the sun and you measure the angle again. You then just do some simple trigonometry (because you know far it is from one side of the sun to the opposite side as earth orbits) and work out the distance to the star.
It's not very accurate because our sun will be moving relative to the other star. But you hope that in 6 months it won't be too far out.
Nothing much in astronomy is all that accurate. Give or take a few billion miles is pretty close to perfection in the grand scheme.
In terms of stars, you cannot know for sure what is happening now. Even with our own sun, it takes about 8 minutes for light to arrive here. So if the sun suddenly ceased to exist, we would still be bathed in light and experience orbital motion for 8 minutes and then it would be darkness and chaos. But I can't think why you would want to know where a star was now or what it was doing now. Such information is purely academic because we only know what we can see.
Over longer distances (intergalactic) you measure by red shift. The assumption is that the chemicals in space are the same chemicals as on earth. Indeed the same all over the universe. There isn't a different periodic table in different galaxies.
This assumption means that each chemical element gives off a specific combination of wavelengths of light. Element A gives off a spectrum of X and element B gives off a spectrum of Y. And - this is important - there is nothing in between X and Y.
So if you see the spectral signature of an element that looks a lot like that of element A except that it is shifted 30% towards the blue end of the spectrum, then such an element does not exist in the periodic table so it must element be A that is producing it but the source of the light is moving towards us at 30% of the speed of light. Such relative motion would cause the spectral signature to be compressed in much the same way as the sound of a train appears high pitched as it approaches you and lower pitched as it hurtles away from you. Most of the distant galaxies we observe are red-shifted, meaning that they are hurtling away from us.
I'm not sure what you mean here.
I'm not sure whether I should think about measuring distance or think about what is a red shift.
I don't know why you would say this.
Why Star Trek and why BBC World News? I am a United States Citizen and I don't have a great interest in Star Trek.
Maybe I should look at the definition of an astronomical unit?
This doesn't help me.
Here is why... I can measure a think using a ruler or a yard stick because we have a standard measure for an inch or a foot. The same could be said for metric units. But light cannot be measured by these distances.
This comes back to what is the speed of light and how do we know.
Then how do we know the distance of any astronomical object?
What is a red shift and how do we know this is relevant to measuring distances?
These are interesting verses. I do not understand them.
Lightning, or a flash or lightening, is very fast.
Interesting how light may involve waves or charges. I can think of direction or casting a shadow. What about the speed of light, how fast it travels?
Caledvwlch
New member
Neither does OCTOBER23, apparently. So angels move as fast as lightning? (Resisting urge to link to a youtube version of Carl Douglas' Kung Fu Fighting) Like, all the time, or just when they feel like it?
Or, as we would say up here in NH, "Wicked fast, bub."
This explains the basics of how these measurements could possibly be done.
I don't know which of these measurements would have been attempted first, perhaps parallax and a near star. But in this method how would you determine the angle from theoretically two locations? Would a person take data for an entire year of days?
Your post here reminds me of this verse for some reason.
Hebrews 1:7 NASB - And of the angels He says, "WHO MAKES HIS ANGELS WINDS, AND HIS MINISTERS A FLAME OF FIRE."
You might start here. There is no one simple answer.
Here is a question, when observing a red shift, if it be possible, does that mean you are seeing red? What exactly is seen that indicates distance?
You just point your telescope at the star and measure the angle it makes with something fixed like true north pole. It doesn't really matter so long as you use the same method to get a bearing when you are on the other side of the sun. You then work out what the angles are to the star from the line that joins the two points in space where you took the measurements from. It's just basic trigonometry.
Earth is about 93 million miles from the sun so you can get a base line of 186m miles. The larger the base line the more accurate you will be. But it wouldn't help much to take a bearing every day because your base line would not be long enough.
To make sure that you had the right base line you must time your measurements so that they each take place on exactly opposite positions to the sun about 6 months apart.
Certainly the length of a half year is half the length of time of a year, which perhaps would need to be determined.
As for an angle, you are talking about more than one. You are using an angle to get an angle somehow?
Jedidiah
New member
Anybody else a bit creeped out that it takes exactly 1,000 seconds for light to travel from one end of the earth's orbit to the other ?
Just me then.
:carryon:
I don't know that it does.
What do you know of that determines this? What is measuring what?
Here is an explanation. It is quicker and easier for you to follow the link than for me to cut and paste.