How do scientists know just by looking at a distant object how many light years it is from earth?

There are a number of ways, depending on the type and distance of the object.





Nearby stars' distances can be triangulated using the ';parallax'; method, in which you carefully measure the position of the star against the distant background at six month intervals, using the Earth's orbit as the base of the triangle.





More distant stars' distance can be measure by observing their spectra. Stars of a particular color and spectral type have known intrinsic (called ';absolute';) brightnesses. By comparing the intrinsic brightness with the observed brightness you know the distance.





Still other types of stars have periodic pulsations in their luminosity. Such stars (called ';Cepheid Variables';) have a known relationship between their intrinsic brightness and their period of pulsation. Hence their distance can be determined as well. Cepheids can be observed at very great distances, and so can be used as ';standard candles'; to measure the distance of other galaxies.How do scientists know just by looking at a distant object how many light years it is from earth?
The best way to do this , is pick a star at around midnight , your local time, when the sun,earth and the star that you picked, form a straight line. Or as close to a straight line as you can.


In 3 months time you observe, and take measurements of the star's position in the sky.


Wait 6 months longer, and do the same thing.


With the information that you have , you can use parallax to figure how distant that star is from our solar system.How do scientists know just by looking at a distant object how many light years it is from earth?
no they need to do some reserch, pinpionting and stuff like that.
dEPENDS ON HOW MUCH zoom they do to see the object for example by a telescope....


image distance=object distance


speed of of light=3.0 x 10^8
For stars they can determine the distance by studying the light. The spectrum of a star can tell you just about everything about it.


Parallax is used for nearby objects and since quite a few stars are close enough to be measured using parallax we have learned a great deal about those stars. And if we then see a star that is much farther away we can find its distance by comparing its spectrum with that of a star for which the distance is known. We can then calculate at which distance the known star would have to be from earth to appear as bright as the star being studied. Some stars, called cepheid variables, allows scientists to measure the distance to the galaxy they are in.


For things like molecular clouds stars are again used as milemarkers. Here the distance to a star in the foreground or the background of the cloud is measured, leving an estimate of the distance to the cloud.
It depends. For really distant objects (billions of light years away) they know the objects will be moving away from us faster the further away they are. This causes the light to be shiftend in frequency (look up the ';Doppler Effect). By measuring the size of the shift, they can estimate the distance.





For stars, etc. closer (in our galaxy) they basically use simple trigonometry. Here's how: astronomers will take (very precise) measurements when the Earth is on one side of the Sun and then 6 months later when its on the other side. That givs us the base of a triangle (with the star at the apex)--and two of the angles. With that imformation you can calculate the lengths of all three sides--and either of the other two will be the distance to the star.


In practice, we have instruments precise enough today you don't have to wait the entire 6 month's (which gives you the longest base for the triangle); just wait long enough for the earth to move a little way along its orbit. But the method is identical either way.
Astronom... is correct, but I'd like to add something. Distances to very distant objects with which cepheids can't be used (because of the distance the cepheids are too faint to resolve) are estimated using just a general estimate.





For example, spiral galaxies have average intrinsic brightnesses, so a very distant one is assumed to have a similar intrinsic brightness. It's apparent brightness versus it's intrinsic brightness gives its apparent distance.





A better estimate can be made if there is a supernova observed in that galaxy. Using it's spectrum a good estimate of the intrinsic brightness of the supernova can be found, and the distance estimated more closely. Years ago, before Cepheids were used, this method was used to determine that the Andromeda Galaxy is, indeed, an object outside our Milky Way. Interestingly, though, the intrinsic brightness of the supernova involved was underestimated, and the distance underestimated accordingly (the distance was still very large).
They see how far it is form other objects and find it that way. I think there is also somthing with blue shifts and red shifts.
Triangulation, I bet.
they don't know...science is nothing more than guesswork, backed up by pompous degrees, tweed jackets with leather elbow patches, and that facial expression of deep thought that also can be seen on constipated dogs...the distance of objects is immaterial...how many light years away a given object is pales in comparison to how well you relate to objects and people in your own life...forget science, it's just another way to distract you from what's real and significant