# B Finding the distance of two points from observer

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1. Feb 15, 2017

### Tahmeed

Lets suppose that two points A and B are very close together compared to their distance from an observer who is very far. Suppose the distance from observer to point A is DA and to point B is DB. How precisely we can find the distances separately.

2. Feb 15, 2017

### BvU

Your question isn't clear to me: Aren't the two observations independent ?

3. Feb 15, 2017

### Tahmeed

There can be aberration.

4. Feb 15, 2017

### BvU

Can you illustrate what you mean with a sketch ?

5. Feb 15, 2017

### Tahmeed

Yes, i will.

6. Feb 15, 2017

### Staff: Mentor

I'm sorry, but as written, the scenario seems like a non-sequitur: There isn't anything described that enables determining distance at all and even if there was, it wouldn't make any difference if the two objects were near to each other or not.

Is this question about Halton Arp?

7. Feb 15, 2017

### Tahmeed

8. Feb 15, 2017

### Tahmeed

how precisely can we measure the DA and DB here?

9. Feb 15, 2017

### Staff: Mentor

Tahmeed, your situation is still unclear. What do points A and B represent? What are the approximate distances you're asking about? The precision and error of our measurements greatly depend on how far away the objects are. Nearby stars are relatively easy to measure, but beyond a few hundred or thousand light years things begin to change since parallax cannot be used. And if you're asking about cosmological distances (greater than a few million light years) then that's a very different scenario indeed...

10. Feb 15, 2017

### newjerseyrunner

Are you trying to simply determine the distance between A and B based on known angle of separation between them and known distances? You can know that exactly using pretty basic geometry.

I would set DB to be your x axis, then use trig to figure out (XA, YA).
The orange vertical is YA.
The orange horizontal is then XA - DB.
The pink line is found using the Pythagorean theorem.

11. Feb 15, 2017

### Simon Peach

if they are a long way away wouldn't you use the small angle approximation?

12. Feb 16, 2017

### Tahmeed

Yes, its kinda like this, but if the error in measurement of DA and DB is higher than their separation, then will we be able to find outt their actual seperation? so i wanted to know how precisely we can measure DA and DB

13. Feb 16, 2017

### newjerseyrunner

No, if you turn those values into variables, then the equation can only be made to provide a range.

Find the values for DB and DA that place them closest together and those that place them furthest apart. That's your range.

14. Feb 16, 2017

### Tahmeed

I got my answer here. thanks