<p>No. If you only know the distance to your object, that tells you absolutely nothing about where it is. That's like saying, I know the grocery store is 5 miles from my house, how can use this information to figure out exactly where it is? The answer is that you just can't, you need more information. – zephyr </p>

<p>In 3D space, you need three coordinates to locate an object. Ra, Dec are two (position on a sphere), z (or some other radial distance measure for nearby objects) is the third. Astronomy usually tries to make do with just Ra, Dec because that's easy to measure. – Alex</p>

<p>(comments converted to answer under cc licence)</p>

CC BY-SA 3.0
2017-06-05T18:28:35.440Z
2017-06-05T18:28:35.440Z
James K
<p>The redshift-distance relation is $cz=H_0d$. So if we have $z$ we can calculate $d$. But is there a way to calculate or covert or write this $d$ in terms of right ascension (RA) and declination (DEC) ?</p>

<p>Thanks in advance</p>

CC BY-SA 3.0
2017-06-05T13:32:35.963Z
No. If you only know the distance to your object, that tells you absolutely nothing about where it is. That's like saying, I know the grocery store is 5 miles from my house, how can use this information to figure out exactly where it is? The answer is that you just can't, you need more information.
zephyr
CC BY-SA 3.0
2017-06-05T14:20:00.743Z
To add to the previous comment: In 3D space you need three coordinates to locate an object. Ra, Dec are two (position on a sphere), z (or some other radial distance measure for nearby objects) is the third. Astronomy usually tries to make do with just Ra, Dec because that's easy to measure.
Alex
<coordinate><redshift>
relative distance and right ascention
user17094
# relative distance and right ascention

(Asked by: username_0 on 2017-06-05)

The redshift-distance relation is $cz=H\_0d$. So if we have $z$ we can calculate $d$. But is there a way to calculate or covert or write this $d$ in terms of right ascension (RA) and declination (DEC) ?

Thanks in advance

**Tags:** coordinate, redshift

> **Comments:**
> **username_1**: No. If you only know the distance to your object, that tells you absolutely nothing about where it is. That's like saying, I know the grocery store is 5 miles from my house, how can use this information to figure out exactly where it is? The answer is that you just can't, you need more information.  
> **username_2**: To add to the previous comment: In 3D space you need three coordinates to locate an object. Ra, Dec are two (position on a sphere), z (or some other radial distance measure for nearby objects) is the third. Astronomy usually tries to make do with just Ra, Dec because that's easy to measure.  

---

## 1 answers

### Answer by username_3 on 2017-06-05. Score: 1

No. If you only know the distance to your object, that tells you absolutely nothing about where it is. That's like saying, I know the grocery store is 5 miles from my house, how can use this information to figure out exactly where it is? The answer is that you just can't, you need more information. – username_1

In 3D space, you need three coordinates to locate an object. Ra, Dec are two (position on a sphere), z (or some other radial distance measure for nearby objects) is the third. Astronomy usually tries to make do with just Ra, Dec because that's easy to measure. – username_2

(comments converted to answer under cc licence)


---

