<p>Projected separation is the minimum physical separation of two astronomical objects, as determined from their angular separation and estimated distance.<a href="https://en.wikipedia.org/wiki/File:Stellarparallax_parsec1.svg" rel="nofollow noreferrer">The Parallax shift of a star at a distance of one parsec as seen from the Earth. Not to scale</a> For planets and double stars, this distance is usually given in Astronomical Units. The actual separation of the two objects depends on the angle of the line between the two objects to the line-of-sight of the observer.</p>

CC BY-SA 4.0
2018-10-21T16:05:43.540Z
2019-07-07T16:17:00.507Z
2019-07-07T16:17:00.507Z
FRQ
<p>It means the separation between the two objects <em>if</em> they were both at the same distance away. This separation is found by multiplying the angular separation (in radians) by the distance to the objects.</p>

<p>Often, the distance to extragalactic objects depends on the assumed value of the Hubble parameter. In this case, $h$ is a dimensionless number corresponding to $h=H_0/100$, where $H_0$ is the Hubble parameter in km/s per Mpc.</p>

<p>The current best value of $H_0$ is about 70 km/s per Mpc, so $h\simeq 0.7$.</p>

CC BY-SA 3.0
2015-10-15T17:42:47.013Z
2015-10-15T17:42:47.013Z
ProfRob
<p>I have been reading <a href="http://arxiv.org/abs/astro-ph/0001141" rel="nofollow">this</a> paper on the separation of a binary quasar in which it states:</p>

<blockquote>
  <p>At a projected separation of 8.3/h kpc (Omega_matter = 1), CTQ 839 would be the smallest projected separation binary quasar currently known.</p>
</blockquote>

<p>What is meant by projected separation and where does this unit come from?</p>

<units><quasars>
What is projected separation and how can I make sense of its unit (h^-1 kpc)?
thodic
# What is projected separation and how can I make sense of its unit (h^-1 kpc)?

(Asked by: username_0 on 2015-10-15)

I have been reading [this](http://arxiv.org/abs/astro-ph/0001141) paper on the separation of a binary quasar in which it states:

> At a projected separation of 8.3/h kpc (Omega\_matter = 1), CTQ 839 would be the smallest projected separation binary quasar currently known.

What is meant by projected separation and where does this unit come from?

**Tags:** units, quasars

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## 2 answers

### Answer by username_2 on 2015-10-15. Score: 5 | Accepted answer

It means the separation between the two objects *if* they were both at the same distance away. This separation is found by multiplying the angular separation (in radians) by the distance to the objects.

Often, the distance to extragalactic objects depends on the assumed value of the Hubble parameter. In this case, $h$ is a dimensionless number corresponding to $h=H\_0/100$, where $H\_0$ is the Hubble parameter in km/s per Mpc.

The current best value of $H\_0$ is about 70 km/s per Mpc, so $h\simeq 0.7$.


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### Answer by username_1 on 2018-10-21. Score: 3

Projected separation is the minimum physical separation of two astronomical objects, as determined from their angular separation and estimated distance.[The Parallax shift of a star at a distance of one parsec as seen from the Earth. Not to scale](https://en.wikipedia.org/wiki/File:Stellarparallax_parsec1.svg) For planets and double stars, this distance is usually given in Astronomical Units. The actual separation of the two objects depends on the angle of the line between the two objects to the line-of-sight of the observer.


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