<p>The terms refer to the <em>viewing angle</em>, i.e. from which direction do we observe a (disk) galaxy. If we happen to be located roughly in its plane of rotation, we see it from the "edge", whereas if we are more or less above or below its plane, we see its "face".</p>

<p>To make a more quantitative statement, the orientation is described by the <em>inclination angle</em> $i$, which is the angle away from "face-on". That is, the inclination of a galaxy that is observed face-on is $i=0$, and the inclination of a galaxy observed edge-on is $i = 90º$. For randomly oriented galaxies, the mean inclination angle will be around $i = 60º$.</p>

<p><a href="https://i.sstatic.net/oZavx.png" rel="noreferrer"><img src="https://i.sstatic.net/oZavx.png" alt="Inclination"></a></p>

<p>Wrt. your second question, you're right that the text is not correct. The three small constellations lie <em>north</em> of hydra. "North" means "toward higher values of declination". Perhaps it's more easily seen in this image (from <a href="https://en.wikipedia.org/wiki/Hydra_(constellation)" rel="noreferrer">Wikipedia</a>):</p>

<p><a href="https://i.sstatic.net/3GY5E.png" rel="noreferrer"><img src="https://i.sstatic.net/3GY5E.png" alt="Sky"></a></p>

CC BY-SA 3.0
2016-11-16T18:41:27.853Z
Regarding the north/south issue, I think you're correct, but you wouldn't believe the crazy inconsistency between the meaning of north/south/east/west in astronomical coordinate systems that I've seen. Maybe the book is "right" using whatever weird way of defining south that they have.
zephyr
CC BY-SA 3.0
2016-11-16T18:56:39.593Z
@zephyr: I agree, astronomical coordinates are a mess to get one's head around. But in general, if you see a map — at least in scientific journals — north is up, and east is left (i.e. R.A increases to the left). This is contrary to a world map where east is right, since the sky is seen "from the inside".
pela
CC BY-SA 3.0
2016-11-16T10:12:02.670Z
2016-11-16T11:00:04.663Z
2016-11-16T11:00:04.663Z
pela
<p>I just want to add pictures to augment <a href="https://astronomy.stackexchange.com/a/19075/14255">pela's answer</a>.</p>

<p>Just google "edge on galaxy" and "face on galaxy" to find results like these, it is entirely about the viewing angle.</p>

<p><strong>Example of a face on galaxy (Messier 74)</strong>
<a href="https://i.sstatic.net/KHER5.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/KHER5.jpg" alt="Messier 74"></a></p>

<p><strong>Example of an edge on galaxy (NGC 891)</strong>
<a href="https://i.sstatic.net/HWu12.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/HWu12.jpg" alt="NGC 891"></a></p>

CC BY-SA 3.0
2016-11-16T18:57:38.973Z
Yes, nice with some real examples!
pela
CC BY-SA 3.0
2016-11-17T05:59:31.930Z
Thank you. I completely understand due to the image you attached so kindly.
Purple Rain Kim
CC BY-SA 3.0
2016-11-16T17:56:52.993Z
2016-11-16T17:56:52.993Z
2017-04-13T12:59:06.910Z
Cody
<p>I already googled a face-on galaxy and an edge-on galaxy, but I couldn't find any proper information. Would you explain them to me?</p>

CC BY-SA 3.0
2016-12-15T08:50:37.023Z
An edge on view of a book is simple, a face on view should be logical. spiral galaxies are flat.
bandybabboon
CC BY-SA 3.0
2016-12-16T07:43:37.280Z
Kim - literally the very first Google result explains this perfectly.
Rory Alsop
<galaxy><constellations>
face-on galaxy and edge-on galaxy
Purple Rain Kim
# face-on galaxy and edge-on galaxy

(Asked by: username_0 on 2016-11-16)

I already googled a face-on galaxy and an edge-on galaxy, but I couldn't find any proper information. Would you explain them to me?

**Tags:** galaxy, constellations

> **Comments:**
> **username_1**: An edge on view of a book is simple, a face on view should be logical. spiral galaxies are flat.  
> **username_2**: Kim - literally the very first Google result explains this perfectly.  

---

## 2 answers

### Answer by username_3 on 2016-11-16. Score: 21

The terms refer to the *viewing angle*, i.e. from which direction do we observe a (disk) galaxy. If we happen to be located roughly in its plane of rotation, we see it from the "edge", whereas if we are more or less above or below its plane, we see its "face".

To make a more quantitative statement, the orientation is described by the *inclination angle* $i$, which is the angle away from "face-on". That is, the inclination of a galaxy that is observed face-on is $i=0$, and the inclination of a galaxy observed edge-on is $i = 90º$. For randomly oriented galaxies, the mean inclination angle will be around $i = 60º$.

[![Inclination](https://i.sstatic.net/oZavx.png)](https://i.sstatic.net/oZavx.png)

Wrt. your second question, you're right that the text is not correct. The three small constellations lie *north* of hydra. "North" means "toward higher values of declination". Perhaps it's more easily seen in this image (from [Wikipedia](https://en.wikipedia.org/wiki/Hydra_(constellation))):

[![Sky](https://i.sstatic.net/3GY5E.png)](https://i.sstatic.net/3GY5E.png)

> **Comments:**
> **username_4**: Regarding the north/south issue, I think you're correct, but you wouldn't believe the crazy inconsistency between the meaning of north/south/east/west in astronomical coordinate systems that I've seen. Maybe the book is "right" using whatever weird way of defining south that they have.  
> **username_3**: @username_4: I agree, astronomical coordinates are a mess to get one's head around. But in general, if you see a map — at least in scientific journals — north is up, and east is left (i.e. R.A increases to the left). This is contrary to a world map where east is right, since the sky is seen "from the inside".  

---

### Answer by username_5 on 2016-11-16. Score: 17

I just want to add pictures to augment [username_3's answer](https://astronomy.stackexchange.com/a/19075/14255).

Just google "edge on galaxy" and "face on galaxy" to find results like these, it is entirely about the viewing angle.

**Example of a face on galaxy (Messier 74)**
[![Messier 74](https://i.sstatic.net/KHER5.jpg)](https://i.sstatic.net/KHER5.jpg)

**Example of an edge on galaxy (NGC 891)**
[![NGC 891](https://i.sstatic.net/HWu12.jpg)](https://i.sstatic.net/HWu12.jpg)

> **Comments:**
> **username_3**: Yes, nice with some real examples!  
> **username_0**: Thank you. I completely understand due to the image you attached so kindly.  

---

