<p>There are a few details here that make this non-trivial. The year doesn't matter much, since the same stars are visible each year. Further, in Stellarium, you can set the date to 1889 so this is not an issue but we have to turn off atmospheric effects to make the stars visible.</p>

<p>More tricky is that the Brazilian flag is designed as if viewed "from above". We can flip the image from Stellarium to simulate this.</p>

<p>Now, the stars are on a globe and the flag is flat, so a projection is needed to represent it. It's not clear what projection the flag designers used, but perhaps it is "orthographic". Stellarium can do that projection.</p>

<p>The projection also seems to be slanted, so "north-south" isn't vertical, although it's not clear by how much. We can estimate this slant from the position of the equator.</p>

<p>So here is an image of the sky from Rio. The Southern Cross is visible towards the bottom of the image.</p>

<p><a href="https://i.sstatic.net/7SeZ0.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/7SeZ0.png" alt="enter image description here"></a></p>

<p>If you want to do this yourself in Pyephem you can create a "fixed body" object for each star (Pyephem has a catalogue of brighter stars), create an observer in Rio, and call "body.compute(observer)" which will calculate the azimuth and altitude of the star. Their orthographic projection is then the application of the formulas found on Wikipedia, and you can plot that using any plotting package. </p>

CC BY-SA 4.0
2018-07-03T16:23:47.540Z
Amazing view. Let me read everything you wrote, try to follow ideas and appreciate the result. Is it possible in Stellarium to filter the stars and plot only those from the flag (I have their names).
Sigur
CC BY-SA 4.0
2018-07-03T17:05:27.283Z
I think north is up; the faint star at bottom center represents pole star $\sigma$ Oct. The white band might represent the ecliptic. Also you can hide the Sun: View > Sky > uncheck Solar System objects.
Mike G
CC BY-SA 4.0
2018-07-03T16:02:18.070Z
2018-07-08T19:14:56.677Z
2018-07-08T19:14:56.677Z
James K
<p>First of all, I have very basic knowledge of astronomy so sorry if I make some mistakes in my terminology.</p>

<p>I'd like to plot the Southern Cross. I can find the name of its 5 stars. Also, I have some knowledge of Python and I've tried to use <a href="http://rhodesmill.org/pyephem/" rel="nofollow noreferrer">PyEphem</a> and <a href="http://www.astropy.org/" rel="nofollow noreferrer">AstroPy</a>. </p>

<p>What I don't know is: what kind of information do I need to plot the cross on the circle? I mean, I'd like to plot it exactly at the right position.</p>

<p>The most difficult part is: the observer should be in <code>Rio de Janeiro</code> and the date should be <code>1889-11-15 08:30:00 UTC-03</code>.</p>

<p>I tried to collect some info with the Astropy function <code>FixedTarget.from_name()</code> but, if I understood correctly, the date is too far in the past.</p>

<p>If anybody could give me some guidelines (maybe an online tool) to create a sky map with the cross in the right position, I'd really appreciate it. </p>

CC BY-SA 4.0
2018-07-03T03:22:06.207Z
08:30 in what time zone?
Mike G
CC BY-SA 4.0
2018-07-03T09:28:18.953Z
To extend on Mike's comment, 08:30 in the early summer in Rio is during the day, and the stars would not be visible, Rio is three hours behind UTC, but what time was used in 1889?  If you just want to get a map you can use Stellarium
James K
CC BY-SA 4.0
2018-07-03T10:42:57.783Z
Dear friends, thanks for helping. So, @JamesK is right. The time is UTC-03. I suppose that time there was no summer time. Also, even if it is not possible to see the stars during the day, is it possible to discover its position, right? With Stellarium, is it possible to save the star positions? I'd like to confirm the position of stars in Brazilian flag (cf. https://tex.stackexchange.com/a/438149/14757)
Sigur
CC BY-SA 4.0
2018-07-03T12:38:59.493Z
The first thing to note about the stars on the Brazillian flag is that they are viewed as they would appear if projected onto the globe, and viewed from above. In other words, they are the mirror image of what would be seen in the sky (where we view the stars from below)
James K
CC BY-SA 4.0
2018-07-05T12:10:00.760Z
@barrycarter, so seems that there is no way to fully represent the flag according to the sky. Maybe the positions are not correct on the flag. Thanks for link.
Sigur
<star><amateur-observing><positional-astronomy><python>
How to construct a sky map
Sigur
# How to construct a sky map

(Asked by: username_0 on 2018-07-02)

First of all, I have very basic knowledge of astronomy so sorry if I make some mistakes in my terminology.

I'd like to plot the Southern Cross. I can find the name of its 5 stars. Also, I have some knowledge of Python and I've tried to use [PyEphem](http://rhodesmill.org/pyephem/) and [AstroPy](http://www.astropy.org/).

What I don't know is: what kind of information do I need to plot the cross on the circle? I mean, I'd like to plot it exactly at the right position.

The most difficult part is: the observer should be in `Rio de Janeiro` and the date should be `1889-11-15 08:30:00 UTC-03`.

I tried to collect some info with the Astropy function `FixedTarget.from_name()` but, if I understood correctly, the date is too far in the past.

If anybody could give me some guidelines (maybe an online tool) to create a sky map with the cross in the right position, I'd really appreciate it.

**Tags:** star, amateur-observing, positional-astronomy, python

> **Comments:**
> **username_1**: 08:30 in what time zone?  
> **username_2**: To extend on username_1's comment, 08:30 in the early summer in Rio is during the day, and the stars would not be visible, Rio is three hours behind UTC, but what time was used in 1889?  If you just want to get a map you can use Stellarium  
> **username_0**: Dear friends, thanks for helping. So, @username_2 is right. The time is UTC-03. I suppose that time there was no summer time. Also, even if it is not possible to see the stars during the day, is it possible to discover its position, right? With Stellarium, is it possible to save the star positions? I'd like to confirm the position of stars in Brazilian flag (cf. https://tex.stackexchange.com/a/438149/14757)  
> **username_2**: The first thing to note about the stars on the Brazillian flag is that they are viewed as they would appear if projected onto the globe, and viewed from above. In other words, they are the mirror image of what would be seen in the sky (where we view the stars from below)  
> **username_0**: @barrycarter, so seems that there is no way to fully represent the flag according to the sky. Maybe the positions are not correct on the flag. Thanks for link.  

---

## 1 answers

### Answer by username_2 on 2018-07-03. Score: 2

There are a few details here that make this non-trivial. The year doesn't matter much, since the same stars are visible each year. Further, in Stellarium, you can set the date to 1889 so this is not an issue but we have to turn off atmospheric effects to make the stars visible.

More tricky is that the Brazilian flag is designed as if viewed "from above". We can flip the image from Stellarium to simulate this.

Now, the stars are on a globe and the flag is flat, so a projection is needed to represent it. It's not clear what projection the flag designers used, but perhaps it is "orthographic". Stellarium can do that projection.

The projection also seems to be slanted, so "north-south" isn't vertical, although it's not clear by how much. We can estimate this slant from the position of the equator.

So here is an image of the sky from Rio. The Southern Cross is visible towards the bottom of the image.

[![enter image description here](https://i.sstatic.net/7SeZ0.png)](https://i.sstatic.net/7SeZ0.png)

If you want to do this yourself in Pyephem you can create a "fixed body" object for each star (Pyephem has a catalogue of brighter stars), create an observer in Rio, and call "body.compute(observer)" which will calculate the azimuth and altitude of the star. Their orthographic projection is then the application of the formulas found on Wikipedia, and you can plot that using any plotting package.

> **Comments:**
> **username_0**: Amazing view. Let me read everything you wrote, try to follow ideas and appreciate the result. Is it possible in Stellarium to filter the stars and plot only those from the flag (I have their names).  
> **username_1**: I think north is up; the faint star at bottom center represents pole star $\sigma$ Oct. The white band might represent the ecliptic. Also you can hide the Sun: View > Sky > uncheck Solar System objects.  

---

