# How to construct a sky map

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 and AstroPy.

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.

• 08:30 in what time zone? Jul 3 '18 at 3:22
• 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 Jul 3 '18 at 9:28
• 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. tex.stackexchange.com/a/438149/14757) Jul 3 '18 at 10:42
• 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) Jul 3 '18 at 12:38
• fotw.info/flags/br_astro.html may or may not be helpful
– user21
Jul 5 '18 at 5:34

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.

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.

• 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). Jul 3 '18 at 16:23
• 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. Jul 3 '18 at 17:05