[AstroPy] Using Astropy to plot skymaps...possible error in tutorial?

William Bridgman wtbsvs at gmail.com
Mon Dec 9 10:50:59 EST 2019

I occasionally need to generate reference charts and plots for some of
my work producing content for general audiences and I was trying to
use the charting tutorial at the bottom of

However, if I plot the Yale Bright star catalog using this algorithm,
I get unrecognizable constellations.

ra,dec,magnitude = YaleStars.ExtractBasic()
coords_icrs = coord.SkyCoord(ra,dec,frame='icrs')
fig = plt.figure(figsize=(8,4.5))
axes = fig.add_subplot(111,projection='mollweide')
axes.scatter(ra.radian, dec.radian,s=(vMagThreshold - magnitude+1)**2)
axes.set_xticklabels(['14h','16h','18h','20h', '22h', '0h',

Checking against my reference charts clearly shows the constellations
reversed (see ICRSflipped.pdf).

A change to the above fixed this issue (ICRS.pdf):

axes.scatter(-ra.radian, dec.radian,s=(vMagThreshold - magnitude+1)**2)
axes.set_xticklabels(['10h','8h','6h','4h', '2h', '0h',

Is there a legitimate convention for plotting skymaps reversed,
perhaps to match instrument view?

What's the procedure for revising tutorials?

Dr. William T."Tom" Bridgman
Scientific Visualization Studio
NASA/Goddard Space Flight Center
Greenbelt, MD  20771
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