Precession, correct movement and eras for ancient or future observers

I have a question about eras. I want to show the night sky as a super-ancient observer will see him or someone in the distant future. Do I have to set the year and epoch the same?

The three scenarios described in Brandon Road Halley's Comet example are as follows.

• my_star.compute(epoch='-100000')
  
  
• my_star.compute(-100000',epoch='-100000')
  
  
• my_star.compute('-100000')
  
  

For me, based on his descriptions of equatorial telescopes and his second example of a description of a modern observer in 1066, I believe that I should choose the second option. That is, to set the era and year in the ancient or future years.

halley.compute (epoch = '1066')
This is probably useless: it calculates halley's current position, but returns coordinates relative to the direction that the earth's axis was pointing in 1066. If you are not using the Conquest era star atlas, this is not useful.

halley.compute ('1066', epoch = '1066')
This is somewhat more promising: it calculates the position of the halo in 1066 and returns coordinates for the orientation of the Earth that year. This can help you visualize how the object was placed over modern observers who considered it a bad omen in the imminent conflict between King Harold of England and William the Bastard. But to plot this position against the background of the stars, you will first need to recalculate the position of each star in coordinates 1066.

halley.compute ('1066')
This is what you will probably use the most; you get the position of halley in 1066, but you are expressed in the coordinates of the year 2000, which your star atlas is probably using.

Why am I so insecure about my images because Jan Ridpat and Rick Pogge got different results. Links to these relevant results are posted on the github issue. Rick says he's also corroborated by the StarryNight results. I don't mind if I'm wrong, I just see Halley's example, which in my opinion is the most common with the results of several people.

I can't post many links on stackoverflow yet (or images!), But I have documented very carefully in my github issue for this at https://github.com/brandon-rhodes/pyephem/issues/61 . I just wanted to be very clear about what I am looking for and show you possible scenarios.

If you want to test two IPython workbooks, see Part1: proper-motion.ipynb

and Part2: big-dipper.ipynb

in the notebooks on github.

There is a big-dipper.ipynb

lot of code in the example , so I will not post it in the tracker, but I will make a blog post soon. See the above github link for all code and images!

I believe that describing Varying Epoch and Year Together is what I want to see the ancient observer seeing the night sky from his perspective and the futuristic observer seeing the night sky from his own perspective. It's kind of tricky to understand sometimes, so I would really love some community input on this.

Here is a small part of the code. The notebook big-dipper.ipynb

shows more detailed scenarios.

%pylab inline
import ephem

UMa = {
'dubhe': 'αUMa', #HIP 54061
'merak': 'βUMa', #HIP 53910
'phecda':'γUMa', #HIP 58001
'megrez':'δUMa', #HIP 59774
'alioth':'εUMa', #HIP 62956
'mizar': 'ζUMa', #HIP 65378
'alcor': '80UMa',#HIP 65477
'alcaid':'ηUMa', #HIP 67301
}

def const(const,year=None,epoch='2000',title=None):
    s = []

    for star in const.keys():
        s.append(ephem.star(star.capitalize()))

    for i in range(len(s)):
        if(year!=None):
            s[i].compute(year,epoch=epoch)
        else:
            s[i].compute(epoch=epoch)

    tau = 2.0 * pi
    degree = tau / 360.0
    hour = tau / 24.0

    ra_list = [star.a_ra / hour for star in s]
    dec_list = [star.a_dec / degree for star in s]

    mag_array = np.array([star.mag for star in s])
    size_array = (5 - mag_array) ** 1.5 * 4

    scatter(ra_list,dec_list,size_array)
    if(title!=None):
        pyplot.title(title)
    gca().xaxis.grid(True)
    gca().yaxis.grid(True)
    gca().invert_xaxis()
    return s