Plotting time on the x-axis using matplotlib in pyton3 reading from CSV

Question

I am measuring the mass of Jupiter based on Kepler's Third Law. I have taken 147 images of Jupiter and its moon, specifically Io. I have written code to take the position vectors from the image file so I can calculate the magnitude of the distance between Io and Jupiter (distance). This will allow me to plot distance over time. ie: plot(t,d)

The code below while not elegant maybe, achieves this wonderfully. Yet I am trying to convert my array of datetimes into a plot-able value. I have several nights worth of data. The plot will be sinusoidal and the period of Io's orbit will be derived by interpreting the plot. I have included the code, followed by the CSV file in .txt format: python3

import numpy as np
import matplotlib.pyplot as plt
import datetime
import matplotlib.dates as mdates
import math

# Declare arrays (lists) for (x,y) pixel position vectors from .fits
# image files. 
jup_x = []
jup_y = []
io_x = []
io_y = []
cal_x = []
cal_y = []
gan_x = []
gan_y = []
eur_x = []
eur_y = []
date_and_time = []

readFile = open('raw_data1.txt','r')

sepFile = readFile.read().split('\n')

readFile.close()

for j_pos_pair in sepFile:
    j_xandy = j_pos_pair.split(',')
    jup_x.append(int(j_xandy[1]))
    jup_y.append(int(j_xandy[2]))

print(jup_x)
print(jup_y)

for io_pos_pair in sepFile:
    io_xandy = io_pos_pair.split(',')
    io_x.append(int(io_xandy[3]))
    io_y.append(int(io_xandy[4]))

print(io_x)
print(io_y)

for cal_pos_pair in sepFile:
    cal_xandy = cal_pos_pair.split(',')
    cal_x.append(int(cal_xandy[5]))
    cal_y.append(int(cal_xandy[6]))

print(cal_x)
print(cal_y)

for gan_pos_pair in sepFile:
    gan_xandy = gan_pos_pair.split(',')
    gan_x.append(int(gan_xandy[7]))
    gan_y.append(int(gan_xandy[8]))

print(gan_x)
print(gan_y)

for eur_pos_pair in sepFile:
    eur_xandy = eur_pos_pair.split(',')
    eur_x.append(int(eur_xandy[9]))
    eur_y.append(int(eur_xandy[10]))

print(eur_x)
print(eur_y)

for date_time in sepFile:
    d_and_t = date_time.split(',')
    date_and_time.append(str(d_and_t[0]))

print(date_and_time)

# make into arrays for vector operations
# date_and_time = np.array(date_and_time)
datetime = np.array(date_and_time)
Jup_x = np.array(jup_x)
Jup_y = np.array(jup_y)

Io_x = np.array(io_x)
Io_y = np.array(io_y)

print(Jup_x)
print(Jup_y)
print(Io_x)
print(Io_y)

# vector subtraction (Position vector between Jupiter and Io)
J_Io_s_x = Jup_x - Io_x  # reads: separation between Io and Jupiter x values
J_Io_s_y = Jup_y - Io_x  # reads: separation between Io and Jupiter y values

print(J_Io_s_x)
print(J_Io_s_y)

M_s_J_Io = ((J_Io_s_x)**2 + (J_Io_s_y)**2)**0.5 # The magnitude of the separation 
                                                # between Jupiter and Io in pixels
print(M_s_J_Io)

from datetime import datetime
datetime.strptime("2015-feb-05 19:00", "%Y-%b-%d %H:%M")

x = np.array([datetime.datetime(2015, 2, 5, i, 0) for i in range(24)])
plot_M_s_J_Io = np.array(M_s_J_Io)

plt.plot(x,plot_M_s_J_Io)
plt.show()

The CSV file: (This is the test run copy, the actual list is waaaay larger)

2015-feb-05 20:11,1150,735,1335,647,307,1123,0000,000,965,818

This CSV file above includes info on all the moons as well. For my testing i restricted it to just info on Jupiter and Io. That is only the first five fields in the CSV file. Its indexed from 0-10.

Any suggestions, I know I am close, I just don't understand the time conversion aspect of it. Full disclosure: this IS for a LAB at NMT, however I am already going above and beyond in this report as I am using python instead of doing this by hand. All help is of course greatly appreciated.

Jesse

Answer 1

Since I don't have your files, I can only presume about your data processing that's going on above the line:

from datetime import datetime

However from that point onwards, you have a few wird quirks in your code.

First your statement datetime.strptime("2015-feb-05 19:00", "%Y-%b-%d %H:%M") does nothing. It runs, but doesn't assign the returned value to a variable. So it's safe to get rid of it.

Secondly is in the line:

x = np.array([datetime.datetime(2015, 2, 5, i, 0) for i in range(24)])

This line will work because you imported the datetime module at the start of your script. But if you didn't do that you would've caused an error message to appear. Since you explicitly imported datetime function of datetime module I'd recommend just using it, for brevity/clarity reasons, instead of datetime.datetime

And the final (and probably the only actual mistake made) is that to plot a time sequence in pyplot you have to use pyplot.plot_dates function and not pyplot.plot function.

Answer 2

This is what is running and working, thanks for the input folks:

from datetime import datetime                   
t = datetime.strptime("2015-feb-05 19:00", "%Y-%b-%d %H:%M")
for line in date_and_time:
    t = datetime.strptime(line, "%Y-%b-%d %H:%M")
    plot_time.append(t.timestamp())
plt.plot(plot_time,M_s_J_Io)
plt.show()