def read_obs(obs_file):
    """
    Function to read in observation data from text file
    each line should be formatted as "T   ill   V   J"
    """
    f = open(obs_file, 'r' )
    f.readline() # there's a header with column labels
    obs_T, obs_ill, obs_V, obs_J = [], [], [], []
    for line in f:
        l=line.split()
        obs_T.append(float(l[0]))
        obs_ill.append(float(l[1]))
        obs_V.append(float(l[2]))
        obs_J.append(float(l[3]))
    f.close()
    return (obs_T, obs_ill, obs_V, obs_J)