Trying to calculate direction in sky of planets at a given time, but getting incorrect answers?

Question

I'm trying to write a program to calculate what planets are above the horizon at a given time, and the direction in the sky the planets are, but I'm getting incorrect results. My code is below, does anyone have any idea what the issue might be?

import numpy as np
import skyfield
from skyfield.api import load
planets = load('de441.bsp')
ts = load.timescale()

earth = planets['earth']
other_planets = [planets['mercury barycenter'], planets['venus barycenter'], planets['mars barycenter'], planets['jupiter barycenter'], planets['saturn barycenter']]
other_planet_names = ["Mercury", "Venus", "Mars", "Jupiter", "Saturn"]
directions = ["North", "Northeast", "East", "Southeast", "South", "Southwest", "West", "Northwest"]

def planets_visible(time):
    numPlanetsViewed = 0
    for i in range (0, 5):
        az, dec, distance = earth.at(time).observe(other_planets[i]).radec()
        if dec._degrees > 0:
            numPlanetsViewed = numPlanetsViewed + 1
            string = other_planet_names[i] + " is " + str(int((round(dec._degrees, 0)))) + " degrees above the horizon!"
            print(string)
            direction = int(np.floor(((az._degrees - 22.5) % 360) / 45))
            string2 = " It is visible to the " + directions[direction] + "."
            print(string2)
            print(" ")
    if numPlanetsViewed == 0:
        print("Sorry, none of the four major planets are visible at this time!")
        
now = ts.now()
planets_visible(now)

I keep getting that, say, right now (4/7/2021, ~4:00 PM), the right planets are visible – yay! – but the azimuths are all off, saying Mars is NE when it's SE, for example (and I can't speak for the right ascensions, since I don't know them at the moment, but I think those are off too.) Does anyone know what' s up?

score 1 · Answer 1 · answered Apr 08 '21 at 03:47

You will want to double-check the documentation: radec() does not return azimuth and altitude, it returns right ascension and declination. Yes, Python is letting you call the return value az, but that’s because Python does not check whether the names you assign to are appropriate; the value is not an azimuth, which is why it is not matching the value you expect. Try reading through the documentation again, and follow its instructions for producing an azimuth; hopefully the numbers will agree better.

Trying to calculate direction in sky of planets at a given time, but getting incorrect answers?

1 Answers1