I’ve been working on new project recently called AstroChallenge. While the details of what exactly AstroChallenge is will have to come later, rest assured, it has to do with Astronomy.

One of the bits of information I’m interested in is whether a particular celestial object is visible in the sky or not. Given an observer’s latitude, longitude and elevation and an object’s right ascension and declination it becomes a straightforward calculation.

However, there are libraries written by smarter people than I and it would be a good idea to use them. So instead if spending my time carefully coding maths, I can simply:

$pip install pyephem

and

import ephem

into my project.

Now the work is done for me. Isn’t the modern age great?

PyEphem is a great python library for performing calculations on all sorts of celestial objects including planets, moons, comets, asteroids, stars and deep space objects. Once you input your observation date, time and location some of the interesting functions you can run include:

• Next transit

• Altitude, Azimuth

• Distance from Earth, Sun, other bodies

• Current Constellation

• Phase, day, month and year

And so on. When you set up an observer, you can even supply dates in the path so you can, for example, find the positions of the moons of Jupiter on February 15, 1564.

Since AstroChallenge is a webapp written in Django we have data models for things like deep space objects on which we can place handy methods to get information from pyephem:

(fields truncated for readability) {{< highlight python >}} class DeepSpaceObject(models.Model): ra_hours = models.IntegerField() ra_minutes = models.FloatField() dec_sign = models.CharField(max_length=1, choices=((‘+’, ‘+’), (‘-‘, ‘-‘)), default=”+”) dec_deg = models.IntegerField() dec_min = models.FloatField()

@property
def fixed_body(self):
    """ Return a FixedBody object which PyEphem uses to perform calculations """
    object = ephem.FixedBody()
    object._ra = "{0}:{1}".format(self.ra_hours, self.ra_minutes)
    object._dec = "{0}{1}:{2}".format(self.dec_sign, self.dec_deg, self.dec_min)
    return object

def observation_info(self, observer):
    """ Given an observer, perform the calculations we are interested in and return them as a dictionary """
    p_object = self.fixed_body
    p_object.compute(observer)
    up = True if ephem.degrees(p_object.alt) > 0 else False
    return {
        'alt': str(p_object.alt),
        'az': str(p_object.az),
        'up': up,
        'neverup': p_object.neverup,
        'rise': timezone.make_aware(observer.next_rising(p_object).datetime(), pytz.UTC) if p_object.rise_time else None,
        'set': timezone.make_aware(observer.next_setting(p_object).datetime(), pytz.UTC) if p_object.set_time else None
    }

{{< / highlight >}}

Some things to note:

• An object is “visible” if it’s Altitude is greater than 0, meaning it is above the horizon. If it still light out, or you live in a light polluted area, you’re probably still out of luck, though,

• PyEphem’s Observer.next_rising/setting methods may return None, that means an object either never rises (as can be determined using Body.neverup) or never sets.

The Observer data can be provided using a simple method on a UserProfile model:

{{< highlight python >}} class UserProfile(models.Model): user = models.OneToOneField(User, editable=False) timezone = TimeZoneField(default=”UTC”) lat = models.FloatField(“latitude”, default=0.0) lng = models.FloatField(“longitude”, default=0.0) elevation = models.IntegerField(default=0)

@property
def observer(self):
    observer = ephem.Observer()
    observer.lat, observer.lon, observer.elevation = str(self.lat), str(self.lng), self.elevation
    return observer

@property
def sunset(self):
    sun = ephem.Sun()
    sun.compute(self.observer)
    return timezone.make_aware(self.observer.next_setting(sun).datetime(), pytz.UTC)

{{< / highlight >}}

Notice the observer property just returns an observer, so we can now supply it in our views to a Celestial object and get the information we need. Another handy property, sunset uses the observer property to compute the time at which the sun will be setting for this user. PyEphem rocks.