Skip to main content
`Ctrl`+`K`

# Determining and plotting the altitude/azimuth of a celestial object

This example demonstrates coordinate transformations and the creation of visibility curves to assist with observing run planning.

In this example, we make a `SkyCoord` instance for M33. The altitude-azimuth coordinates are then found using `astropy.coordinates.EarthLocation` and `astropy.time.Time` objects.

This example is meant to demonstrate the capabilities of the `astropy.coordinates` package. For more convenient and/or complex observation planning, consider the astroplan package.

Let’s suppose you are planning to visit picturesque Bear Mountain State Park in New York, USA. You are bringing your telescope with you (of course), and someone told you M33 is a great target to observe there. You happen to know you are free at 11:00 PM local time, and you want to know if it will be up. Astropy can answer that.

    >>> import matplotlib.pyplot as plt
    >>> import numpy as np
    >>> from astropy import units as u
    >>> from astropy.coordinates import AltAz, EarthLocation, SkyCoord, get_body, get_sun
    >>> from astropy.time import Time
    >>> from astropy.visualization import quantity_support

`astropy.coordinates.SkyCoord.from_name()` uses Simbad to resolve object names and retrieve coordinates.

Get the coordinates of M33:

    >>> # m33 = SkyCoord.from_name("M33")
    >>> m33 = SkyCoord(23.46206906, 30.66017511, unit="deg")

Use `astropy.coordinates.EarthLocation` to provide the location of Bear Mountain and set the time to 11pm Eastern Daylight Time (EDT) on 2012 July 12:

    >>> bear_mountain = EarthLocation(lat=41.3 * u.deg, lon=-74 * u.deg, height=390 * u.m)
    >>> utcoffset = -4 * u.hour  # EDT
    >>> time = Time("2012-7-12 23:00:00") - utcoffset

`astropy.coordinates.EarthLocation.get_site_names()` can be used to get locations of major observatories.

Use `astropy.coordinates` to find the Alt, Az coordinates of M33 at as observed from Bear Mountain at 11pm on 2012 July 12:

    >>> m33altaz = m33.transform_to(AltAz(obstime=time, location=bear_mountain))
    >>> print(f"M33's Altitude = {m33altaz.alt:.2}")
    M33's Altitude = 0.13 deg

This is helpful since it turns out M33 is barely above the horizon at this time. It is more informative to find M33’s airmass over the course of the night.

Find the Alt, Az coordinates of M33 at 100 times evenly spaced between 10 PM and 7 AM EDT:

    >>> midnight = Time("2012-7-13 00:00:00") - utcoffset
    >>> delta_midnight = np.linspace(-2, 10, 100) * u.hour
    >>> frame_July13night = AltAz(obstime=midnight + delta_midnight, location=bear_mountain)
    >>> m33altazs_July13night = m33.transform_to(frame_July13night)

Convert Alt, Az to airmass with `secz` attribute:

    >>> m33airmasss_July13night = m33altazs_July13night.secz

Plot the airmass as a function of time:

    >>> with quantity_support():
    ...     fig, ax = plt.subplots(1, 1, figsize=(12, 6))
    ...     ax.plot(delta_midnight, m33airmasss_July13night)
    ...     ax.set_xlim(-2, 10)
    ...     ax.set_ylim(1, 4)
    ...     ax.set_xlabel("Hours from EDT Midnight")
    ...     ax.set_ylabel("Airmass [Sec(z)]")
    ...     plt.draw()

Use `get_sun()` to find the location of the Sun at 1000 evenly spaced times between noon on July 12 and noon on July 13:

    >>> delta_midnight = np.linspace(-12, 12, 1000) * u.hour
    >>> times_July12_to_13 = midnight + delta_midnight
    >>> frame_July12_to_13 = AltAz(obstime=times_July12_to_13, location=bear_mountain)
    >>> sunaltazs_July12_to_13 = get_sun(times_July12_to_13).transform_to(frame_July12_to_13)

Do the same with `get_body()` to find when the moon is up. Be aware that this will need to download a 10 MB file from the internet to get a precise location of the moon.

    >>> moon_July12_to_13 = get_body("moon", times_July12_to_13)
    >>> moonaltazs_July12_to_13 = moon_July12_to_13.transform_to(frame_July12_to_13)

Find the Alt, Az coordinates of M33 at those same times:

    >>> m33altazs_July12_to_13 = m33.transform_to(frame_July12_to_13)

Make a figure illustrating nighttime and the altitudes of M33 and the Sun over that time:

    >>> with quantity_support():
    ...     fig, ax = plt.subplots(1, 1, figsize=(12, 6))
    ...     ax.plot(delta_midnight, sunaltazs_July12_to_13.alt, color="r", label="Sun")
    ...     ax.plot(
    ...         delta_midnight, moonaltazs_July12_to_13.alt, color=[0.75] * 3, ls="--", label="Moon"
    ...     )
    ...     mappable = ax.scatter(
    ...         delta_midnight,
    ...         m33altazs_July12_to_13.alt,
    ...         c=m33altazs_July12_to_13.az.value,
    ...         label="M33",
    ...         lw=0,
    ...         s=8,
    ...         cmap="viridis",
    ...     )
    ...     ax.fill_between(
    ...         delta_midnight,
    ...         0 * u.deg,
    ...         90 * u.deg,
    ...         sunaltazs_July12_to_13.alt < (-0 * u.deg),
    ...         color="0.5",
    ...         zorder=0,
    ...     )
    ...     ax.fill_between(
    ...         delta_midnight,
    ...         0 * u.deg,
    ...         90 * u.deg,
    ...         sunaltazs_July12_to_13.alt < (-18 * u.deg),
    ...         color="k",
    ...         zorder=0,
    ...     )
    ...     fig.colorbar(mappable).set_label("Azimuth [deg]")
    ...     ax.legend(loc="upper left")
    ...     ax.set_xlim(-12 * u.hour, 12 * u.hour)
    ...     ax.set_xticks((np.arange(13) * 2 - 12) * u.hour)
    ...     ax.set_ylim(0 * u.deg, 90 * u.deg)
    ...     ax.set_xlabel("Hours from EDT Midnight")
    ...     ax.set_ylabel("Altitude [deg]")
    ...     ax.grid(visible=True)
    ...     plt.draw()

(`png`, `svg`, `pdf`)

(`png`, `svg`, `pdf`)