def _coord_names(frame):
    """Name the coordinates based on the frame.

    Examples
    --------
    >>> _coord_names('icrs')[0]
    'ra'
    >>> _coord_names('altaz')[0]
    'delta_az'
    >>> _coord_names('sun')[0]
    'hpln'
    >>> _coord_names('galactic')[0]
    'glon'
    >>> _coord_names('ecliptic')[0]
    'elon'
    >>> _coord_names('turuturu')[0]
    Traceback (most recent call last):
       ...
    ValueError: turuturu: Unknown frame

    """
    if frame in ["icrs", "fk5"]:
        hor, ver = "ra", "dec"
    elif frame == "altaz":
        hor, ver = "delta_az", "delta_el"
    elif frame == "sun":
        hor, ver = "hpln", "hplt"
    elif frame == "galactic":
        hor, ver = "glon", "glat"
    elif frame == "ecliptic":
        hor, ver = "elon", "elat"
    else:
        raise ValueError(f"{frame}: Unknown frame")

    return hor, ver