def get_grid_data(df):
    
    """
    Prunes dataframe to rows whose longitudes are multiples of 60
    degrees and whose years are multiples of 10. This includes only
    lat/lon grid locations for which we have USNO data for all eight
    types of twilight events.
    """
    
    bools = (df['Longitude'] % 60 == 0) & (df['Year'] % 10 == 0)
    return df[bools]