'Geocentric' object has no attribute 'observe'

[Cannikev]

Apologies for what I think will turn out to be a silly question. I've spent a bit of time trying to find the answer, but cannot seem to resolve by myself:
I'm trying to determine the position of an object, but I'm receiving an error:
observer.at(t).observe(body) #AttributeError: 'Geocentric' object has no attribute 'observe'

I'm following the example on this page:
Positions
and have come up with this:

from skyfield.api import N,S,E,W, wgs84, Loader
planets = Loader('de421.bsp')
earth, mars = planets['earth'], planets['mars']
ts = planets.timescale()
# Altitude and azimuth in the sky of a
# specific geographic location

boston = earth + wgs84.latlon(42.3583 * N, 71.0603 * W, elevation_m=43) #AttributeError: 'Geocentric' object has no attribute 'observe'
astro = boston.at(ts.utc(1980, 3, 1)).observe('mars')
app = astro.apparent()

alt, az, distance = app.altaz()
print(alt.dstr())
print(az.dstr())
print(distance)

I'm running skyfield version (1.46), which I think is the latest, what did I miss?

[Bernmeister]

Made a few changes...

#!/usr/bin/env python3

import skyfield
print( skyfield.__version__ )

from skyfield.api import N,S,E,W, wgs84, load
planets = load('de421.bsp')
earth, mars = planets['earth'], planets['mars barycenter']
ts = load.timescale()
# Altitude and azimuth in the sky of a
# specific geographic location

boston = earth + wgs84.latlon(42.3583 * N, 71.0603 * W, elevation_m=43) #AttributeError: 'Geocentric' object has no attribute 'observe'
astro = boston.at(ts.utc(1980, 3, 1)).observe( mars )
app = astro.apparent()

alt, az, distance = app.altaz()
print(alt.dstr())
print(az.dstr())
print(distance)

and got the results

1.46
24deg 30' 27.2"
93deg 04' 29.5"
0.678874 au

[Cannikev]

Thank you