Comet Observation using a Cross-staff (Jacob's Staff) and Torquetum
from Peter Apian. Ein kurtzer bericht d' Observation
unnd urtels / des Jüngst erschinnen
Cometen / im weinmon un wintermon dises XXXII. Jars. Ingolstadt (1532).
This delightful illustration is incredibly rich in information. Apian demonstrates diagramatically how the tail of the comet always points directly away from the sun (he was the first to note this phenomena), showing both the comets position in the sky (including its approximate position along the ecliptic) and the sun's position on the ecliptic below the horizon (and thus invisible to the observer!). He also illustrates how a cross-staff is used to determine the angle between a celestial object such as a comet and a bright star (Cauda Leonis).