ON APPARENT MAGNITUDE

The larger luminous body is D. The same must be understood regarding the Moon.
III. If you seek the method of observation, look at the following figure.

A technical geometric diagram illustrates an astronomical measurement device. On the left, a large circle represents a celestial body such as the Sun or Moon. Dashed lines representing light rays extend from the top and bottom of the circle, passing through a rectangular frame labeled with points L, M, and N. These rays continue to a second, larger rectangular frame labeled with points O, P, Q, and R. Both frames are mounted vertically on a long horizontal bar labeled H I. The diagram shows how the light from the large source projects through the first aperture to create a specific pattern or shadow on the second surface.

This represents a very simple, yet suitable instrument, which I often use to track the diameters of the Sun and the Moon. Namely, H I represents a smooth beam: trabecula of nearly four fathoms: orgyiae, or Parisian toises: a pre-metric unit of length equal to six feet. K L M N and O P Q R are two equal sight-vanes: pinnacidia, or plates used for alignment and measurement nearly half a foot wide. These are erected perpendicular to the plane of the beam and its ends. I raise and support the beam with the sight-vanes (doing this inside a building as dark as possible otherwise) so that the beam is aligned according to the ray of the Sun. With the sight-vane K L M N made the upper one, the Sun directs its shadow onto the lower one, O P Q R. Because the Sun is larger than the upper sight-vane, its shadow must decrease as it moves forward. When it is projected toward the lower sight-vane, it is no longer equal to it. Instead, it appears as a clearly shortened dark area within that surface. Therefore, by taking into account both this shortening and the distance between the sight-vanes, I calculate the diameter, and consequently the apparent magnitude of the luminary. Indeed, as the distance between the sight-vanes is to the half of the light on the sides...