Eclipse of the moon - September 2, 1830

Abstract

This article provides a record and explanation of the appearances observed during the Total Lunar Eclipse of September 2, 1830. The author prefaces the description by noting the superior clarity and distinctness of celestial objects when viewed through the pure atmosphere of the observation location, compared to those in the North.

The paper first explains the mechanics of an eclipse, defining the two parts of the Earth's shadow: the dark inner umbra and the lighter outer penumbra. The primary focus, however, is on the effects of the Earth's atmosphere, which significantly modifies the Moon's appearance during an eclipse:

1. Refraction: The bending of the Sun's rays as they pass through the atmosphere shortens the umbral cone, ensuring the Moon rarely disappears completely, as it receives light even in the darkest part of the shadow. The author references La Place's calculation that this refracted light makes the Moon's centre only about one-fiftieth of its full brilliance.

2. Dispersion: The scattering of light into different colours (like a prism) is responsible for the Moon's colour. During the greatest obscurity, the Moon exhibited a "fiery brazzy hue" (reddish-orange), caused by the lower, denser atmosphere refracting mainly red rays inward. The penumbra had a purple tint.

The author also makes observations on the Moon's surface during the eclipse, noting that different portions shone with different degrees of brightness, particularly a ring of greater illumination around the disc, broadest toward the poles. This observation leads to a discussion of the possibility of water/snow on the Moon, concluding that due to the rare, shallow atmosphere and cold temperatures, any water must exist as a "covering of changeless snow."