Eclipse From the Blue into the Blue: Unpacking Lunar Eclipse Colors

During the total lunar eclipse on September 7, 2025, a remarkable celestial event unfolded over Záhradné, eastern Slovakia, offering a unique opportunity to explore the science behind the colors we see in the Earth's shadow.

The Science of Shadow Colors

We know that our sky appears blue due to Rayleigh scattering, a phenomenon where shorter blue wavelengths of sunlight are scattered more effectively by the molecules in our atmosphere. During a lunar eclipse, the Earth passes between the Sun and the Moon, casting its shadow onto the lunar surface. While we often associate the reddish hue of a 'blood moon' with the light that filters through the Earth's atmosphere, this particular eclipse provided a deeper insight.

A Blue Beginning and End

This time-lapse mosaic beautifully captures the entire totality of the eclipse, moving from right to left. As expected, the characteristic colors were present. However, at the conclusion of the totality, as the sky darkened, a subtle but distinct blue tint appeared along the lunar edge. This intriguing phenomenon is thought to be caused by the ozone layer in our atmosphere. Unlike other atmospheric components that filter out blue light, ozone allows blue light to pass through, and this filtered blue light then continues its journey to the edge of the Moon.

This eclipse truly lived up to its descriptive title, transitioning from blue into the blue, showcasing the complex interplay of light and our planet's atmosphere on a grand cosmic scale.

Equipment Used: Nikon Z6III, ISO 1000, 3s exposures, 10-inch Newtonian Telescope on EQ-6 mount.