If you’re not a morning person, you might want to consider becoming one, if only for tomorrow. Just before dawn, we’re going to get a rare lunar eclipse coinciding with the sunrise.
A simultaneous total lunar eclipse and sunrise is an incredibly rare event called a “selenelion,” rare because geometry says it can’t possibly happen. For the Moon to be fully eclipsed by the Earth, both have to be perfectly aligned with the Sun, an alignment caked a “syzygy” (say that five times fast). With all three bodies in a line and the Sun and Moon 180 degrees apart in the sky, it should be impossible to see an eclipsed Moon and rising sun, right? Yes. But it’s not. Our atmosphere does some weird things to light and makes it possible to see both the Sun and the Moon at the same time during a total eclipse.
The key is refraction. Light bends as it passes through the particles that make up our atmosphere, and because the atmospheric density varies at different altitudes, the amount of refraction varies as well. A trick of the bending light means that the images of both the Sun and the Moon will be apparent on the horizon at the same time, the Sun for a few minutes before it rises and the Moon for a few minutes after it sets.
But that’s not the only thing that makes this eclipse exciting. Full lunar eclipses come with a red Moon, another really neat trick of light. From a lunar perspective, when the Earth passes between the Sun and the Moon the planet would become circled in a ring of fire. This light passes through the stratosphere, which scatters the light and turns it copper coloured. That light then moves on to the Moon, turning it red. There’s also some turquoise associated with a full lunar eclipses, rooted in the sunlight passing through the upper stratosphere and ozone layer. This region of our atmosphere scatters the sunlight and turns it blueish, and that light also hits the Moon to give it a blueish hue.
Like all eclipses, you need to be in the right place at the right time to see it. And in the case of a selenelion, the right time is a vanishingly short window. This eclipse is only visible on the east coast, and you’ll have about 10 minutes before and after sunrise to scan the hopefully clear horizon with binoculars for signs for a slightly mottled looking Sun and red moon. But if you can see it, it’ll be worth braving the early morning for.
Feature image: Ron Delvaux via The Virtual Telescope Project