This year’s second total lunar eclipse on Saturday, Dec. 10, will offer a rare chance to see a strange celestial sight traditionally thought impossible.
Over the central regions of the United States, the moon will set as it becomes progressively immersed in the Earth’s umbral shadow. The Rocky Mountain states and the prairie provinces will see the moon set in total eclipse, while out west the moon will start to emerge from the shadow as it sets.
The moon passes through the southern part of the Earth’s shadow, with totality beginning at 6:06 a.m. PST and lasting 51 minutes. [Total Eclipse of the Moon (Infographic)]
For most places in the United States and Canada, there will be a chance to observe an unusual effect, one that celestial geometry seems to dictate can’t happen. The little-used name for this effect is a “selenelion” (or “selenehelion”) and occurs when both the sun and the eclipsed moon can be seen at the same time.
Seeing the impossible
But wait! How is this possible? When we have a lunar eclipse, the sun, Earth and moon are in a geometrically straight line in space, with the Earth in the middle. So if the sun is above the horizon, the moon must be below the horizon and completely out of sight (or vice versa).
And indeed, during a lunar eclipse, the sun and moon are exactly 180 degrees apart in the sky; so in a perfect alignment like this (a “syzygy”) such an observation would seem impossible.
But it is atmospheric refraction that makes a selenelion possible.
Atmospheric refraction causes astronomical objects to appear higher in the sky than they are in reality.
For example: when you see the sun sitting on the horizon, it is not there really. It’s actually below the edge of the horizon, but our atmosphere acts like a lens and bends the sun’s image just above the horizon, allowing us to see it.
This effect actually lengthens the amount of daylight for several minutes or more each day; we end up seeing the sun for a few minutes in the morning before it has actually risen and for a few extra minutes in the evening after it actually already has set.
The same holds true with the moon, as well.
As a consequence of this atmospheric trick, for many localities there will be an unusual chance to observe a senelion firsthand with Saturday morning’s shadowy event. There will be a short window of roughly 1-to-6 minutes (depending on your location) when you may be able to simultaneously spot the sun rising in the east-southeast and the eclipsed full moon setting in the west-northwest.