Second 'tetrad' total lunar eclipse on Wednesday morning could be rare 'selenelion' for parts of Canada.

Tuesday, October 7, 2014, 10:10 PM - Wednesday morning's total lunar eclipse should be quite a spectacular sight for anyone in the western half of North America or somewhere on the waters of the Pacific Ocean. However, even if those on the east coast won't be able to see the entire eclipse, they'll still be in for a rare astronomical treat.

At just before dawn on Wednesday, in the Eastern time zone, the Moon will be setting on the western horizon and the sun will be rising along the eastern horizon, and just for a few minutes it will be possible to see both the Sun and the Moon in the sky at the same time. Meanwhile, the Moon will likely still be a dull shade of red as it just completes a total lunar eclipse.

If a total lunar eclipse happens when the Sun and the Moon are on exact opposite sides of the Earth from one another, how is this possible?

By Tomruen, via Wikimedia Commons

When the Moon passes through Earth's shadow, it is on the exact opposite of the Earth from the Sun. So, even if the Sun and the Moon are both near the horizon, one should set as the other rises. You shouldn't be able to see both entirely above the horizon at the same time. However, during a 'selenelion' - or horizontal eclipse - that's exactly what you can see for a short time. The reason? Mainly, it's actually just an optical illusion.

One big part of this is that the Moon isn't perfectly centred in Earth's shadow (see the animation to the right). With the Moon slightly offset like that, it makes the illusion easier. Now, what's the illusion all about? It has to do with how the light passing through the atmosphere is bent, or refracted. As the sun rises on one horizon and the Moon sets on the other, to someone standing in the middle between them, the light rays from both are passing through the greatest amount of atmosphere. Thus, those light rays are bent as much as possible along the curvature of the Earth (and thus the curvature of the atmosphere). This bending of the light means that we can still see the Moon after it has already set, and we see a 'preview' of the Sun before it rises. Thus, it appears as though they are both above the horizon at the same time.

This illusion will only be visible for a very short time, though, only in the Eastern time zone (the timing doesn't seem to match up properly anywhere else), and only if you can see both horizons at the same time, with no obstructions. Sky conditions will also be an important factor. The image below gives you an idea of what to expect on Wednesday morning.

In addition to the unusual aspects of this lunar eclipse, Wednesday morning's eclipse is the second of four total lunar eclipses in a row - known as a tetrad. The first eclipse of the tetrad was on the morning of April 15, and all of North and South America had an excellent view of the event. Wednesday's eclipse and the one on April 4, 2015 will be visible mainly from the west coast of North America, the Pacific Ocean, eastern Asia and Australia. The last eclipse of this tetrad will be on Sept 28, 2015, and it should, once again, be a great show for anyone in North and South America.