See the 'impossible' as sunrise and a total lunar eclipse appear at the same time on March 3

This year's first lunar eclipse on Tuesday, March 3, offers a rare chance to see a strange celestial sight traditionally thought impossible: the rising sun and the eclipsed moon in the sky at the same time.

Views of the total phase of this eclipse favor locations near and around the Pacific Rim. For North America, places within the Eastern Time Zone will see the moon set during dawn's early light during the total phase; places farther west will be able to catch the moon emerging from the Earth's shadow as it sets, while for sites out in the Far West, the eclipse will be visible from start to finish. Hawaiians will see the moon almost overhead as totality takes place in the hours after midnight. Meanwhile, during local evening hours, Central Asia and western Australia will see the moon rise as it emerges from the Earth's dark shadow. Eastern Australia, Papua New Guinea, as well as much of Japan and eastern Siberia, will see it all during convenient evening hours.

The moon passes through the southern part of the Earth's shadow, with totality beginning at 3:03 a.m. PST and lasting 59 minutes. Across the eastern half of 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.

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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; thus, 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 really are.

For example, when you see the sun sitting on the horizon, it is not there. It is, in reality, sitting just 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 also 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 has actually already set.

The same holds true with the moon, as well.

Because of this atmospheric trick, for many localities, there will be an unusual chance to observe a selenelion firsthand with this impending shadowy event. There will be a short window of roughly 1-to-3 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.

Read more: Where to see the total lunar eclipse in the early hours of March 3

Regions of visibility

For places to the west of the Continental Divide, this effect, unfortunately, may not be visible. For most places in the Mountain and Pacific Time Zones, the moon will have moved completely free of the dark umbral shadow of the Earth before it sets. Those in the Mountain Time Zone will see the moon set while it is still within the Earth's penumbral shadow.

This shadow is so faint that at least 50-70% of the moon must be immersed within it before you have a chance of detecting it visually, either with your naked eyes or using an optical aid. For places in the southern and central Rockies, such as Santa Fe, New Mexico or Denver, Colorado, the lower-right portion of the moon will appear somewhat darker or "smudged" as it begins to disappear beyond the western horizon.

Farther north, however, from Jackson, Wyoming and Butte, Montana, the moon will look perfectly normal as it sets.

Across most places in the Central Time Zone, the total phase will have passed, and the moon will be emerging from the Earth's umbra. Depending on your location, the moon may resemble a crescent, a half-moon, or just a "bite" taken out of its lower right limb.

For most locations in the Eastern Time Zone, the moon will set completely immersed in the Earth's shadow, while for the Atlantic Canada provinces (Nova Scotia, New Brunswick, Prince Edward Island and Newfoundland), only the opening partial stages will be visible with the moon setting before totality occurs.

This table shows the local times of sunrise and moonset, along with the percentage of the moon's diameter that is within the dark umbral shadow at the time of moonset, for 11 selected cities. An asterisk (*) indicates that totality has already occurred and that the moon is emerging from the umbral shadow. Note that locations farther to the west have the moon and sun together in the sky for a noticeably longer interval. That's because after mid-eclipse, the moon's orbital motion has carried it a bit more to the east and thus higher up in the sky, so it remains in view a bit longer.

Important facts to consider

To observe the selenelion, you should make sure that both your eastern and western horizons are free of any tall obstructions that might block your views of the setting moon or rising sun.

Also, be aware that depending on the clarity of your sky, you might lose sight of the moon about 10 or 15 minutes before sunrise, thanks to the brightening morning twilight and the moon sinking into any horizon haze (atmospheric "schmutz").

Keep in mind that this holds only for the uneclipsed portion of the moon. Indeed, if the moon is totally eclipsed at moonset, you will probably have to scan the western horizon as the twilight increases to detect the darkened moon, which will perhaps resemble a dim and eerily illuminated softball.