Is this the third of four Blood Moon eclipses?
Depending on the algorithm being used, the April 4 lunar eclipse can be regarded as a very shallow total lunar eclipse or a very deep partial lunar eclipse. The eclipse master, Fred Espenak, assures us that it’s actually a total eclipse of very short duration, despite some sites calling it a partial eclipse. Good thing, or else this April 4 eclipse wouldn’t rate as a so-called Blood Moon. This eclipse counts as the third in a series of four straight total lunar eclipses – a lunar tetrad – all of which are visible from North America!
Who will see a partial lunar eclipse? A partial lunar eclipse precedes the total eclipse for one hour and 42 minutes, and follows totality for one hour and 42 minutes.
So, from start to finish, the moon takes 3 hours and 29 minutes to totally cross Earth’s dark umbral shadow. Eastern North America and western South America can see beginning stages of the partial umbral eclipse low in the west before sunrise April 4, whereas middle Asia (India, western China, mid-Asian Russia) can view the ending stages of the partial umbral eclipse low in the east after sunset April 4. Greenland, Iceland, Europe, Africa and the Middle East won’t be able see this eclipse at all.
Incidentally, a very light penumbral eclipse comes before and after the dark (umbral) stage of the lunar eclipse. But this sort of eclipse is so faint that many people won’t even notice it. The penumbral eclipse would be more fun to watch from the moon, where it would be seen as a partial eclipse of the sun.
What causes a lunar eclipse? A lunar eclipse can only happen at full moon. Only then is it possible for the moon to be directly opposite the sun in our sky, and to pass into the Earth’s dark umbral shadow. Most of the time, however, the full moon eludes the Earth’s shadow by swinging to the north of it, or south of it. For instance, the March 2015 full moon swung south of the Earth’s shadow. Next month – in May 2015 – the full moon will swing north of the Earth’s shadow.
The moon’s orbital plane around Earth is actually inclined at 5o to the ecliptic – Earth’s orbital plane around the sun. However, the moon’s orbit intersects the ecliptic at two points called nodes. It’s an ascending node where it crosses the Earth’s orbital plane going from south to north, and a descending node where it crosses the Earth’s orbital plane, going from north to south.
In short, a lunar eclipse happens when the full moon closely coincides with one of its nodes, and a solar eclipse happens when a new moon does likewise. It’s not a perfect alignment this time around, with the moon crossing its ascending node about 9 hours before the moon turns full. But that’s close enough for the moon to stage a total lunar eclipse, even if it happens to be the shortest one of the 21st century!
By Bruce McClure
Animation of the 2015 April 4 total lunar eclipse. The moon travels eastward through the Earth’s penumbra (light outside shadow) and umbra (dark inner shadow) shadow. The yellow line depicts the ecliptic – Earth’s orbital plane. Although the moon, at least in part, spends about 3.5 hours within the umbra, it is only totally submerged in the umbra (dark shadow) for a short while, or less than 5 minutes.