What is a lunar eclipse? What causes it? And how can you safely observe one?
Like a solar eclipse, a lunar eclipse is caused by a perfect alignment of Earth, the Sun, and the Moon.
However, a solar eclipse only occurs when the Sun and Earth are on opposite sides of the Moon
with the Sun casting the Moon's shadow onto Earth.
On the other hand, a lunar eclipse occurs when Earth and the Sun are on the same side of the Moon
so that the Sun is casting the shadow of Earth onto the Moon.
What's really unusual about lunar eclipses, particularly total lunar eclipses,
is that when the Moon passes completely into Earth's shadow, it doesn't just darken and
disappear against the blackness of the night sky. Instead it turns a coppery red color.
It was this reddish color that made some ancient cultures think that the Moon was bleeding—
the result of an attack by some evil demon.
Today we know why that color appears. It's the color of Earth's shadow.
You might expect our shadow to be completely dark, but it isn't. As the Sun's light shines
on our planet and casts its shadow into space, it passes through our atmosphere where dust particles
and air molecules scatter out the blue color in its light, producing our blue skies.
That leaves the mostly reddish light to make it all the way through.
Because the Sun has a perceptible diameter and is not a point source of light, it doesn't cast a clear razor sharp shadow.
Instead, the shadow it casts has two parts—
a dark inner shadow, or "umbra," and the faint outer shadow, or "penumbra."
During a lunar eclipse, when the Moon enters the penumbra of Earth's shadow,
most observers can't perceive the very subtle shading that results.
It's much more obvious in photographs.
The entry of the Moon into the umbra is much more dramatic.
The moment this happens is known as "first contact" and shortly after this,
the Earth's shadow can be seen gradually encroaching on the Moon's disc, creeping in from the Moon's western edge.
While the Moon is entering the umbra, observers will notice the curved shape of Earth's shadow.
The fact that it always has this shape convinced the ancient Greeks that Earth is a spherical object.
The moment the entire disk of the Moon is enveloped in the umbra is called "second contact"
and it marks the start of totality.
This is when the Moon is completely immersed in Earth's shadow and looks a deep coppery red.
As observed from the Moon, Earth would be surrounded by a ring of reddish light
—the color of all the sunrises and sunsets happening at that moment.
During totality, the Moon's light is dimmed considerably,
allowing you to see other objects in the sky that perhaps just an hour earlier
were being washed from view by its bright glow.
Depending on the angle that the Moon takes through Earth's shadow,
the duration of totality may vary.
If the Moon takes the longest path through the very center of the umbra,
the partial phases—that is, when it's entering or exiting the umbra—may take about an hour each.
Totality may last as long as an hour and a half because the diameter of Earth's umbra
is several times larger than that of the Moon itself.
On the other hand, if the Moon skims just inside the edge of the umbra,
totality may last only a matter of minutes as the Moon starts exiting soon after it has entered.
If the Moon doesn't completely enter the umbra, totality doesn't occur.
This is a partial lunar eclipse.
Totality ends when the Moon starts exiting the umbra,
revealing a bright white edge. That moment is called "third contact" and the entire eclipse ends
when the Moon is completely out of the umbra and back to its normal brightness, "fourth contact."
Lunar eclipses only occur at full moon and there may be as many as four lunar eclipses per year,
usually in some combination of total, partial, and the barely noticeable penumbral eclipses.
Remember that while solar eclipses can be seen only from certain areas on Earth's surface,
lunar eclipses can be seen from the entire side of the planet from which the Moon is visible.
And lunar eclipses are perfectly safe to observe and do not require any special eye protection or filters.
Some ancient cultures grew to understand eclipses enough
—or at least to recognize long-term patterns and their occurrences—
to foretell the times when they might likely take place.
And people figured that out without even realizing how the Sun, Moon, and Earth move with respect to one another.
Today, we understand the Universe well enough to predict eclipses to the second,
but this doesn't detract from our appreciation of
the celestial motions that make these remarkable alignments possible.