What is a Lunar Eclipse?

Just before dawn on the morning of Saturday, April 4, 2015, Acadiana sky watchers will—just barely—have a chance to see a total lunar eclipse. 

Lunar eclipses happen when the moon goes through Earth’s shadow.  This typically happens a couple times every year somewhere in the world, and can only happen at full moon.  Because the moon itself passes through Earth’s shadow, the eclipse can be seen over nearly half the earth—anywhere the moon is visible during the event.  By contrast, solar eclipses—when the moon passes directly between Earth and the sun—can only be seen where the moon’s shadow touches the earth.

Lunar eclipses don’t happen every month because the moon’s orbit is tilted compared to the plane of the ecliptic (the approximate plane of our planetary system and the region where Earth’s shadow must always fall).  In most months the full moon appears a little above or below Earth’s shadow so that no eclipse occurs.  A lunar eclipse requires a fairly precise alignment of the sun, Earth, and moon.

There are three kinds of lunar eclipses: penumbral, partial, and total.  A penumbral eclipse happens when the moon passes only through the outer shadow, or penumbra, of Earth.  A partial eclipse happens when the moon passes partially through the dark inner shadow, or umbra, of Earth.  Total eclipses begin penumbral and go through a time of partial eclipse, but then completely enter Earth’s umbra during totality; as the moon continues to move around Earth, totality ends and is followed by another partial phase and by another penumbral phase.  The whole event lasts several hours.

It may seem odd that Earth could have both an inner and outer shadow, but that happens because the sun is bigger than Earth.  Sunlight from one side of the sun creates a shadow behind Earth that lies in a slightly different direction from the shadow caused by light from the other side of the sun.  The inner, umbral shadow is that region that is completely shadowed from the sun while the outer, penumbral shadow is the region that is shadowed from some (but not all) of the sun.

During a total lunar eclipse the moon turns color and may appear anywhere from a very dark red to a brighter coppery-orange.  This is caused by the scattering of sunlight passing through Earth’s atmosphere, with the reddish colors then being bent onto the moon.  It’s hard to predict the color of any particular eclipse because it depends on Earth’s worldwide atmospheric conditions at the moment of the eclipse!

The Internet can make anything seem wacky. Over the last several years the phrase “blood moon” has been used to describe lunar eclipses. Perhaps this phrase was designed to make the moon passing through Earth’s shadow seem somehow more dangerous than, say, walking through the shadow of a building. The scientific term for these events is “total lunar eclipses.” Besides, if your blood actually is the color of a lunar eclipse, you might want to see your doctor!

How to See This Lunar Eclipse

Lunar eclipses can be seen with just the unaided eye.  No other equipment is needed, and the eclipse is completely safe to watch (it’s a solar eclipse that will damage your vision).  That being true, the April 4 lunar eclipse is in a poor position for viewing from Acadiana, and a pair of binoculars might be helpful.  In any case, you’ll need a clear horizon to the west.

The essentially unnoticeable penumbral eclipse will begin minutes after 4 a.m. CDT, but there won’t be much to see until the partial phase starts at 5:15.  The moon will appear to move into Earth’s shadow as the moon begins to set and twilight begins to interfere.  Totality starts at 6:58 a.m., but the moon sets at 7:01!  In fact, the moon may be blocked earlier than that by trees or building along the horizon.  This means that under the best conditions only a minute or two of totality will be seen, and that will happen not only as the moon is setting but also as the sun is rising.  If you have a beautifully clear horizon, it might be interesting to see how long you can see the moon before the eclipse is overwhelmed by sky conditions.

Although the planetarium does not plan to have public telescopes out for this eclipse due to the time and sky conditions, observers will be able to see whatever happens from wherever they live.


Meanwhile, mark your calendar for the evening of September 27 when Acadiana will experience a far better prime time lunar eclipse!  And don’t be confused—while that eclipse will officially be on September 28, in our time zone it will be on September 27 from roughly 7 p.m. to midnight with a great view of totality from about 9:15 to 10:15 p.m.