This image explains how the Earth's shadow causes a lunar eclipse. Notice the difference between the umbra and the penumbra.
Click on image for full size
Windows to the Universe original image
Lunar eclipses are special events that only occur when certain conditions are met. First and foremost, the Moon must be in full phase. Secondly, the Sun, Earth and Moon must be in a perfectly straight line. If both of these are met, then the Earth's shadow can block the Sun's light from hitting the Moon.
The only time these three objects are in a straight line is when the Sun and Moon are in the line of nodes. This line is the intersection of two planes: the Earth's orbit and the Moon's orbit. The best way to explain this is to use your hands. Take your right hand and hold it flat and level. This will represent the Earth's orbit, because it moves around in a circle in this plane.
Now, place your flat, left hand next to your right, and tilt your fingers up and your palm down. This represents the Moon's orbit, because it's orbit is slightly tilted compared to the Earth's. Now, notice the line where your two hands cross. This is the line of nodes!
There are three types of lunar eclipses. Which one we will see depends on the alignment of the three celestial objects. But first, you need to know that the Earth's shadow is broken up into two parts. The umbra is the darker part of the shadow, where no part of the Sun can been seen. The penumbra is lighter than the umbra, because part of the Sun can be seen.
So, when part of the Moon passes through the umbra, this is called a partial eclipse. When all of the Moon passes through the umbra, this is called a total eclipse. Finally, when the Moon only passes through the penumbra, this is called a penumbral eclipse.
Now that you know all about eclipses, you can step outside and enjoy the show! Remember that lunar eclipses are not dangerous to the eye, so you don't need protection to watch. Just find a nice, cozy spot and enjoy!
Shop Windows to the Universe Science Store!
Our online store
on science education, ranging from evolution
, classroom research
, and the need for science and math literacy
You might also be interested in:
The last lunar eclipse of the millennium in North America (or the first of the new millennium, if you prefer)is this week! On January 20, 2000, which is Thursday for those living in North America, a total...more
Some people would say that advanced astronomical techniques were practiced at Stonehenge and other megalithic structures (megalith is Greek for "large stone"). But, it is not likely that ancient observers...more
A solar eclipse was visible from a small area on Earth on May 31, 2003. Parts of Scotland, the Faeroe Islands, Iceland, and Greenland were within the central section of the Moon's shadow that produced...more
The first eclipse of 2003 will occur on the night of Thursday, May 15th (or early in the morning on May 16th if you live in Europe!). This eclipse is a total lunar eclipse, so the Earth's shadow will darken...more
The second total lunar eclipse of 2003 will occur on the night of Saturday, November 8th. The Earth's shadow will darken the Moon for a total of more than four hours, while the "total eclipse phase"...more
There will be a total lunar eclipse on Wednesday night, October 27, 2004. The Earth's shadow will darken the Moon for more than three and a half hours, while the "total eclipse phase" will span a period...more
When it is noon where you live, it is midnight on the opposite side of the world. Usually when we think of time, we mean "the time of day where I live". If we say something happened at 9 AM, we mean it...more