A total eclipse of the moon
Will happen we think pretty soon
When the moon turns to red
Just as Lord Rayleigh said
It can make an astronomer swoon
There’s a blood moon tonight. It has nothing to do with vampires, and the moon has not been the loser in a fight. The moon appears red in eclipse for the same reason the sun appears red at sunrise and sunset — an English Lord decreed it so.
Lunar Eclipses By The Numbers
There is a partial or total lunar eclipse approximately every nine months on average. There can be as many as three, or none at all, each year. Since about half of all eclipses are total, there is a total eclipse of the moon every year and a half. The longest possible time of totality is around two hours; the time of totality of tonight’s eclipse will be an hour and 18 minutes.
The eclipse will begin at 1:58 a.m. EDT, and the total phase will start at 3:07 a.m.
Totality will end at 4:25, and at 5:33 a.m., the eclipse will be over.
For those who just want to see the moon at its darkest and bloodiest, 3:46 Eastern time is the witching hour.
Why Is The Eclipsed Moon Red?
The moon emits no light of its own. The image we see of the moon is provided by light from the sun, which is reflected to us. When the moon hides behind the Earth, the direct sunlight is cut off. So why isn’t the moon black in eclipse? Some sunlight, bent slightly through the earth’s atmosphere, reaches the eclipsed moon and reflects back to Earth.
John William Strutt, Third Baron Rayleigh (better known as Lord Rayleigh) developed the equation for the scattering of solar radiation by the atmosphere. It is a long equation, but the important thing is that the scattering is inversely proportional to the fourth power of the wavelength.
That means that the longest wavelengths (red) are scattered the least and the shortest (blue) the most. This is why the sky is blue and the sun is reddish at sunrise and sunset.
As the sun’s radiation passes through the Earth’s atmosphere during a lunar eclipse, some red light gets through, to be reflected back to Earth… and the moon looks bloody as a result.
Relationship Between Solar And Lunar Eclipses
Simple geometry explains why total eclipses of the sun are much rarer than eclipses of the moon. It is an astronomical coincidence that, viewed from the earth, the moon and sun occupy the same extent of the sky.
Each extends approximately 1/2 degree from limb to limb.
Because the moon’s orbit around the earth is not round, there are two possible kinds of solar eclipse, depending on whether the moon is in the part of its orbit closer to or farther from the Earth:
- Total eclipse occurs when the moon entirely covers the sun.
- Annular eclipse occurs when we can still see a sliver of sun along the edge of the moon.
The totality of a solar eclipse lasts a very brief time. As viewed from the sun, however, the Earth is much bigger than the moon, so the moon takes several hours to traverse the shadow of the earth.
Why Don’t We Get An Eclipse Every Month?
If shapes and angles in the solar system were regular and symmetrical, there would be an eclipse of the sun and moon every month. But they’re not.
The earth is tipped at an angle of 23 degrees to its direction of motion around the sun. Most planets and satellites orbit close to the plane of rotation of their primaries. However, the moon does not orbit at the Earth’s equator; rather it stays within five degrees of the plane of the earth’s motion around the sun. This variation of up to five degrees from a line between the sun and earth means that the three bodies are lined up closely enough for a lunar eclipse only once every nine passes.
Who Can See Tonight’s Eclipse?
Anyone in North and South America will have a good view. Unfortunately for Europeans, the moon will be setting at the time of eclipse. In the States, the sky will have to be at least partly clear for viewing to take place. The best viewing will be in the dry and cloudless southwest, though most of the country from the west coast to the Mississippi Valley will have favorable conditions except for the Pacific northwest.
From the Mississippi River Eastward, the front associated with Winter Storm Zephyr will obscure viewing in most places except right along the Atlantic coast.
And What If You Miss It?
Since lunar eclipses are fairly common, most viewers get multiple opportunities to see one. The next total lunar eclipse will be visible in North America on October 8 of this year, and the following chance will be on September 28, 2015. But neither of those will have as long a duration of totality as tonight’s eclipse. So, if you can rouse yourself in the wee hours of tomorrow, this ‘Blood Moon’ lunar eclipse will be worth watching.
Decoding Science. One article at a time.