Lunar Eclipse Next Week
Published on Feb 13, 2008 at 11:59 pm.
3 Comments.
Filed under eclipses, skywatching.
Next week, on Wednesday evening here in North America, there will be a total lunar eclipse. Lunar eclipses are fun to watch, and if the skies are clear, we’ll be watching the eclipse from campus. You can see the eclipse just fine with your naked eye, but it is even more fun to watch through binoculars or a telescope.
Almost a year ago, we had a lunar eclipse with the Moon rising eclipsed and coming out of eclipse after it had risen. I posted some photos that I took of that eclipse. There was another lunar eclipse about five and a half months ago. For that eclipse, the Moon entered eclipse and was totally eclipsed as viewed from here. I posted photos of that one, too. One of my readers, David Breadsell, though, got a nice series of photos of that eclipse from beginning to end. This time, we are having a big public eclipse watch, so I may be too busy to take a lot of cool photographs.
Lunar eclipses happen whenever the Moon passes into the Earth’s shadow. That means that the Moon has to be on the opposite side of Earth from the Sun. That is the same condition, though, as the Full Moon. So, lunar eclipses happen only at the Full Moon. But, they don’t happen every Full Moon because the Moon’s orbit around Earth is tilted a few degrees from Earth’s orbit around the Sun. That means that most of the time, the Moon misses Earth’s shadow, passing either below or above it. A lunar eclipse can only happen when the alignment of the orbits is right. But, the Earth’s shadow is big enough that the alignment doesn’t have to be perfect, just pretty close. So, an eclipse can happen anytime within a couple weeks of the alignment. This window of time when an eclipse can occur is called an eclipse season. An eclipse near the beginning or end of the eclipse season is most likely only a partial eclipse, but that counts! You might think that eclipse seasons would be exactly six months apart, but that isn’t the case. Because of precession of the Moon’s orbit, the eclipse seasons actually come slightly less than six months apart.
The Earth’s shadow has two parts, the umbra and the penumbra. The umbra is the part of the shadow where the Sun is completely blocked. However, the Sun is not a point source. So, that means that it is possible to block part of it, but not all of it. The penumbra is a sort of partial shadow, where the Earth blocks part of the Sun, but not all of it. When the Moon enters the penumbra, it is an penumbral eclipse. A penumbral eclipse is often pretty underwhelming, since part of the Sun is still shining past the Earth. That means that sunlight is still falling on the Moon, just not as much sunlight. So, the Moon looks a bit dimmer, but that is all. But, if the Moon passes into the umbra, then you notice it. When part of the Moon is in the umbra, it is a partial lunar eclipse. When all of the Moon is in the umbra, then it is a total lunar eclipse. A diagram showing the path of the Moon through Earth’s shadow for next week’s eclipse has been created by Fred Espenak of NASA’s Goddard Space Flight Center and reproduced below.
The Moon moves about its own width across the sky in an hour’s time. So, it takes a while for the eclipse to transpire, as you can see from the times listed above. Note, these times are for Central Standard Time. Go to the NASA Eclipse Page for next week’s eclipse to find other diagrams with times listed for other time zones.
In some of the photographs of the past lunar eclipses, such as mine or David Breadsell’s of the one at the end of last August, you will notice that at full eclipse the Moon often looks rather orange or red. You might suspect that when the Moon is fully eclipsed, and no sunlight is shining directly on it, that the Moon would be invisible. Well, that is not what really happens. Remember that Earth has an atmosphere. So, that atmosphere scatters light. That is why the sky does not get instantly dark at sunset. Instead twilight is filled with orange and red colors. The same happens during a total lunar eclipse. The Earth’s atmosphere scatters light, so a ring of twilight surrounding Earth shines on the Moon, giving it a reddish or orange glow. Sometimes the eclipse is very dark, and sometimes not so dark. Sometimes the Moon appears blood red, and sometimes a bright orange. The French astronomer Andre Danjon created a scale for rating the appearance of the Moon at full eclipse. I wrote about that for the eclipse a year ago.
But, this eclipse holds something a bit unusual. As the Moon is passing through Earth’s shadow, it will be very near the planet Saturn in the sky, as seen in the following sky view created using Starry Night Pro:
This view was created for Fort Worth, Texas, at about 10:40pm Central Standard Time. As you can see, Saturn will be the very bright “star” very near the Moon during the eclipse. So, we will have some telescopes pointed at Saturn during our public eclipse watch next week. After all, who doesn’t think seeing the rings of Saturn is cool? But remember, you don’t have to have a telescope to watch the eclipse. You can do that with your own two eyes. It is even better with a pair of binoculars. But, if you have a telescope, it is interesting to watch the Moon and measure what time various craters enter and exit the Earth’s umbra. Sometimes that is a bit easier said than done, though, because Earth’s shadow does not normally have a clearly defined edge. Remember, Earth has an atmosphere, so its shadow is a bit fuzzy. Still, I’ll have my students timing when these events occur.
So, be sure to go out and enjoy the eclipse next Wednesday night.
-Astroprof
drew on February 21, 2008 at 6:04 pm: 1
thank you from a back yard in Tujunga,Calif.
drew on February 21, 2008 at 6:06 pm: 2
thanks,from my back yard in Tujunga
drew on February 21, 2008 at 6:07 pm: 3
I like stars a lot it gives me hope for the world.