A total eclipse of the moon will occur very early Tuesday morning, Dec. 21, 2010. Houstonians will be able to see the whole event, which begins shortly after midnight. If you happen to be traveling for the holidays, don’t worry – our entire continent has a full view of this eclipse.
When will the Lunar Eclipse Occur?
Note the timing, which might throw off some people. The night we’re talking about is Monday night, December 20, and Tuesday morning, December 21. For people in Houston, the eclipse starts at about 12:30 a.m., so the date is December 21. If you go out on the night of the 21st looking for this event, you will have missed it by one day.
All times are listed in CST:
||Tuesday, Dec. 21
Why do Lunar Eclipses Occur?
Lunar eclipses occur when the full moon moves into the Earth’s shadow. The orbits of the Earth and moon are not exactly in the same plane; the moon’s orbit is inclined to ours by about 5 degrees. This may not seem like much, but it is enough to make the full moon miss the Earth’s shadow most of the time.
About twice a year, the full moon is close enough to Earth’s orbital plane to encounter the shadow rather than passing above it or below it. Even at that, a slightly imprecise alignment may cause the moon to skirt the edge of the shadow (resulting in only a partial eclipse) or even just the penumbra (no noticeable change for us). For example, on the morning of June 26, the moon never fully entered Earth’s shadow, and we saw it set in partial eclipse.
Finally, when the moon does go all the way into the Earth’s shadow, we see the event only if it happens during our nighttime, and if skies are clear. During the last total lunar eclipse visible in America, on February 20, 2008, we were clouded out in Houston. Given all of these factors, the opportunity to watch a lunar eclipse is quite special. Let’s all hope the weather cooperates and we all can take some time to enjoy this spectacular sight.
What to Watch For: Observing An Eclipse
Observing the eclipse is quite simple; everyone who sees the moon sees the eclipse. Unlike the sun, the moon presents no danger to our eyes when observed directly. No special observing equipment is necessary. Unlike dimmer objects such as meteors, the moon shows up clearly even among bright city lights. You can enjoy the eclipse from your backyard, even if this is in the middle of Houston.
As the Earth blocks sunlight and casts a shadow across space, we can define two regions. The umbra is the region of space in which Earth completely blocks the sun. The penumbra is the region of space in which Earth only partially blocks the sun.
The first part of the Earth’s shadow that the moon will encounter is the penumbra. The penumbral shadow’s faintness means that even sharp-eyed observers will notice only a slight dimming of the moon between 11:28 p.m. on Monday night and 12:32 a.m. Tuesday. The moon moves into the darkest part of the earth’s shadow, the umbra, at 12:32 a.m, and will be totally eclipsed by 1:40 a.m. Totality lasts 73 minutes because once in, the moon takes until 2:53 a.m. to cross to the other side of the umbra. The moon then takes just over an hour to re-emerge from the umbra. By 4:01 a.m., the moon has moved out of the Earth’s shadow, and the eclipse is over.
How Bright will the Eclipse Be?
The moon’s brightness during a total eclipse depends on the amount of dust particles in the atmosphere. A large amount of dust from a volcanic eruption, for example, can make the totally-eclipsed moon almost invisible. With little dust in our atmosphere, the moon glows reddish-orange during totality. This is because only the sun’s red light is bent enough by the Earth’s atmosphere that it lands on the moon even while it is in the Earth’s shadow.
As this diagram shows, the moon will pass through the northern part of the shadow, for about 73 minutes of totality. As a result, the bottom (southern) limb, closer to the center of Earth’s shadow, will appear darker.
Last Chance to See A Lunar Eclipse?
There are two lunar eclipses in 2011, but Houstonians won’t get a good look at either one. The eclipse of June 15, 2011 occurs while we’re having daytime. On the morning of December 10, 2001, an eclipse gets underway just as the moon sets for us. In Houston, we see only the first 15 minutes of that eclipse. (Observers farther north and west see more.) We won’t see our next total lunar eclipse in Houston until just after midnight on April 15, 2014.