Observer's Notebook

Total Lunar Eclipse, January 20, 2000

by John Rummel

Thursday evening, January 20th, most of the western hemisphere will be treated to its first total lunar eclipse since September of 1996. All of North and South America are placed perfectly to see the total event. Because totality comes well before midnight, this event will be easily viewable by hundreds of millions of people.

A lunar eclipse occurs at full moon, when the moon passes into the shadow cone of the earth. It can be seen from any place where the moon is visible above the horizon. A total lunar eclipse occurs when the moon enters completely into the umbra of the earth's shadow. The moon can usually be seen throughout totality, being illuminated by sunlight refracted by the earth's atmosphere into the shadow area. Since the bluer wavelengths are removed by scattering, the moon has a coppery-red color.

   
 Path of the moon through the Earth's shadow January 20, 2000. Illustration (not to scale) of the Earth's shadow cone.

There are usually two or three lunar eclipses per year, giving every location on earth an average of one lunar eclipse every three or four years.

The lunar eclipse has long been regarded as second rate when compared to its wealthy cousin, the solar eclipse. This reputation is undeserved, however. During a total lunar eclipse, the spectacularly bright disk of the full moon is shrouded in Earth's shadow and turns a deep shade of red or orange. The spectacle of the Moon's disk half-enveloped in the shadow is an unforgettable sight, and one not to be missed by casual observers.

Observations of lunar eclipses played an important in early reckonings of the size of the moon and its distance from earth. The ancient Greek astronomer Aristarchus was able to deduce, from measurements taken during lunar and solar eclipses, the size and distance of the moon - a feat that is even more impressive because he did his work almost 2,000 years before the invention of the telescope

Historians are aware of many eclipses that played an important role in shaping world events. In 1503, during Columbus's fourth voyage to the new world, his ship was damaged while exploring Central America. Taking on water, the crew tried to reach the Spanish settlement at what is today the Dominican Republic. They made it as far as Jamaica, where they were stranded for several months while the crew made repairs. Supplies ran low but the crew was able to trade with the natives for food. This arrangement worked for awhile until several incidents of mistreatment of the natives caused Columbus' welcome to wear out. By January of 1504, the natives were actively planning to cut off Columbus's supply of food and possibly even attempt an attack on his crew.

Desperate, Columbus used his knowledge of astronomy in a clever and dangerous gamble. Onboard ship, he had a copy of Johannes Muller's astronomical tables, published about 1474. It contained predictions of lunar eclipses for many years ahead. It revealed that a lunar eclipse was due on 29 February 1504, now just a few days away. The night of the 29th, Columbus arranged an evening meeting with the tribal leaders to coincide with the beginning of the eclipse and announced that God would show his displeasure toward them by taking away the moon. Right on cue a dark shadow began to pass over the face of the moon. When the frightened tribesmen pleaded for its restoration Columbus retired to his ship to 'confer' with God - Columbus knew exactly how long the total phase would last so he just needed to kill some time before announcing that God would indeed restore the moon, if the natives agreed to give Columbus and his men all they needed. The moon - and the food supply - were promptly restored, and Columbus had no further trouble with the natives.

While you probably won't be able to extract any food or gifts from your neighbors, maybe you can impress them with your knowledge of the January 2000 event. Although this month's eclipse is not central (the Moon's northern limb just misses the "center" of the shadow), the total phase still lasts about 75 minutes. The eclipse begins at 8:05 pm (all times local to Madison) with first penumbral contact. An hour later, the partial eclipse commences with first umbral contact at 9:03 pm. The total umbral eclipse begins at 10:06 pm and ends at 11:21 pm. The partial phase ends at 12:23 am and the Moon leaves the penumbral shadow at 1:21 am. The Moon's path through Earth's shadows is shown above.

The eclipse occurs just as the Moon is entering the constellation Cancer; the Beehive cluster is just a few degrees to the left and should be visible during peak eclipse. Other winter constellations are well placed and will make for a beautiful sky show that night.

Since this is not a central eclipse (at deepest penetration, the Moon will lie just below the central point of the umbra), the northern regions of the Moon will probably appear much darker than the southern regions since they lie deeper in the shadow. It is likely the moon's appearance will change dramatically as the eclipse progresses, though these things are difficult to predict. Surely it will be a show worth staying up for. Even the kids can enjoy the beginning stages before heading off to bed.

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