Feb 21, 2008

Rahu strikes again!

Last night there was a full eclipse of the moon. A very eerie sight; as the shadow of the Earth moves across the face of the moon it doesn't disappear, but turns a dull melancholy red. One can well understand how this sight must have been regarded as a bad omen before the astronomy was understood.

In Thailand, the traditional belief, derived like much of Thai culture originally from India, was that in primeval times the monster Rahu swallowed the sun and moon, depriving the world of light. Vishnu saved the universe by slaying the monster, cutting off it's head. Or not quite slaying, because Rahu is an immortal and his severed head wanders around in space, being the eighth planet in Thai astrology. Occasionally it swallows the sun or moon and we experience an eclipse, but happily the luminous orb in a short while emerges from the monster's neck.

Three questions about lunar eclipses occur to me. Two I think I can answer from first principles. One; is a lunar eclipse visible from the whole night side of the planet Earth? I would think yes; it's different from a solar eclipse in that it is essentially the shadow of the Earth moving across the moon, whereas a solar eclipse is when the moon blocks the line of sight of the sun from the Earth, and given the astronomically close position of the moon, parallax makes a significant difference from different localities. Two; how would it look if you were standing on the surface of the moon? I think the Earth would eclipse the sun totally, but I imagine one would see some kind of diffuse "halo" around the rim due to atmospheric diffusion.

Three is not so easy to answer; what was the scientific explanation for eclipses given before the emergence of the heliocentric model? I mean in learned western thought. How did the Aristotelians with their spheres and epicycles explain that? How did Tycho Brahe with his mixed model (sun and moon orbit the Earth, all other planets orbit the sun)? I have no idea.


Robert said...

The red light reflecting off the moon during the eclipse is basically the red light of all of the "sunsets" on earth reflecting off the moon. So, if you were on the moon, you would see a red "sunset" around the whole edge of the earth's disk. Essentially you'd witness all sunsets on earth at once, if I understand all this correctly.

Robert said...

(Actually, to be completely correct, I guess you'd be witnessing all sunsets and sunrises on earth at once.)

Angulimalo said...

My understanding....as limited as it is. Is that the moon always sees all the sunrises and sunsets on earth. Eclipses are just when we are directly in front of the moon instead of slightly off to one side.

Robert said...

No, that's wrong. Some part of the day/night transition line is pretty much always visible from the moon (line between night and day), but in this case we're talking about the red light of a sunset/sunrise, filtered through the atmosphere, actually hitting the moon. That doesn't normally happen. The moon has to be in the shadow of the earth.

Basically a full moon should be facing the dark side of the earth, particularly during an eclipse. The whole earth is dark (you only see the "night" part.) The red sunset/sunrise light should be visible from the moon during this as a red ring around the earth. Most likely the intensity of different parts of the red ring effect will be different at different times.

Some of this is me guessing and most is based on stuff I read on astronomy web sites.

Joe said...

I couldn't cite anything for you off hand, but I have always understood that Aristotle actually used the fact that the Earth cast a curved shadow on the moon as the basis for saying that the Earth was spherical and not flat, something I don't think many people really thought or cared about, except when someone got a bug up their butt to talk about it.

I don't know how that fits into a geocentric versus a heliocentric model of the cosmos though. As far as explaining lunar eclipses go, though, it doesn't seem hard to fudge an idea of how it works in either system. As soon as you start having to deal with, I don't know, everything else is when you run into problems.

Like you already explained in the Thai explain, to the extent that anyone really cared about such phenomena, the explanations prior to any modern conception of the solar system were probably religious in form and significance.

Xing Ping said...

Actually, it was the moon's descending node, Ketu, which was involved in this lunar eclipse in sidereal Aquarius. Rahu was with the Sun and Saturn in sidereal Leo, beneath the horizon at the time when the eclipse was visible. I don't think it was visible here in Hawaii.

Saturn was the moon's ruler in this eclipse, so we have an emphasis on discipline and renunciation, which really applies to the entire lunar Year of the Earth Rat, of which this was the first new moon. Ketu-Moon has much the same effect. It's going to be a year of letting go. The prominence of Buddhism in the coming year is signed by all of the above, and by the involvement of the sign Leo, since the Buddhadharma is sometimes called "The Lion's Roar."

In a more secular sense, it's going to be a year of challenge for leaders. In the current primary election, these astral tides favor the Leo Obama. The speed and craziness of his political success is typical of the influence of Rahu, found with the Sun Swakshetra (own sign) and Saturn in Leo here.

Namu Amida Butsu
Xing Ping


Larry said...


Artist's interpretation of a lunar view of a lunar eclipse. Actually, what Earth residents think of as a "lunar eclipse" is what Lunar resdients would see as a "solar eclipse!"