Friday, October 10, 2008

Galileo And Neptune

This weekend will be an interesting and frustrating weekend for astronomy buffs who enjoy observing Neptune.

Tonight, 10/10, the Moon will pass less than one degree north of Neptune. (The Moon, full, is about half a degree wide. So, tonight the Moon will be about two lunar diameters away from Neptune.)

This isn’t an Earth-shaking event, it happens not infrequently, however it is unusual. For one night observers will have the most distant easily visible solar system object, Neptune, in the same telescopic field of view as the nearest solar system object, the Moon.

That’s pretty cool.

The bad news, however, the frustrating bit, is that the Moon now is very bright. We are only four days away from the next full Moon. Bright light from the Moon probably will wash out dim Neptune for most observers.

I’ll check it out with my 60mm refractor.

Neptune Last Weekend

Toward the end of last week we had patchy moments of reasonably good seeing. I got to observe Neptune from my tripod-mounted 10x50 binoculars.

My binoculars are wide angle, taking in something like 6.5 or 7 degrees. I did this quick crayon sketch [an exaggerated impression of the scene (I’ve exaggerated the brightness and size of the objects, not the field-of-view)] to remember what I saw. That’s Delta and Gamma Capricorni at the bottom. Above Delta are the three stars about 5.5 magnitude, 44, 45 and 42 Capricorni. Just west of 44 Capricorni are two dim field stars of about 8 magnitude. Neptune is a third star in that line, about 7.8 magnitude.

This makes for a great scene for finding Neptune. For the rest of this month Neptune will continue retrograde, moving away from the two dim field stars. Then Neptune will resume its normal motion and move back east. By the end of the year Neptune will have moved back past the two dim stars. With so much motion going on, from week to week you can be sure you’re observing Neptune.

There were moments Thursday or Friday when I thought I could glimpse just a hint of color for Neptune through my binoculars. It was fleeting and I’m generally certain I imagined it, however I have a sliver of doubt that I may have actually observed the color because it looked just like the unexpected color I saw last year through my telescope, a kind of greenish-blue glint, coming and going. (Neptune is a blue planet so it’s odd to see it as greenish-blue, but with colors in astronomy you just note what you think you see because the colors are so transitory that sometimes they take you by surprise.)

Galileo And Neptune

Although this isn’t an Earth-shaking occasion, the Moon being so close to Neptune, just about four hundred years ago Neptune passed very close to Jupiter. That wasn’t exactly Earth-shaking, but hundreds of years later it is still big news.

It’s big news because at the end of 1612 and the beginning of 1613 Galileo was observing the moons of Jupiter that he had discovered. Galileo was observing the moons of Jupiter very carefully because back then nobody really knew what was happening in the sky. The Church had one view. Superstitious astrologers had another view. Scientists trying to think rigorously had worked out quite a few different possible views and were attempting to gather evidence to sort out what really was happening.

Galileo’s observations of the moons of Jupiter, specifically how the planet Jupiter eclipsed the moons and cast its shadow on them, would be a pivotal bit of evidence for convincing most scientists that, in fact, the Sun is at the center of the solar system. It also, of course, caused a heck of a lot of trouble between Galileo and the Church, but that’s another story.

So, Galileo was making very careful observations of Jupiter and the moons of Jupiter. He was drawing diagrams and measuring the distances of the moons from Jupiter using a device that he inserted into his telescope to ascertain graphically how wide Jupiter appeared to be and how far away the moons appeared to be, using as units the apparent width of Jupiter.

For a few nights, late in 1612 and early in 1613, Galileo observed that Jupiter passed very close to a dim ‘fixed’ star. At the same time, Galileo noticed a second star in the same area and, strangely, the second star appeared to move from night to night.

Galileo made some diagrams of the odd arrangement of celestial bodies but, unfortunately, it appears that bad weather made it impossible for him to perform follow-up observations.

However, Galileo’s observations and diagrams were so accurate that astronomers today can easily identify the fixed star Galileo observed and astronomers today know that the second star, the star that appeared to move from night to night, was actually Neptune.

Galileo was a real scientist. An amazing guy. Nobody back then even knew what planets and moons were. Even the people who were beginning to suspect some hints of the truth were still pretty unclear. Kepler, for instance, the guy who would work out the real geometry of the solar system, suspected that Venus was actually burning by its own light, sort of like a miniature version of the Sun.

But Galileo went about his work so carefully, with such dedication and with such active interest in what he was doing, that his results are not only still interesting today, but still can be useful today.

Astronomers who are trying to locate possible new planets in the outer solar system have been trying to use the data Galileo set down to calculate possible disturbances to Neptune’s orbit that could point to the location of a new planet.

Unfortunately, as careful as Galileo was, because he only had two or three opportunities to observe Neptune, astronomers today are not exactly clear on how to interpret, I mean exactly interpret, his diagrams. One researcher puts it like this: “If Galileo’s diagram is indeed drawn to scale, as Kowal and Drake believe, then the presence of a tenth planet in the solar system is almost a certainty. The modern ephemerides of Neptune cannot contain an error as large as 3 Jovian semidiameters (more than one arc minute). However, if Galileo did not bother with scale and drew the asterisks only to show alignment with Jupiter, then his diagram demonstrates the accuracy of modern planet position tables—which are based on the gravity of only nine planets.” [from “Planets Beyond” below]

This is one of the coolest episodes in the history of science that I am aware of. Galileo, working away four hundred years ago, worked so carefully that scientists today are still able to use Galileo’s data to help us sort out our understanding of the solar system.

It pays to do things right. I mean, right. You just never know what role the stuff you do might someday play in a bigger picture.

Incidentally, this episode touches on many other interesting episodes. The whole business of Galileo observing the moons of Jupiter is an interesting story in itself and can be read in: Galileo: Pioneer Scientist, by Stillman Drake. The astronomer who first learned that Galileo ‘accidently’ observed Neptune didn’t learn it by chance. He tells the story himself on the net here: Galileo’s Observations of Neptune, by Charles T. Kowal. And the search—the quest, really—for understanding the outer solar system is still happening right now, every bit as dynamic and even more interesting now than it was four hundred years ago. Some of that story can be found in: Planets Beyond: Discovering the Outer Solar System, by Mark Littmann . Finally, the actual discovery of Neptune in the ninteenth century was a very strange (and strangely modern!) combination of mathemtics, observational astronomy and oddball politics. People are still writing books trying to sort out exactly what happened. The links at Wikipedia’s Neptune page point to some good titles for further reading.

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