August 29, 2003
A: FROM MENTOR JOAN LUSK IN RI
Your last question is a biggie! We have to accept _some_ things as true in order to go on living, I think you'll agree. Otherwise we'd all be unable to get up in the morning, lying there in bed wondering if our legs would support us and our blood supply still reach our brains when we stood up.

In the case of the distance to Mars, I _think_ the different estimates of when it was last this close are a matter of precision. I heard on NPR the 60,000-year estimate, with the distance given to the nearest foot. (When will the ordinary public go metric???) If instead the distance had been given to the nearest mile, very likely Mars would have been that close more often. Perhaps an astronomer among us can set me straight if there is more to it than that. The physics involved is very well understood, so the orbits are pretty accurately known. Some uncertainty must come from the passage of previously identified comets, for example, and calculations that included all the gravitation of every asteroid would be pretty complex and lead to uncertainty too.
Scientific theories get accepted when they make sufficiently accurate predictions. In the case of planetary orbits, we've been able to predict the locations quite well for hundreds of years. When a planet isn't exactly where we expect it to be, one possibility is that gravity from a new, previously undiscovered planet is attracting it. that possibility prompts astronomers to look for a new planet. See

http://www-gap.dcs.st-and.ac.uk/~history/HistTopics/Neptune_and_Pluto.html

The discovery of Neptune ... did not come from ... chance observations. Rather it came from a mathematical analysis of the deviation of Uranus from its predicted orbit. Delambre computed tables of planetary positions Tables du Soleil, de Jupiter, de Saturne, d'Uranus et des satellites de Jupiter published in 1792. However discrepancies began to arise in the predicted position of Uranus. Bouvard (1767-1843), a French astronomer who was director of the Paris Observatory, had already published accurate tables of the orbits of Jupiter and Saturn in 1808 and he now undertook to produce a corrected version of Delambre's tables for Uranus. However he could not make all the observations fit, even after taking the perturbations of the other planets into account. He published his new tables of Uranus in 1821 but wrote

... I leave it to the future the task of discovering whether the difficulty of reconciling [the data] is connected with the ancient observations, or whether it depends on some foreign and unperceived cause which may have been acting upon the planet.

This site and the following one report that the same kind of reasoning prompted a search for Pluto - but when Pluto was found it was not actually capable of causing the perturbations that led to the search! I'd been unaware of this bit of history - I'd remembered a more logical version of the story.
See also
http://www.solarviews.com/eng/pluto.htm

The path toward [Pluto's] discovery is credited to Percival Lowell who founded the Lowell Observatory in Flagstaff, Arizona and funded three separate searches for "Planet X." Lowell made numerous unsuccessful calculations to find it, believing it could be detected from the effect it would have on Neptune's orbit. Dr. Vesto Slipher, the observatory director, hired Clyde Tombaugh for the third search and Clyde took sets of photographs of the plane of the solar system (ecliptic) one to two weeks apart and looked for anything that shifted against the backdrop of stars. This systematic approach was successful and Pluto was discovered by this young (born 4 Feb 1906) 24 year old Kansas lab assistant on February 18, 1930. Pluto is actually too small to be the "Planet X" Percival Lowell had hoped to find. Pluto's was a serendipitous discovery.

But I'd like to get back to your big question: when should we believe a theory? You've probably heard that science can only definitively _disprove_ theories, by finding that their predictions are false. When their predictions are true we grow more and more confident that the theory is true too - but there is always the chance that the "predicted" results are merely coincidence. At some point we decide that the theory's being true is more likely that the million coincidences that are the alternative... but every theory is a draft in progress. Some are old, established drafts with plenty of supporting evidence, and some are rough first drafts with little evidence (yet). We have to test our ideas to see which ones make enough good predictions to deserve to be called theories at all. Perhaps you can follow up on the story of Lowell's calculations and Pluto's failure to explain them; I doubt that anyone suggested that the discrepancy meens that the laws of gravitation are wrong, but why wasn't there a visible 'planet X" where predicted?

When we can imagine a chain of events, each one of which is consistent with well-tested theories, then it's relatively easy to accept the theory devised to explain this new chain of events. That doesn't mean the theory has to be right. But a competing theory for which some of the explanatory events don't fit with the rest of our theoretical and experimental scheme will be much more difficulty to believe in.
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A: FROM MENTOR LESLIE WAITE IN CA
Hi Alexis;
These are excellent questions! They go right to the heart of how
science works, and how to interpret science and scientific statements.

Unfortunately, I am not a physicist or an astronomer, so I can't
directly answer your questions about why we lack the information to
accurately predict past locations of Mars, but feel we can put a date
on future locations. What I can say is that in all fields of science,
we frequently lack information on past events and items because the
progress of technology allows us to examine things with modern
samples in a way that we couldn't in the past. Think of ocean
exploration: underwater cameras allow us to take pictures of things
in the ocean that could never have been photographed 100 years ago,
and new unmanned submarines allow us to go to depths that we could
never get to before. As a result, we can see things we never saw
before and save those images of what we see to show others and get
their opinions to help us do our science and answer the questions we
may have about ocean life. Before, we just had to guess what might be
"in the deep". Thus, new technology lets us study things and make
predictions about things now, yet still be unable to fully know how
things were before. It is possible this is the case with the Mars
location predictions.

As to your broader question: "Why should science theories be believed
or accepted as truth?"
The simple answer is: they shouldn't! Well, OK, that's a bit extreme.
But it is important to remember that scientific theories are just
that- theories. We call them theories because we don't know with
absolute certainty the explanation for what we are discussing (in
this case, the location of Mars in 200 years). But theories are not
just "made up" out of the blue. They are based on the evidence that
we have from studying a problem, and thinking about how to explain
it, and designing and doing experiments that will test whether or not
our ideas are correct. Then, once those experiments are done, and we
feel we have an idea of what is happening, we tell other scientists
what we are thinking. These other scientists can discuss and critique
our ideas and theories and help us to develop more experiments to
better test our theories, or point out flaws in our logic or other
aspects of the problem we have not considered. This process of
evaluation and scrutiny by other scientists is what prevents someone
from just "making a theory", as you say in your question. Such a
careless scientist would not be respected nor listened to, and would
have a very hard time getting a job in science!
So-even though a a scientific theory should not be taken as the
absolute truth and final word on a subject, it should be respected
and considered to be the "best guess" we have right now. New
experiments and new technologies may allow us to accept, refine,
alter or even discard a theory later, but that is what science is
about- getting closer and closer to what is really going on.
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A: FROM MENTOR MAYLING WONG IN IL
The questions that you ask are quite valid. As you sensed from the
seemingly conflict of facts, the way these predictions are made are very
complicated. I note that what was written in the newspapers may not
always be the complete picture. If you were to go to the source of the
facts, such as the scientist or scientists who made the calculations, then
you would likely learn about all of the facts. What I gather from what is
being written about Mars is that 1) Mars so close to Earth that we can see
it in the sky with the naked eye, and 2) This phenomenon happens once (or
maybe twice) in my lifetime, so I will take advantage of the moment!

I also applaud you for keeping track of what you read and asking questions
when things do not make sense. If only everyone were as perceptive as
you! Regarding the question on why scientific theories should be
believed, my response is that you actually believe many scientific
principles are true without realizing it. You believe that medicine or
vaccines that are taken will help your health. You believe the lights
will turn on when you flip the switch. You believe the car or the plane
will run without falling apart. The difference is that these scientific
theories are applied in everyday life. But all scientific principles and
theories start off with questions, experimentation, and proofs. Who knows
how today's science experiments will be applied in tomorrow's technology?