Critical Thinking Questions
(Think about these carefully before you consult the answers below)
QUESTIONS
1) What is a natural human ability through which most people use parallax every day? <answer>
2) Is parallax purely a visual phenomenon? <answer>
3) What are the major limitations on the use of parallax in astronomy? <answer>
4) What would be the advantages and disadvantages of measuring parallax from the surface of Mars as opposed to Earth? <answer>
5) Try to imagine the most accurate possible set up for us today in measuring parallax of nearby stars. What set up would you arrange? <answer>
ANSWERS
1) The ability to judge distances through visual clues, known as "depth perception," is the body's natural application of parallax.
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2) We normally think of parallax in terms of visual observations, and that tends to be the most useful application, but other forms of radiated energy can be used. Although not useful for astronomy, even sound waves can be used to locate objects (why do you think you have two ears to hear in stereo?).
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3) Probably the greatest limitation is the obscuring effect of the Earth's atmosphere, which makes measuring the extremely small angles involved very difficult or impossible. Another limitation is our baseline for measurements, which is the diameter of the Earth's orbit. The Hipparchos satellite overcame the first limitation by observing from orbit. A future space-born project aims at using spacecraft to gauge accurate distances to a billion stars in the Milky Way. Spacecraft orbiting the Sun at distances greater than Earth's or in orbit about Mars or farther planets will someday overcome the second limitation.
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4) Not considering the remoteness and trouble in setting up such a station, measurements from Mars could provide better numbers than those from the surface of Earth simply because Mars is farther from the Sun, which is one of the limitations listed above. With a greater baseline, more accurate measurements of parallax and hence distance can be made. However, since Mars does have an atmosphere -- although much thinner than Earth's -- the better choice would be to observe from orbit above Mars, rather than from the surface. One disadvantage of measuring parallax from Mars is simply that the measurements would take longer since Mars' year is longer than Earth's year.
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5) Of course we can't know what you would design, but perhaps the best arrangement within the range of current technology would be a pair of spacecraft simultaneously measuring stellar positions from widely separated positions in the solar system. Two probes, orbiting on opposite edges of the solar system would spectacularly increase our knowledge. However, such far ranging craft may not be needed, as advances in electronics and techniques will greatly increase the accuracy of even Earth-orbiting observatories in the next few years.
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