Today in class : The Sky and Celestial Motion The Planetary Motion
: Celestial Motion
Question concepts Question #1: During a single day or night, the Sun, Moon, stars and planets all appear to move across the sky together. Their relative positions change relatively slowly from night to night. Question #2: As the Earth orbits the Sun, the night side of the Earth is oriented in a different direction in space, always away from the Sun. As a result, at different times of year different constellations are visible in the night sky. Question #3: Using a figure like that below, where the earth may be located in various positions around its orbit, characterize the location of constellations in the sky at various times (sunset, midnight, noon). For example, the constellation directly on the opposite side of the earth from the sun will be highest in the sky at midnight. Gemini Taurus Aries Earth Pisces Sun Aquarius Capricornus Sagittarius Cancer Leo Virgo Libra Scorpius
Science vs. the scientific method Intro to scientific models of the heliocentric model of the solar system More about scientific practice
The ancient question, resolved in 1600 s - Apparent Retrograde Planetary Motion Planets appear to turn around and back up at certain points during their movement among the stars. Due to the motion of the earth and planets around the Sun.
Science vs. the Science: The body of scientific information (facts and inferences) and their confidence levels, relationships The community supporting and refining these ideas The scientific method: hypothesize, test, evaluate, refine, repeat The process by which ideas are increased (or decreased) in confidence level
Scientific model nomenclature Models/hypotheses/theories/laws are largely synonyms example: s laws of planetary motion are not physical laws at all, are observed empirical relations Newton s theory of gravity is incorrect (Einstein s is better ) but we still call it a theory and call its constituent principles laws. Distinctions are sometimes made in certain cases, but these are largely contextual in this lecture sun-centered solar system is hypothesis even though actually proven fact A model can be both right and wrong Even an incorrect model can continue to have utility A model can be just wrong there are many incorrect models that have no utility utility = ability to predict outcomes of situations
The ancient question, resolved in 1600 s - Apparent Retrograde Planetary Motion Planets appear to turn around and back up at certain points during their movement among the stars. Due to the motion of the earth and planets around the Sun.
The Models (at this point hypotheses ) Sun-centered projection effect Earth-centered circles on circles (epicycles)
Ptolemaic model - c. 100 Based on ideas developed in ancient Greece Greeks developed both earth and sun-centered models Earth-centered model favored because it was thought that the earth must be static Stars don t shift as Earth moves - In fact they do, it is just very small (parallax) Movement of Earth is inconsistent with motion of objects on Earth - In fact, Aristotle s laws of motion were wrong Provided reasonably accurate prediction
Copernican model - 1500 s formalized existing proposal in 1500 s to allow better comparison He was convinced sun-centered provided a simpler explanation for retrograde motion To provide accurate predictions with circular motions, just as complicated as Ptolemaic model (circles on circles were needed) Still concerned about a moving Earth
Question concepts Question #4: A sun-centered solar system had been considered as a possibility since Greek times, even though it was disfavored until the 1600s. Question #5: When formalized the sun-centered solar system, it inferred a different movement in space of the planets. However, it was not significantly simpler in its initial form, and also the observations of planet motion at the time were consistent with either the sun- or earth-centered model.
10 1 1 measured by multiple independent parties conjecture "reasonable" speculation Unlikely speculation Disproven earth-centered had slightly higher confidence at the time because it was thought self-evident that the earth wasn t moving The scale of scientific confidence: tool to help you categorize scientific ideas position of idea on scale can depend on person At high levels of confidence, answer is universal at lower levels of confidence, scientists can disagree Experimental evidence (from multiple sources) is given the highest confidence different confidence in trustworthy and not trustworthy sources of evidence
Brahe - late 1500 s Realized need for more accurate measurements and performed them In the face of competing theories which are both unsatisfactory, gather better data! This might point the way to a theory better than both In this case both Ptolemaic and Copernican models were wrong in detailed comparisons to data Shows importance of technical capability to advancing scientific understanding
- early 1600 s By abandoning circles was able to develop a highly accurate sun-centered model Stated in 3 laws (will be discussed more later) Orbits are ellipses Equal area in equal time (sets orbital speed) Quantitative relation between period and orbit size
When Tides Change in Science The new precision of s measurements (facts) made a earth-centered solar system an unlikely inference. s laws, which were sun-centered, provided a more compelling inference because they were very consistent with s observations of planet motion in the sky. This drove a historical shift in opinion toward a sun-centered solar system. Elevated the confidence of sun-centered model over Earth-centered. But the motion of the Earth had not been detected (actually it wouldn t be until the 1800 s).
Facts and Inferences Inference can t have higher confidence than facts on which it is based, and may have much lower 10 1 1 measured by multiple independent parties conjecture "reasonable" speculation Unlikely speculation Disproven Facts: observations bare results of experiments (what happened) line can get blurry for extremely well tested theories Example: the motion of the planets in the sky Inferences: often first called hypotheses may have very high or low confidence Example: (historically) the motion of the planets/sun in space