Observing the Sky: The Birth of Astronomy Flashcards
Describe the main features of the celestial sphere
Zenith- the top of the dome, directly above you
Horizon- where the dome meets Earth
Celestial sphere- the dome of the sky (apart of a dome) is turning around you ← wrong (Earth turns as the day goes on)
Explain the system astronomers use to describe the sky
Constellations?
Describe how motions of the stars appear to us on Earth
The rising and setting of the sun
- Stars circle during the daytime ← sunlight is scattered by molecules in the atmosphere, filling the sky with light and hiding the stars
- Sun changes position gradually on the celestial sphere (about 1 degree to the east); changes day to day
Ecliptic- the path the sun takes around the celestial sphere each year (rises 4 min later each day)
Describe how motions of the Sun, Moon, and planets appear to us on Earth
From the Earth’s perspective, the planets are observed to move closely along a circle known as the ecliptic - the apparent path of the Sun’s movement on the celestial sphere as seen from the Earth. The (((rise and set of a planet)))) as observed from the Earth is related to its apparent motion relative to the Sun.
Understand the modern meaning of the term constellation
One of 88 sectors in which to divide the sky
Describe Galileo’s discoveries concerning the study of motion and forces
1) It was familiar to all persons then, as it is to us now, that if something is at rest, it tends to remain at rest
and requires some outside influence to start it in motion. Rest was thus generally regarded as the natural state
of matter
2) Also studied the way objects accelerate—change their speed or direction of motion (He found that such objects accelerate uniformly; that is, in equal intervals of time they gain equal increments in speed.)
Explain how Galileo’s discoveries tilted the balance of evidence in favor of the Copernican model
Galileo found four moons revolving about Jupiter in times ranging from just under 2 days to about 17 days
1) This discovery was particularly important because it showed that not everything has to revolve around Earth
2) It demonstrated that there could be centers of motion that are themselves in motion.
- Defenders of the geocentric view had argued that if Earth was in motion, then the Moon would be left behind because it could hardly keep up with a rapidly moving planet. Yet, here were Jupiter’s moons doing
exactly that. - Test Copernican theory w/ telescope, based
on the phases of Venus. Within a few months, he had found that Venus goes through phases like the Moon,
showing that it must revolve around the Sun so that we see different parts of its daylight side at different times - These observations could not be reconciled with Ptolemy’s model, in which Venus circled
about Earth. In Ptolemy’s model, Venus could also show phases, but they were the wrong phases in the wrong
order from what Galileo observed.
Explain Kepler’s three laws of planetary motion
1) Orbits are elliptical; Every planet’s orbit is an ellipse with the Sun at a focus
2) Equal areas in equal times; A line joining the Sun and a planet sweeps out equal areas in equal times
3) The square of a planet’s orbital period is proportional to the cube of the semi-major axis of its orbit. As it’s the third which is most often used, Kepler’s law usually means Kepler’s third law (of planetary motion)
When was Earth known to be round?
Since ancient times
How did people in ancient times know the Earth was round?
The shape of the Earth’s shadow on the moon
What is the truth about constellations?
With constellations, stars are farther apart than they seem
Why are stars sphere-shaped?
Gravity pulls stars into the most compact shape, which is a sphere
How many foci are in every ellipse?
2 foci (one is the sun)
What is the second name for Kepler’s 2nd Law?
The Law of Equal Areas (the sun’s gravity pulls and wains on the planet’s in orbit)
What are the features of Kepler’s 3rd Law?
P (orbital period) has to be in years
a (semi-major axis) has to be in astronomical units (AU), which is the distance away from the sun
