chapter 2 Flashcards
What does the universe look like from Earth?
Stars and other celestial objects appear to lie on a great celestial sphere surrounding Earth. We divide the celestial sphere into constellations with well-defined borders. From any location on Earth, we see half the celestial sphere at any one time as the dome of our local sky, in which the horizon is the boundary between Earth and sky, the zenith is the point directly overhead, and the meridian runs from due south to due north through the zenith.
Why do stars rise and set?
Earth’s rotation makes stars appear to circle around Earth each day. A star whose complete circle lies above our horizon is said to be circumpolar star. Other stars have circles that cross the horizon, making them rise in the east and set in the west each day.
Why do the constellations we see depend on latitude and time of year?
The visible constellations vary with time of year because our night sky lies in different directions in space as we orbit the Sun. The constellations vary with latitude because your latitude determines the orientation of your horizon relative to the celestial sphere. The sky does not vary with longitude.
What causes the seasons?
The tilt of Earth’s axis causes the seasons. The axis points in the same direction throughout the year, so as Earth orbits the Sun, sunlight hits different parts of Earth more directly at different times of year.
How does the orientation of Earth’s axis change with time?
Earth’s 26,000-year cycle of precession changes the orientation of the axis in space, although the tilt remains about The changing orientation of the axis does not affect the pattern of seasons, but it changes the identity of the North Star and shifts the locations of thesolstices and equinoxes in Earth’s orbit.
precession
The gradual wobble of the axis of a rotating object around a vertical line
Why do we see phases of the Moon?
The phase of the Moon depends on its position relative to the Sun as it orbits Earth. The half of the Moon facing the Sun is always illuminated while the other half is dark, but from Earth we see varying combinations of the illuminated and dark faces.
What causes eclipses?
We see a lunar eclipse when Earth’s shadow falls on the Moon and a solar eclipse when the Moon blocks our view of the Sun. We do not see an eclipse at every new and full moon because the Moon’s orbit is slightly inclined to the ecliptic plane.
Why was planetary motion so hard to explain?
Planets generally move eastward relative to the stars over the course of the year, but for weeks or months they reverse course during periods of apparent retrograde motion. This motion occurs when Earth passes by (or is passed by) another planet in its orbit, but it posed a major mystery to ancient people who assumed Earth to be at the center of the universe.
Why did the ancient Greeks reject the real explanation for planetary motion?
The Greeks rejected the idea that Earth goes around the Sun in part because they could not detect stellar parallax—slight apparent shifts in stellar positions over the course of the year. To most Greeks, it seemed unlikely that the stars could be so far away as to make parallax undetectable to the naked eye, even though that is, in fact, the case.
