Astronomy Ch 2

What are constellations? How did they get their names?

Constellation is a region of the sky with well defined borders. All 88 constellations received their names and borders in 1928 by the International Astronomical Union.

12.Briefly describe the moon's cycle of phases. Can you ever see a full moon at noon?

In each cycle of lunar phases, the appearance of the moon and the times it arises and sets changes depending on its relative location to the sun as it orbits Earth. Each cycle takes about 29.5 days. Half of the moon always faces the Sun and is illuminated. But the amount of this illuminated half we see depends on where the Moon is in its orbit. As a result we see a combo of the bright and dark faces of the moon. We can't see a full Moon at noon, because the full Moon occurs when it is opposite the Sun in the sky. The full moon reaches its highest point in the sky at midnight . Thus it arises around the sunset and sets around the sunrise.

Describe the conditions needed for a solar or lunar eclipse.

The Moon's orbit is inclined by about 5 degrees to the ecliptic plane, Earth's orbit around the Sun. Because of this inclination, the Moon spends most of the time either above or below this ecliptic plane. The moon crosses through the plane only twice per orbit, one coming up and once going down. These positions are the nodes. it is only when the Moon is near its node and aligned with the sun and earth where an eclipse can occur. 1. The phase of the Moon must be full for a lunar eclipse and new for a solar eclipse. 2. At the same time, when the Moon is either full or new, the nodes of the moon's orbit need to be aligned with the Sun and Earth.

Explain why we can measure only angular sizes and angular distances for objects in the sky. What are arcminutes and arcseconds?

The angular size of an object is the angle it appears to span in your field of vision. The angular distance between a pair of objects is the angle that appears to separate them. We use angular size and angular distance to describe an objects place in the sky because our lack of depth perception on the celestial sphere, hampering our ability to judge the true sizes or separation of objects. Arcminutes and arcseconds are more precise forms of astronomical measurement. Each degree is separated into arcminutes and further subdivided into arcseconds

.What do we mean by apparent retrograde motion of planets? What was it difficult for ancient astronomers to explain but is easy for us to explain.

The planets move eastward through the constellations most of the time. However, they occasionally reverse their course and move westward through the zodiac. This is what we mean by retrograde motion. It was difficult for ancient astronomers becasue they thought the earth was the center of the unvierse.

What is precession, and how does it effect what we see in our sky?

The precession is a gradual wobble that changes the orientation of the Earth's axis in space. Earth rotates around every 24 hours and its axis precesses every 26,000 years. It affects our view of the sky because it changes the constellations associated with solstices and equinoxes. Since those two correspond to points on the Earth's orbit, which depend on the direction the axis points to in the space, the positions in the orbits gradually shift along Earth's orbit.

Briefly describe key facts about the solstices and the equinoxes.

The summer solstice in June is when the northern hemisphere is most directly tipped towards the sun and receives most direct sunlight. The winter solstice in December is when the northern hemisphere receives the least direct sunlight. The spring equinox in March is when the northern hemisphere goes from being tipped slightly away from the sun to slightly towards it. The fall equinox in September is when the northern hemisphere is starting to tip away from the sun

Why do we always see the same face of the moon?

The time taken by the astronomical body to cover its own axis is equal to the time taken by that body to revolve around another object. This phenom is known as tidal locking. Earth's moon undergoes this and the time period of rotation of moon on its own axis is 29.5 days which is the same period it takes to revolve around Earth. Therefore its visible throughout the year.

What is the zodiac, and why do we see different parts of it at different times of year?

The zodiac are the constellations that make up the ecliptic. It is the sun's apparent location along the ecliptic that determines which constellations we see at night

What is stellar parallax? How did an inability to detect it support the ancient belief in an earth-centered universe?

When a nearby star is viewed from Earth at different locations of its orbit, it should appear to shift back and forth against the background of of more distant stars. If the earth moves around the sun than stellar parallax should be there. THe ancient greeks did not detect stellar parallax because the stars were too far for the stellar parallax to be observable.

What are circumpolar stars? Are more stars circumpolar at the North Pole or in the United States? Explain.

circumpolar star is a star that always remains above the horizon for a particular latitude.