Astronomy Assignment 1 LU

Chapter 1 Answers Page 1




5/21/2014

P a g e | 1

Chapter 1

9. The straight path that the Sun traces from day to day on the celestial sphere is called the

ecliptic. The path of the Earth on the celestial sphere as seen from the Sun is precisely the same as the

path of the Sun as seen from the Earth and therefore is also called the ecliptic.

It is tilted with respect to the celestial equator because the Earth’s rotation axis is tilted 23 ½° away from

a line perpendicular to the ecliptic. The line of celestial equator and ecliptic intersect only at two points

which are exactly opposite each other on the celestial sphere and are called equinox.

Source:

Comins, Neil F., and William J. Kaufmann III. "Discovering the Night Sky." Discovering the Universe.

Eighth ed. New York: W. H. Freeman and Company, 2008. 16. Print.

16. The beginning of winter in the northern hemisphere occurs when Sun is lowest in the northern

sky on the winter solstice. The amount of daylight increases daily as the Sun moves northward. The

beginning of spring takes places when the vernal equinox marks a midpoint in the amount of heat from

the Sun onto the northern hemisphere. As the Sun continues to move and reaches the summer solstice,

it is highest in the northern sky and is above the horizon for the most hours of any day of the year.

When the Sun crosses the celestial equator on the autumnal equinox returning southward, it marks the

beginning of fall.

Source:



P a g e | 2

27. A solar eclipse occurs when the Moon’s shadow moves across the Earth’s surface. As seen from

Earth, the Moon moves in front of the Sun. This configuration occurs during the phase of a new Moon.

A lunar eclipse occurs when the Moon passes through the Earth’s shadow. This can happen only when

the Sun, Earth, and Moon are in a straight line at full Moon.

Source:



P a g e | 3

Chapter 2

9. A planet’s sidereal period is the time it would take an observer fixed at the Sun’s location

watching that planet move through the background stars to go from one point on the celestial sphere,

around the sphere, and back to that same point again. The sidereal period is the length of a year for

each planet.

The synodic period is the time that elapses between two successive identical configurations as seen

from Earth. It can be from one opposition to the next, or from one conjunction to the next. It tells us

when to expect a planet to be closest to Earth and, therefore, most easily studied.

Source:

Comins, Neil F., and William J. Kaufmann III. "Gravitation and the Motion of Planets." Discovering the

Universe. Eighth ed. New York: W. H. Freeman and Company, 2008. 46-47. Print.

14. Newton’s law of gravitation concluded that gravitational force decreases with distance. Despite

this weakening, the force of gravity from every each object extends throughout the universe. When

scientists discovered that Uranus was not following the orbit predicted by Newton’s laws, they

independently calculated that the deviations of Uranus from its predicted orbit could be explained by

the gravitational pull of a then-unknown, more distant planet. Thus, the finding of Neptune by

astronomers confirmed Newton’s law of gravitation.

Source:

Comins, Neil F., and William J. Kaufmann III. "Gravitation and the Motion of Planets." Discovering the

Universe. Eighth ed. New York: W. H. Freeman and Company, 2008. 58-59. Print.

P a g e | 4

Chapter 3

6. The three major functions of a telescope are providing astronomers with images as bright as

possible, to reveal greater detail of the objects that are more than just points of light, and to make

objects larger.

The light-gathering power of a telescope is directly related to the area of the telescope’s primary mirror;

Area = (Pi x diameter2) / 4.

Angular resolution measures the clarity of images. The angular resolution of a telescope is measured as

the arc angle between two adjacent stars. The smaller the angle, the sharper the image.

The magnification of a reflecting telescope is equal to the focal length of the primary mirror divided by

the focal length of the eyepiece lens.

Source:

Comins, Neil F., and William J. Kaufmann III. "Light and Telescopes." Discovering the Universe. Eighth ed.

New York: W. H. Freeman and Company, 2008. 78-79. Print.

11. “Radio telescopes record radio signals from the sky. Just as a mirror reflects visible light, the

metal surfaces of radio telescopes reflect radio waves. Each radio telescope has a large concave dish

that focuses radio photons in the same way that an optical telescope mirror focuses visible photons.”

While optical telescopes are located in places where light pollution can be avoided, radio telescopes are

similarly located in areas where electromagnetic interference is absolutely minimized.

“Radio waves have the longest wavelengths of all electromagnetic radiation. The angular resolution of a

telescope decreases as its wavelength increases and therefore even the largest radio dish in existence

cannot come close to the resolution of the best optical telescopes.” Radio telescopes have allowed

P a g e | 5

astronomers to detect radio waves in regions (interstellar gas clouds) from where no visible light is

emitted.

Source:


New York: W. H. Freeman and Company, 2008. 90-91. Print.

13. X-rays from space interact strongly with the particles in Earth’s atmosphere, preventing these

dangerous radiations from reaching our planet’s surface. Direct observations of astronomical sources

that emit these extremely short wavelengths, therefore, must be made from space. Direct observation

of x-rays through satellites and Earth orbiting observatories also prevent any atmospheric interference,

thereby providing better information.

Source:


New York: W. H. Freeman and Company, 2008. 95. Print.

P a g e | 6

Chapter 4

7. According to Wein’s Law, the peak wavelength of radiation emitted by a blackbody is inversely

proportional to its temperature. This means that as an object is heated more, the color of the object

changes from red-hot (coolest of all glowing bodies) to blue-hot (hottest state) and the peak of the

emission spectrum shifts from longer wavelengths to shorter wavelengths.

Stefan-Boltzmann Law explains that the energy emitted by an object per unit area is the proportional at

a rate equal to the fourth power of its temperature. In other words, as the temperature of an object is

increased and the color is shifted from red to blue, the energy emitted each second of the object’s

surface is increased by a factor of 4. This corresponds to a change in intensity from longer wavelengths

to shorter wavelengths.

11. Electrons change orbits by emitting or absorbing photons. If the electron starts in its ground

state, then it must get exactly the energy necessary to move it to an excited state. Otherwise, the

photons will pass right through the atom. Absorption lines are therefore caused by photons being taken

out of the stream of light by electrons, which thereby move into higher-energy allowed orbits. The

emitted photons have the same set of wavelengths as the absorbed photons.

In the case of hydrogen atom, when the atom absorbs a Hα (red) photon, the absorbed energy causes

electrons to jump from a lower state to a higher state. When this happens, a specific color is absorbed.

Then, electrons fall back from that higher state to the lower state and the energy that is lost goes into

emitting the same Hα (red) photon. When this happens, the same color that was absorbed is emitted in

the emission spectrum.

Source:

Comins, Neil F., and William J. Kaufmann III. "Atomic Physics and Spectra." Discovering the Universe.

Eighth ed. New York: W. H. Freeman and Company, 2008. 116-117. Print.

Documents

Astronomy Assignment 1 LU