© 2004 pearson education inc., publishing as addison-wesley a few quick reminders

© 2004 Pearson Education Inc., publishing as Addison-Wesley

A few quick reminders


Announcement

No quiz the first week back after spring break

-no extra homework set this week


Basics about the sky…

• What is a constellation?• A constellation is a region of the sky. The sky is

divided into 88 official constellations.

• What is the celestial sphere?• An imaginary sphere surrounding the Earth upon

which the stars, Sun, Moon, and planets appear to reside.

• Why do we see a band of light called the Milky Way in our sky?• It traces the Galactic plane as it appears from our

location in the Milky Way Galaxy.


What have we learned?• Describe the basic features of the local sky.

• The horizon is the boundary between Earth and sky. The meridian traces a half circle from due south on your horizon, through the zenith (the point directly overhead), to due north on your horizon. Any point in the sky can be located by its altitude and direction.


What have we learned?• Why are some stars above the horizon at all times?

• All stars appear to make a daily circle. Circumpolar stars are those for which their entire daily circles are above the horizon, which depends on latitude.

• What is the cause of the seasons on Earth?• As the Earth orbits the sun, the tilt of the axis causes

different portions of the Earth to receive more or less direct sunlight at different times of year. The two hemispheres have opposite seasons. The summer solstice is the time when the northern hemisphere gets its most direct sunlight; the winter solstice is the time when the southern hemisphere gets its most direct sunlight. The two hemispheres get equally direct sunlight on the spring and fall equinoxes.


What have we learned?

• Why do we see phases of the Moon?• At any time, half the Moon is illuminated by the Sun

and half is in darkness. The face of the Moon that we see is some combination of these two portions, determined by the relative locations of the Sun, Earth, and Moon.

• What conditions are necessary for an eclipse?• An eclipse can occur only when the nodes of the

Moon’s orbit are nearly aligned with the Earth and the Sun. When this condition is met, we can get a solar eclipse at new moon and a lunar eclipse at full moon.


What have we learned? • Why do planets

sometimes seem to move backwards relative to the stars?• Apparent retrograde

motion occurs over a period of a few weeks to a few months as the earth passes by another planet in its orbit.


• Why did the ancient Greeks reject the idea that the Earth goes around the Sun, even though it offers a more natural explanation for planetary motion?

…their inability to detect stellar parallax --- the slight shifting of nearby stars against the background of more distant stars that occurs as the Earth orbits the Sun.

To the 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. They instead explained the lack of detectable parallax by imagining the Earth to be stationary at the center of the Universe.


Matter & Energy

• What is matter? What is energy?• Matter is material. Energy is what makes matter

move.

• What is a joule?• A joule is the standard unit of energy. It is used to

quantify and compare energies.

• What are the three basic categories of energy?• Kinetic energy is energy of motion. Potential energy

is stored energy that can be released later. Radiative energy is energy carried by light.


What have we learned?• What is temperature, and how is it different from

heat?• Temperature is a measure of the average kinetic

energy of the many individual atoms or molecules and a substance. At a particular temperature, a denser substance contains more thermal energy (heat).

lower T higher T

same T

less heat more heat


What have we learned?• What is gravitational potential energy?

• It is energy stored because of an object’s ability to fall from its current position. The amount of an object’s gravitational potential energy depends on its mass, the strength of gravity, and how far it could fall.

m

d

g

•It depends on:–the object’s mass (m)–the strength of gravity (g)–the distance which it falls (d)


What have we learned?• Explain the formula E = mc2.

• It describes the potential energy of mass itself. E is the energy stored in a piece of matter of mass m, and c is the speed of light.

• Why is the law of conservation of energy so important?• It tells us that energy can be neither created nor destroyed and

instead can only be exchanged between objects or transformed from one form to another. We can, therefore, understand many processes in the Universe by tracking the path of energy.


What have we learned?• How do phases of matter

change with increasing temperature?• Most substances are solid at

low temperature. As temperature rises, the substance may melt into liquid and then evaporate into gas. As temperature rises further, molecules (if any) will dissociate and atoms will be ionized to make a plasma.


Light etc• What is the difference between energy and

power?• Power is the rate at which energy is used. The

standard unit of power is 1 watt = 1 joule/s.

• What are the four ways in which light and matter can interact?• Matter can emit, absorb, transmit, or reflect light.

• In what way is light a wave?• Light is an electromagnetic wave – a wave of vibrating

electric & magnetic fields – characterized by a wavelength and a frequency and traveling at the speed of light.


What have we learned?• In what way is light made of particles?

• Light comes in individual photons, each with a specific energy that depends on its frequency.

• How are wavelength, frequency, and energy related for photons of light?• Frequency increases when wavelength decreases, and

vice versa. Energy is proportional to frequency.

Light as a particle E = hf photon

Light as a wave f = c


Light as a Wave• For a wave, its speed: s =

f • But the speed of light is a

constant, c.

• For light: f = c

• The higher f is, the smaller is, and vice versa.

• Our eyes recognize f (or ) as color!

•The energy carried by each photon depends on its frequency (color)

E = hf = hc / [“h” is called Planck’s Constant]


What have we learned?• List the various forms of light that make up the

electromagnetic spectrum.• In order of increasing frequency (energy), the forms of

light are: radio, infrared, visible light, ultraviolet, X-rays, and gamma-rays.


What is thermal radiation?• The spectrum from a dense/opaque body that depends

only on its temperatureonly on its temperature

• Photons trying to escape a dense body hit so many atoms on the way out they ‘bounce around’ and randomize their energies->so, the distribution of photons finally emerging depends how energetic the atoms themselves are to start with, ie the temperature of the objectthe temperature of the object


What have we learned?

• What are the two rules of thermal radiation?(1) Hotter objects emit more total radiation per unit

area.

Stephan-Boltzmann LawE = T4

(2) Hotter objects emit photons with a higher average energy.

Wien Lawmax = 2.9 x 106 / T(K) [nm]


What have we learned?(1) Hotter objects emit more total radiation per unit

area.

(2) Hotter objects emit photons with a higher average energy.


Doppler Effect

• What do we learn from a redshift or blueshift?• It tells us how fast the object is moving away from us

(redshift) or toward us (blueshift). The Doppler shift does not tell us about motion across our line of sight.

• How does a star’s rotation affect its spectral lines?• Because of Doppler shifts, faster rotating stars have

broader spectral lines.

v c

=

© 2004 pearson education inc., publishing as addison-wesley a few quick reminders

Documents

pearson education

distant stars

circumpolar stars

direct sunlight

new moon

local sky

lunar eclipse

solar eclipse