Exam 3

Question 0

Result of Precession

Answer 0

The north celestial pole traces out a circle 47 degrees in diameter relative to the stars in our Northern sky.
Consequently, Polaris has not always been our North Star, and the right ascension and declination of any given star slowly changes. Also, the sun no longer appears in your zodiacal constellation on your birthday!

Question 1

Precession

Answer 1

Of the earths rotation axis is analogous to the wobbling of a gyroscope, and result from the fact that

(a) the earth is spinning, so that it has rotational angular momentum
(b) the earths axis is tilted at an angle of 23.5 degrees. As a result of all of these, the sun and Moon exert torques on the earth’s equatorial bulge. The period of the Earth’s precession Is approximately 26,000 years.

Question 2

Hipparchus

Answer 2

Discovered in the 2nd century that all the positions he’d measured for the brighter naked-eye Stars has shifted systematically relative to those derived about 200 years earlier.

Question 3

Tides

Answer 3

Result from the fact that the forces of gravity diminishes with distance

Question 4

Tidal bulges

Answer 4

The moon raises two on opposite sides of the Earth; the the Sun also raises tidal bulges, but they are less significant than those raised by the Moon

Question 5

Spring tides

Answer 5

Happened on the days of New Moon and Full Moon. The moon and sun “work together”

Question 6

Body tides

Answer 6

The solid ground rising and sinking about eight inches twice a day

Question 7

Tidal friction

Answer 7

Between the oceans and the ocean floor is gradually slowing the Earths rotation. Our day is getting longer, by approximately 0.0023 seconds per century!
To compensate, the Moon is gradually drifting away from Earth,at a rate 3.4 centimeters per year. The tides which the Earth raises on the Moon are stronger than those which the Moon raises on Earth, because the Earth is more massive.

Question 8

Synchronous Rotation

Answer 8

The Moon’s rotation has slowed to the point where it is locked. The moons rotation period equals it’s orbital; period of 27.3 days. Consequently, the same side of the moon aways faces earth. This is a common phenomenon; all the large moons of the outer planets are locked into synchronous rotation!

Question 9

Phobos

Answer 9

A tiny moon orbiting Mars, is so close to the planet that it orbits in less ban a Martian day. Consequently tidal drag is drawing Phobos inward,and in about 11,000,000 years it will crash onto the Martian surface.

Question 10

Center of mass

Answer 10

In any system of two bodies bound by gravity, this imaginary point is located on the line joining their centers, but is closer to the more massive body.

Question 11

In the case of the Sun and Jupiter..

Answer 11

The Suns mass is 1000 times that of Jupiter. The center of mass lies 1000 times close to the center of the Sun than to the center of Jupiter, and is just above the surface of the sun! As a result Jupiter moves in a big elliptical orbit about the center of mass over a period of 11.9 years, while the Sun moves in a tiny orbit.

Question 12

Kepler’s Second Law

Answer 12

(The Law of Area) results from the fact that gravity cannot change the angular momentum of a system. For a planet orbiting the sun in a nearly circular orbit, the planet’s angular momentum is the product of the planet’s mass, it’s orbital speed, and its distance from the Sun.

Question 13

Kepler’s third law

Answer 13

(Harmonic law)

A^3/P^2=Ma+Mb

We can only determine the mass of a star or planet by studying its gravitational effect on a neighboring body.

Question 14

Conic sections

Answer 14

In any two-body systems in which the bodies move under the influence of their mutual gravitational attraction, they follow paths which are conic sections. Since the ratio of their distances from the center of mass is fixed

rA/rB=mB/mA= constant

The orbits of the two have the same shape or eccentricity.

Question 15

Four conic sections/ how it’s determined

Answer 15

Circle e=0
Ellipse 01

The shape of its orbits is determined by the total energy and angular momentum of the system.

Question 16

Circular velocity

Answer 16

The faster the speed with which the satellite is launched the larger the orbit of the satellite, and the greater the orbital period.

Question 17

Escape velocity

Answer 17

The orbit is a parabola and the satellite never returns to Earth! Note that the escape velocity is greater than the circular velocity by a factor of squareroot2 ; near the Earths surface the escape velocity is about 25,000 mph or 7 miles per second. Finally, if the speed exceeds the escape velocity the orbit is a hyperbola.

Question 18

Olaus Roemer

Answer 18

Observing the moons of Jupiter in 1675; that the light travels at a finite speed. The speed of light (in a vacuum) is 299,792.50 km/sec, or about 186,000 miles per second. In a vacuum all colors of light travel at that speed.

Question 19

Thomas young

Answer 19

1801 demonstrated that light is a form of a wave. The observed bright and dark fringes produced by a beam of light passed through two slits could be explained only in terms of the phenomena of constructive interference and destructive interference.

Question 20

Wave

Answer 20

Is a disturbance pattern which propagates at some speed (c) through a medium

Question 21

Wavelength

Answer 21

Is the distance over which the pattern repeats itself

Question 22

Frequency

Answer 22

Is the number of vibrations per second at a given point; is is measured in HERTZ (Hz)
1Hz=1 vibration per second

Question 23

James C. Maxwell

Answer 23

Light is fundamentally different from sound, which cannot travel through a vacuum,-‘s the question arose as to what actually vibrates within a light wave.
1870 Maxwell; light is a form of electromagnetic wave; consisting of vibrating electric fields and magnetic fields.

Question 24

How is the color of visible light determined?

Answer 24

By its wavelength: in order of decreasing wavelength.. Red, orange, green...

Question 25

EM (electromagnetic) radiation

Answer 25

The human eye is sensitive to wavelengths of EM radiation ranging from about 380nm(violet) to 740nm(red). There are of course EM waves who frequencies make them undetectable by the human eye! At shorter wavelengths we have ultraviolet light, X-rays, and gamma rays. At longer wavelengths we have infrared light, microwaves, and radio waves.

Question 26

Photoelectric effect

Answer 26

By 1905 light was known to have both wave particle natures. The wave model of light could not account for the way light transfers energy to electrons in the photoelectric effect.

Question 27

Photons

Answer 27

A beam of light consists of particles called photons; each has an associates wave which determines the probability of the photons being in a given place at a particular time. The more energy a photon has, the smaller its wavelengths is.

Question 28

Electromagnetic wave

Answer 28

Consisting of vibrating electric and magnetic fields. According to his theory such a wave would travel through empty space station prescribed speed (c=299,792.50 km/sec), regardless of the frame of reference in which that speed is measured!

Question 29

Michelson-Morley

Answer 29

Experimentally verified the consistency of the speed of light in vacuum in 1887

Question 30

Special Theory of Relativity

Answer 30

Einstein's it treats the relative motion of observers, in the special case where there is no acceleration. It is based upon two postulates (assumptions)

Question 31

Einstein's First Postulate

Answer 31

The laws of physics are the same in all inertial frames of reference. (There are no preferred reference frames!)

Question 32

Einsteins Second Postulate

Answer 32

The speed of light in a vacuum has the same value in all directions, in all inertial reference frames.

Question 33

If two observers are moving relative to one another they will, in general, not agree as the the distance between two points or the time interval between two events. The most important results are that:

Answer 33

(1)no material object may travel at a speed equal to, or greater than, the speed of light in a vacuum and that (2) energy and mass are equivalent: E=mc^2

Question 34

E=mc^2

Answer 34

The quantity c^2=8.988E16 joules/kilograms is a huge number; even a tiny quantity of matter contains a tremendous amount of energy!

Question 35

Time dilation

Answer 35

We always see time pass more slowly for other, moving people than for ourselves!

Question 36

Length contraction

Answer 36

The contraction of a measured length of an object or a distance in a frame of reference.

Question 37

Einsteins General Theory of Relativity

Answer 37

Provides a more complete description of the phenomenon of gravitation than does Newtons law of Universal Gravitation.

Question 38

Principle of Equivalence

Answer 38

(The fundamental principle of general relativity) a uniform gravitational field is equivalent to a uniformly accelerating frame of reference

Question 39

Sir Arthur Eddington

Answer 39

Observed the bending of light by gravitation during a total solar eclipse in 1919. A prediction made by this principle is that a beak of light will be deflected by a gravitational field. However, since nothing can travel faster than light, by definition he path light follows from one point to another is the shortest path between those points. Since these shortest paths are not straight lines, it follows that the presence of mass must curve the surrounding space!

Question 40

Compare and contrast Newtons Law of Universal Gravity and Einsteins General Theory of Relativity

Answer 40

Newtons law: Gravity is described as an invisible force of attraction btwn any two objects which has mass. Einsteins Theory: a manifestation of the curvature of space introduction by the presence of mass. Both are valid descriptions of gravity, but the latter is the "more complete". Einstein's theory predicts that a beam of light will curve in the presence of a gravitational field, and also that time passes more slowly the stronger the field is; both predictions have been verified by experiment.

Question 41

The primary function of an astronomical telescope

Answer 41

Is to collect as much light as possible (so that we may observe faint objects) and to bring that light to a focus. Light may be focused by the processes of refraction and reflection.

Question 42

Electromagnetic radiation travels

Answer 42

At the same speed (c) in a vacuum. It travels more slowly when passing through a material medium. The speed of light in a material is c\n where n is the materials index of refraction.

Question 43

Index of refraction

Answer 43

In c/n n is the index of refraction. For example, since n=2 for diamond, light travels at a speed of 1/2 c when passing through a diamond. When a light ray passes from one material into one with a higher refractive index( and in which light moves more slowly) the ray bends closer to a line perpendicular to the interface between the substances.

Question 44

Refracting telescope

Answer 44

Uses a convex lens to collect and focus light

Question 45

Reflecting telescope

Answer 45

Uses a concave mirror to collect and focus light

Question 46

Chromatic aberration

Answer 46

Early refracting telescopes suffered from this, single lens focused different colors of light at slightly different points, resulting in blurred images. This can be remedied (albeit expensively) through use of multiple lenses.

Question 47

Yerkes observatory

Answer 47

The largest refracting telescope in the world. 40 inch aperture. In Wisconsin complete in 1896. No refracting telescopes larger than 40 inches in diameter have been built: a larger lens would sag under its own weight!

Question 48

Reflecting telescope information

Answer 48

Was invented by Isaac Newton. Mirrors do not suffer from chromatic aberration and, because they can be supported across their entire backside, there is no theoretical limit to how large a mirror can be. May be designed so that light is collected at the prime focus, Newtonian focus, cassegrain focus, and coude focus.

Question 49

Largest reflectors of the past century

Answer 49

Have been 5-meter (200 inch) Hale telescope at Paloma's observatory (1948) 6 meter telescope in Russia (1978) and the twin 10 meter. Keck telescopes atop Mauna Kea in Hawaii (late 1990s)

Question 50

Light gathering power

Answer 50

Of a telescope is proportional t the square of the the objective diameter. As an example, a 12 inch aperture telescope collects nine times as much light as a 4 inch telescope (since 12/4=3 and 3^2 =9)

Question 51

Resolution

Answer 51

Of a telescope (or of any optical instrument) refers to its ability to discern fine detail. For example, the unaided human eye cannot resolve mizar as a double star but a small telescope can. In principle larger telescopes have better resolution; in practice the resolution of any ground based telescope larger in aperture than about ten inches is limited by atmospheric turbulence

Question 52

Adaptive optics

Answer 52

Technique that allows astronomers to correct for atmospheric distortion in real time, allowing ground- based telescopes to match the resolution of telescopes located in space.

Question 53

Magnification

Answer 53

Of a telescope is the ratio of the focal length of the telescope objective to the eyepiece focal length Telescope aperture x the focal length x 25.4/ eye piece focal length

Question 54

Focal ratio

Answer 54

Of a telescope objective is its focal length dived by its diameter

Question 55

Quantum efficiency

Answer 55

The human eye has very low QE. Even at a wavelength of 550 nm ( green light ) where the human eye is most sensitive, only about 5 percent of the photons striking the retina actually produce a signal in the optic nerve. In addition, the eye cannot make time exposures (it collect light for extended periods) and it creates no permanent record

Question 56

Cameras and CCD Cameras

Answer 56

Record two dimensional images; however photographic emulsions ado suffer from low quantum Effie cues (2-4%). Nearly all astronomical photography is now done with electronic CCD ( charge-coupled device) cameras, which may have quantum efficiencies up to 80%! Note that using light more efficiently yields the same result as collecting more light with a larger telescope, and at much lower cost!

Question 56

Spectrographs

Answer 56

Collect and disperse light from a star or other celestial body, and provide an image of the resulting spectrum.

Question 57

Photometers

Answer 57

Are fancy light meters; they are used to measure brightness of stats (usually with different colored filters m) one a mainstay of any astronomical observatory, they now for the most part, obsolete. Stellar Magnitudes for all the stars on a CCD image can be easily obtained using simple software programs

Question 58

Radio telescope

Answer 58

Collects and focuses radio waves from celestial bodies to form an image.

Question 59

Radio telescope

Answer 59

Collects and focuses radio waves from celestial bodies to form an image

Question 60

Interferometer

Answer 60

Because radio waves have such long wavelengths a single dish has very poor resolving power. However, two or more radio dishes can be combined to form a interferometer.

Question 61

Interferometer baseline

Answer 61

The resulting resolution is equivalent to that of a single antenna whose aperture is equal to be baseline. Which is the maximum distance between antennae

Question 62

Two motivations for placing telescopes in space: (outside our atmosphere)

Answer 62

(1) to obtain sharply defined images and (2) to avoid the light pollution which plagues ground-based observatories! Also, since our atmosphere is opaque to gamma rays, X-Rays, and most infrared and ultraviolet light, is JS necessary to place telescopes in space on order to observe at these wavelengths.

Question 63

The largest orbiting telescopes currently:

Answer 63

Hubble (visible, near-infrared, near-ultraviolet) Spitter (infrared) Chandra (X-ray)

Question 64

Temperature of Fahrenheit and Celsius scales

Answer 64

The temperature of a substance is proportional to the average kinetic energy per molecule within that substance; the higher the temperature, the faster the molecules move on average.

Question 65

The kelvin or absolute temperature scale

Answer 65

Is similar to the Celsius scale, except that the zero point is at absolute zero; to convert from Celsius to Kelvin add 273 degrees. There are no negative temperatures on the Kelvin scale!

Question 66

Blackbody

Answer 66

Hypothetical object which absorbs all electromagnetic radiation which falls upon it. Nothing in Nature Is a perfect Blackbody; every thing reflects some radiation!

Question 67

If a black bodies temperature increases..

Answer 67

The color balance shifts to shorter wavelengths . If s Blackbody's temperature were to keep increasing, it's color would change from red to orange, then to yellow, then to white and finally to blue.

Question 68

The relation between color and temperature..

Answer 68

Does not apply to objects seen by rejected light, not to light emitted by low-density gases (such as candle flames)

Question 69

Wien's Law

Answer 69

Relates the wavelengths at which a Blackbody's spectrum peaks to the Blackbody's temperature: the temperature must be on the Kelvin scale.

Question 70

Stefan-Boltzmann Law

Answer 70

States that the flux (the power radiated by each square meter) of a Blackbody's surface is proportional to the fourth power of its temperature. If the temperature doubles, the energy becomes sixteen times greater. If the temperature triples, then the energy becomes 81 times greater (since 3x3x3x3=81)

Question 71

Luminosity

Answer 71

(It's total power output) of a spherical Blackbody's (such as a star) depends only on its size and its surface temperature.

Question 72

Absolute error

Answer 72

Difference between the measured and known value

Question 73

Relative error

Answer 73

Absolute error/ by the true value expresses as a percentage

Question 74

At longer wavelengths (EM)

Answer 74

Infrared light, microwaves, and radio waves

Question 75

Shorter wavelengths (EM)

Answer 75

Ultraviolet light, X-rays, gamma rays