Astronomy Final - Prof Moody

How much greater is the light-gathering power of a telescope with a 20 inch primary mirror than one with a 10 inch primary mirror?

(20/10)2 = 4

The visible universe is about 14 billion light- years across. The visible universe is then _______ times larger than the Milky Way.

140,000

Suppose you have a telescope with a focal length of 40 inches. What is the magnification when using an eyepiece with a focal length of 2 inches?

20

How much greater is the resolving power of a telescope with a 20 inch primary mirror than one with a 10 inch primary mirror?

20/10 = 2

The speed of light in a vacuum is approximately ____________ km/sec. This value was first measured accurately by ______ ________ who used eclipses of the ______ for his estimate.

300,000, Olaus Roemer, moons of Jupiter

The solar system is about 1 light day across. The Milky Way galaxy is about 100,000 light-years across. The Milky Way is about __ times larger than the solar system.

36,500,000

Explain what a Doppler shift is and how it affects light from a galaxy that is a) going away from us, b) coming toward us and c) moving rapidly past us across the sky but not coming closer or farther. Draw a picture if it helps.

A Doppler shift is the stretching or compression of wavelength when the emitting or receiving sources are moving either toward or away from each other. a) moving away causes the colors to become redder. b) moving toward causes the colors to become bluer. c) moving across the sky does not affect the wavelengths. So Doppler shifting only tells us motion toward or away from us.

absorption spectrum

A continuous spectrum of light missing energy at a few wave lengths.

resolving power

A measure of the ability of a telescope to see fine detail.

photon

A particle of light.

We measure objects in the sky by their angular size. For example the Sun and Moon both have an angular diameter of 0.5 degrees. The distance from the horizon to the zenith is 90 degrees and the size of a fist held at arm's length is about 10 degrees. Can a penny held at arm's length completely cover the Sun? A penny is about 0.75 inches in diameter.

A penny held at arm's length is about a degree in diameter and can easily cover the Sun.

blackbody

A perfect absorber and radiator of electromagnetic radiation.

inferior planet

A planet that orbits closer to the Sun than the Earth

superior planet

A planet that orbits farther from the Sun than the Earth.

seeing

A quantity measuring the stability of the Earth's atmosphere.

emission spectrum

A spectrum of light with energy at only a few wavelengths.

continuous spectrum

A spread of light with an uninterrupted wavelength distribution of energy.

electron

A subatomic particle with a negative charge. It creates light.

active optics

A technique using computer-controlled mirrors to sharpen images distorted by the atmosphere

Describe what a theory is and how it differs from an hypothesis.

A theory is a tested and refined explanation while an hypothesis is an educated guess that does not have the depth and breadth of a theory. Hypotheses are tentative while theories have stood up to more careful scrutiny.

Why is a total eclipse of the Sun visible only from certain parts of the Earth?

A total eclipse is where the umbra of the Moon's shadow touches the Earth's surface. It only touches specific places and not the entire surface, so total eclipses are visible only at those locations.

Is the same true for a total eclipse of the Moon? i.e. is it only visible from certain places on Earth? Why or why not.

A total eclipse of the Moon is when the entire lunar surface is in the umbra of the Earth's shadow. Being in the shadow, it will be dark no matter where on Earth you are looking from. Of course since the Moon is only visible to half the Earth at a time, lunar eclipses also will only be visible to half the Earth as well.

electromagnetic radiation

All possible types of energy that can be emitted and absorbed by atoms.

blackbody curve

All wavelengths of light emitted by a blackbody.

annular eclipse

An eclipse of the Sun that occurs when the angular diameter of the Moon is less than the angular diameter of the Sun.

acceleration

Any change in the speed or direction of an object's motion

Explain with the use of a diagram why some constellations seen from Provo are circumpolar and some are never visible, yet from the equator all constellations are visible at some time during the year.

As a person's position on Earth shifts close to the equator, the circumpolar cone gets smaller and the nonvisible southern cone in the south also gets smaller. Right at the equator they both disappear which means all stars are visible from the equator.

When a full Moon sets in the west

At sunrise

When a 1st quarter Moon is directly overhead

At sunset

geocentric

Centered on the Earth

heliocentric

Centered on the Sun

There are 88 constellations in the sky. We categorize them by the season in which they are visible after sunset with circumpolar stars being those that are always above the horizon. a) Why did the ancient people divide the sky into constellations? b) Why do modern astronomers continue to use them?

Constellations were originally a means of preserving knowledge and legends that were passed down from generation to generation among people who did not necessarily know how to read or write. We use them today to divide the sky into sectors like the states divide up the USA or like counties divide up a state.

Explain how individual atoms create both emission and absorption line spectra.

Emission line spectra arise from electrons falling from higher to lower levels in atoms of low density gasses. They jump to the high levels from either collisions caused by heating or by absorbing light from a continuous spectrum. When they absorb light, it is at the same energy and hence the same wavelength, as the energy emitted when the electron fell down. But since absorption removes energy from the spectrum it creates an absorption line.

Stefan-Boltzmann law

Energy = (Temperature/5800K)^4 the total energy given off by every square centimeter of a blackbody is equal to a constant multiplied by the fourth power of the temperature.

Define and explain the relationship between a) frequency, b) wavelength, c) speed, and d) energy.

Frequency (f) is the number of waves that pass a point per second. Wavelength (λ) is the spatial extent of a wave. Speed is frequency times wavelength and has a constant value for all electromagnetic radiation traveling in a vacuum. This means that as frequency increases, wavelength must decrease. Energy is proportional to frequency. So photons of gamma rays have much more energy than photons of radio waves.

semimajor axis

Half of the longest diameter across an ellipse.

The special theory of relativity predicts that nothing can travel faster than this speed. However, under what circumstances can it travel slower than this?

If it is in a denser medium, such as glass, it will move slower. This change is speed is what causes light to refract.

deferent

In Ptolemy's geocentric solar system, the large circle on which a planet's epicycle moved around the Earth

epicycle

In Ptolemy's geocentric solar system, the small circle on which a planet moved

light pollution

Light scattered through the atmosphere that degrades astronomical images.

When a 3rd quarter Moon rises in the east

Midnight

When a new Moon is directly overhead

Noon

What is precession and how does it affect a) the north star b) when the constellations are visible?

Precession is the slow, regular "wobble" of the Earth's axis with respect to the stars. a) It causes the north celestial pole to change position, going in a circle with a period of 26,000 yrs. b) Because the pointing direction of the Earth's rotation axis changes, in 13,000 years the northern hemisphere will tilt toward the Sun when the Earth is on the opposite side of its orbit from where it is now in summer. Calendars are adjusted to keep the seasons on the same days of the years as they are now. So in 13,000 years summer will still be in July and winter in January. However the night-time constellations will change and in 13,000 years the current winter constellations will appear in July and the current summer constellations will appear in January.

Light with all of its colors is part of a family of radiant energy called electromagnetic radiation. List below the types of this energy, ordered from longest wavelength to shortest wavelength. Be sure to include light.

Radio Microwave Infrared Visible Light Ultraviolet Light X-ray Gamma-ray

If you lived on the Moon would you see the Earth rise and set or would it stay in the same place? Explain your answer

Since the Moon is in synchronous orbit with the Earth and keeps one face always pointed toward it, you would always see the Earth in the same place in the sky.

Suppose you see the Moon in the sky on two successive nights. How has its position changed with respect to the stars on the second night? Why?

The Moon will be further to the east by about 12 degrees. This is because the Moon orbits about the Earth.

At the summer solstice, the Sun is at the highest possible point in the north with a declination of 23.5° north of the celestial equator. At the winter solstice, the declination of the Sun is -23.5° which is south of the celestial equator. At the time of the autumnal and vernal equinoxes the declination of the Sun will be 0° which is on the celestial equator or in other words directly over the Earth's equator. The tropical zone on Earth is defined as being the band around the equator between 23.5°. a) What is unique about the Sun inside that zone? b) Now describe how the Sun relates to the Arctic and Antarctic circles.

The Sun is seen at the zenith only at points inside this zone. The Arctic and Antarctic circles are at 23.5 degrees from the north and south poles respectively. While the tropical zone is the latitude where the Sun can be seen directly overhead at the zenith, north of the Arctic circle and south of the Antarctic circle are the zones where it is possible to go through a day without the Sun ever setting below the horizon in summer or rising above the horizon in winter.

Give two reasons why it is hot during the summer in the northern hemisphere.

The Sun's rays are more direct, depositing more energy per square meter than in the winter. The days are longer.

Explain how the Vernal Equinox can be both a time of the year and a place in the sky.

The Vernal Equinox is the location in the sky where the Sun crosses the celestial equator going north. Really, it is when the Earth is at the place in its orbit so that the Sun appears to be exactly on the celestial equator in spring. It does so on March 21 so we refer to both the time and the place as the Vernal Equinox.

retrograde motion

The apparent backward motion of a planet against the background of stars

ecliptic

The apparent path of the Sun through the stars on the celestial sphere.

astronomical unit

The average distance between the Earth and the Sun (=1.5 x 108 km).

Is the dark side of the Moon the same as the far side of the Moon? Explain your answer.

The dark side is the side facing away from the Sun. The far side is the side facing away from the Earth. As the Moon orbits the Earth the far side is always the same physical side but the dark side is not. The dark half constantly changes with the lunar phases. At new Moon the dark side is the near side and at full Moon the dark side is the far side.

focal length

The distance between a lens and its focal plane.

light-year

The distance light travels in one year.(=9.46 x 1012 km).

celestial sphere

The imaginary sphere centered on the Earth that holds the stars.

Wien's law

The law stating that hotter blackbodies look bluer than cooler blackbodies.

Stephen-Boltzman law

The law that says light energy from a blackbody increases as (temperature)^4.

energy level

The location around an atom where an electron resides.

direct motion

The normal eastward movement of a planet against the background of distant stars

meridian

The north-south line passing directly overhead through the zenith.

celestial pole

The place in the sky that the Earth's axis points toward (can be either north or south).

zenith

The point directly overhead.

aphelion

The point in its orbit where a planet is farthest from the Sun

perihelion

The point in its orbit where a planet is nearest the Sun.

opposition

The point where a superior planet is as far away from the Sun as it can be (as seen from the Earth).

greatest elongation

The point where an inferior planet is as far away from the Sun as it can be (as seen from the Earth).

celestial equator

The projection of the Earth's equator onto the celestial sphere.

synodic day

The rotation period of the Earth measured relative to the Sun.

sidereal day

The rotation period of the Earth measured relative to the stars.

spectroscopy

The science of measuring light energy by wavelength.

umbra

The shadow area behind the Earth or Moon where the Sun is completely obscured.

penumbra

The shadow area behind the Earth or Moon where the Sun is partially obscured.

precession

The slow wobble of the Earth on its rotation axis.

Suppose you go outside after sunset and see the stars. You then arise before sunrise and see the stars again. How have they shifted and why?

The stars have all rotated to the west. The western-most stars have set and new ones have risen in the east. This is because the Earth spins on its axis.

spectral lines

The wavelengths where a specific element can absorb or emit light

conjunction

When a planet lines up with the Sun in the sky.

synchronous rotation

When one side of a body always faces the planet it revolves around

A sidereal day is defined as being the interval between times when a star is directly overhead and is about 23 hours 56 minutes. This means that if a star crosses the meridian at 8:00 PM one night, the next night it will cross the meridian at 7:56. What causes the difference between these two periods?

When the Earth spins on its axis it also orbits the Sun. The advancement along the orbital path causes the stars to reappear at the same location after one spin (i.e. the arrows 1 and 2a are in the same direction) slightly sooner than the Sun does. (i.e. the Earth must spin a little farther to reach arrow 2b) This difference is 4 minutes a day.

total eclipse

When the Moon entirely blocks the Sun.

summer solstice

When the Sun is farthest north of the celestial equator (about June 22)

winter solstice

When the Sun is farthest south of the celestial equator (about December 22).

autumnal equinox

When the Sun moves from north to south across the celestial equator (about September 23).

vernal equinox

When the Sun moves from south to north across the celestial equator (about March 21).

Proxima Centauri is 4.22 light-years away. The Sun is 8 light minutes away. Proxima Centauri is then ____ times farther away than the Sun.

about 277,000

A glowing blackbody produces a ______ spectrum of electromagnetic radiation. If this radiation passes through a rarified gas cloud, then it will have _______ created in it. When the gas re-radiates the subtracted light, it creates a(n) ____ spectrum.

continuous, absorption lines, emission line

The_______ coordinate is divided into degrees, minutes, and seconds. Its zero line is the ________ _______. The greatest value it can have is plus or minus _____ degrees.

declination, celestial equator, 90

When we see exactly half illuminated it is a ________ Moon.

first quarter

The____ , which is the intersection of the plane of the Moon's orbit with the plane of the Earth's orbit, must be pointing at the Sun for an eclipse to occur. The yearly path that the Sun traces on the celestial sphere is called the _____ because this is the only place where eclipses can occur.

line of nodes, ecliptic

A __ eclipse occurs when the Moon passes into the Earth's shadow. This can be either a __ eclipse if the Moon is not completely within the umbra of the Earth's shadow or a __ eclipse if it is entirely obscured. These eclipses only occur when the Moon is at the __ phase.

lunar, partial, total, full

Wien's law

peak wavelength = (500)(5800)/Temperature nm the peak wavelength of the light from a blackbody multiplied by its temperature (in degrees Kelvin) is equal to a constant.

The_____ ________ coordinate is divided into hours, minutes, and seconds. Its zero point is the Vernal Equinox. The greatest value it can have is ______ hours.

right ascension, 24

The sky is mapped out by a grid of two coordinates called _____ _____ and ______.

right ascension, declination

Similarly, a __ eclipse occurs when the Earth passes into the Moon's shadow. These eclipses are called __ at places on the Earth that are inside the umbra of the shadow, __ at places that are inside the penumbra of the shadow and __ if the shadow umbra doesn't touch the surface of the Earth at all. These eclipses only occur when the Moon is at the __ phase.

solar, total, partial, annular, new

sidereal day

the interval between times when a star is directly overhead and is about 23 hours 56 minutes

solar, or synodic, day

the time interval between when the Sun is directly overhead and is exactly 24 hours.

It continues to decrease in illumination as a _______________ Moon until it arrives again at the new Moon phase.

waning crescent

As it begins to decrease in illumination, it is in the ________ portion until it is again half illuminated as a _____________ Moon.

waning gibbous, third quarter

A Moon that is new and getting more illuminated every night, is in the _________ portion of its cycle.

waxing crescent

After this it enters the ___________ portion until the entire face is illuminated at the _____________ phase.

waxing gibbous, full Moon