Astronomy Chapter 1

Front 1

How many constellations are there?

Back 1

88. 12 are in the Ancient Zodiac that astrologers use, and the "modern" zodiac that astronomers use there are 13 (the usual 12 plus Ophiuchus)

Front 2

Where does the Celestial Equator lie?

Back 2

Above Earth's Equator. The Earth's Equator and the Celestial Equator lie in the same plane.

Front 3

What is the ecliptic and why is it tilted with respect to the celestial equator?

Back 3

The Ecliptic corresponds to the plane of the Solar System, more specifically the Earth’s orbit around the Sun (or equivalently the Sun’s path around the Celestial Sphere). This is tilted 23.5 degrees with respect to the Celestial equator because the Earth’s axis is tilted 23.5 degrees with respect to the Earth’s orbit.

Front 4

Through how many constellations does the sun move every year? Which ones are they?

Back 4

The Sun moves through 13 constellations, the constellations of the (modern) Zodiac. Ophiuchus is the 13th.

Front 5

What are the vernal and autumnal equinoxes? What are the summer and winter solstices? How are these four events related to the eclptic and the celestial equator?

Back 5

The equinoxes are those points halfway between Solstices at which all points on the globe have 12 hours of daylight and 12 hours of night. The summer solstice occurs when the Sun is highest in the Northern sky, and the winter solstice occurs when the Sun is lowest in the Northern Sky.

On the equinoxes, the ecliptic and the celestial equator intersect. On the solstices the celestial equator and ecliptic are the farthest away.

Front 6

Why is it warmer in the summer than in the winter?

Back 6

Due to the tilt of the Earth's axis with respect to the ecliptic, in the Northern Hemisphere on the Summer Solstice, for example, the Sun is high in the sky and its flux bears down more on the surface, resulting in greater warming. On the Winter Solstice, the Sun is low in the sky and its flux basically gets spread out over more area, resulting in lower warming. In the Southern Hemisphere the situation is reversed.

Front 7

What is the difference between the umbra and penumbra of a shadow?

Back 7

The umbra is the darkest part of a shadow in which all of the light source is blocked. The Penumbra is the part of the shadow in which the source is only partially blocked.

Front 8

Which type of eclipse- lunar or solar- have most people seen?

Why?

Back 8

Lunar. If a Lunar eclipse is taking place, then everyone who can observe the Full Moon can see the eclipse. But to see a solar eclipse, one must be on a very narrow strip on the Earth's surface that just happens to be in the Moon's shadow. So, Lunar Eclipses are more likely to be observed than Solar.

Front 9

How do you describe the shape of Earth's orbit around the Sun?

Back 9

An ellipse. This is Kepler's 1st Law.

Front 10

How long does it take Earth to complete a sidereal orbit of the Sun?

Back 10

The "sidereal period" is the just the year, the time it takes to orbit the Sun once. This is 1 year, or 365.25 days for Earth.

Front 11

At what configuration (superior conjunction, greatest western elongation, etc.) would it be best to observe Mercury or Venus from an Earth based telescope?

At what configuration would it be best to observe Mars, Jupiter, or Saturn?

Back 11

A planet is at OPPOSITION when the Earth is between the planet and the Sun. This situation can only occur for planets further from the Sun than the Earth. Therefore the planets Mercury and Venus cannot be seen at opposition.

Venus and Mercury are best seen at MAXIMUM ELONGATION, as they are furthest away from the blinding Sun.

A planet is at INFERIOR CONJUNCTION when it lies between the Earth and the Sun. Therefore only planets closer to the Sun than the Earth can be seen at inferior conjunction. So the planets MARS, JUPITER, SATURN, URANUS, and NEPTUNE cannot be seen at inferior conjunction.

So it is best to see the outer planets at OPPOSITION, when the Sun is on the other side of the Earth as the planet.

Front 12

What are Kepler's three laws?

Why are they important?

Back 12

First, they are important because they explain fully the motion of all the planets around the Sun. They also provided the first precise description of the motion of objects in the Solar system.

1st- The orbit of a planet around the Sun is an ellipse with the Sun at one focus.

2nd- A line joining a planet and the Sun sweeps out equal areas in intervals of time.

3rd- The square of a planet's sidereal period around the Sun is directly proportional to the cube of the length of its orbit's semimajor axis. Aka P squared = A cubed

Front 13

Describe reflection and refraction.

How do these processes enable astronomers to build telescopes?

Back 13

Refraction: When light enters a transparent material like glass it bends toward the normal. This bending of light is the principle behind prisms and, more importantly to astronomy, it is how lenses work.

Reflection: when light is incident on a reflective surface, like a mirror, it reflects at an angle that is the same as the incident angel. This principle applies to both flat mirrors and curved mirrors (like makeup and shaving mirrors that magnify.)

The principles of reflection and refraction enable astronomers to build telescopes based on lenses and curved mirrors which allow distant objects to be magnified.

Front 14

Give everyday examples of refraction and reflection.

Back 14

Refraction: put a pencil in a glass of water. It will appear to be bent at the surface. Also a person’s legs appear to be short when they stand in water. The bottom of a swimming pool appears closer to the surface because of refraction. A magnifying glass works on the principle of refraction, as do all lenses.

Reflection: When you look in a plane (flat) mirror, you see your reflection, as if coming from an identical but backwards twin standing behind the mirror as far as you are in front of the mirror. In a shaving mirror (or makeup mirror…I don’t use either…) you see an enlarged image of your face, due to the magnifying properties of a concave mirror. “Objects in mirror are closer than they appear” is written on your passenger-side rear view mirror, because it is convex and de-magnifies (makes things smaller…so they don’t appear as close).

Front 15

How much more light does a 3-m diameter telescope collect than a 1 m diameter telescope?

Back 15

9 times more. The area of a circle is in terms of the diameter. Since the area is proportional to the SQUARE of the diameter, a 3m diameter telescope can collect 3 squared = 9 times as much light as a 1m diameter telescope.

Front 16

Explain some of the advantages of reflecting telescopes over refracting telescopes.

Back 16

A reflector can easily be made with a much larger radius, and so a telescope with better resolution can be made. A very large diameter lens actually sags under its own weight, and this creates optical distortions.

Front 17

Compare an optical telescope to a radio telescope.

What do they have in common?

Different?

Back 17

Optical astronomers cannot work effectively during the day because the blinding effect of the Sun and the glowing blue sky make astronomical objects hard to see. So they spend lots of lonely nights at the observatory.

A radio telescope does not have this difficulty, since the background of radio waves during the day isn’t much more than at night. However, Earth-based radio signals can cause unwanted interference. This is why some radio-bands are kept empty, to pacify radio astronomers who would prefer low-radio background in certain key regions of the radio spectrum.

Front 18

Why must astronomers use satellites and Earth-orbiting observatories to study the heavens at X-ray wavelengths?

Back 18

The atmosphere filters out X-ray radiation, and so X-ray telescopes must be outside of the Earth’s atmosphere.

Front 19

What factors would you consider in selecting a new site for an observatory?

Back 19

Probably the major factor is weather, as one would want a large number of viewing days. Another factor is light pollution, and future light pollution, which should be minimized – so locate in a remote area. Another factor is altitude : high altitude observatories (there’s a nice one on top a volcano in Hawaii) have less of a problem of distortions and filtering due to the atmosphere. This is sometimes fixed with adaptive optics.

Consider the Moon!

Advantages: The moon has no atmosphere, and so adaptive optics would not be necessary. Also the moon has a lot less light pollution than on Earth (e.g. like the chemical plants in the Golden Triangle area). The lower gravity on the moon would even lessen the distortion due to “sagging” of telescope optics due to their weight, compared with on Earth.

Front 20

What color blackbody indicates it is the hottest?

Back 20

Violet light because it is the shortest wavelength given, corresponding to the highest temp blackbody.

Front 21

What photons have the lowest energy?

The highest?

Back 21

Infrared has the longest wavelength (lowest frequency), corresponding to the lowest energy.

Ultraviolet (UV) photons have the shortest wavelength and therefore have the highest energy photons.

Front 22

What spectrum of object will show a blueshift?

Back 22

Only when there is motion toward or away from the Earth can the Doppler effect occur. Motion toward the Earth would create a blue shift.

Front 23

What is a blackbody?

What does it mean to say that a star appears almost like a blackbody?

If stars appear to be like blackbodies, why are they not black?

Back 23

A blackbody is an object that absorbs any light incident upon it.

Now stars are extremely hot, glowing balls of gas. BUT think about light incident ON THE STAR. What are the odds that the light will bounce off? Almost zero - it is bound to get absorbed by the gas atoms.

So why are stars not black? Because they are extremely hot, and emit their own radiation, in a spectrum that depends on the temp (for example the higher the temp, the brighter, or more luminous the star.) So "black" really just means "highly absorbing".

Front 24

Using Wien's Law and the Stefan-Boltzmann's law, state the changes in color and intensity that are observed as the temp of a hot glowing object increases.

Back 24

As an object gets hotter, Wien’s Law states that the peak wavelength gets shorter, that is, more towards the blue part of the spectrum. At the same time the luminosity (energy emitted) gets much larger, so the object is brighter. So a relatively cool object may peak in the IR, a slightly hotter object would be red, hotter still orange and brighter, and very hot might look white or even blue and very bright. This applies to stars (our main concern), but also to more everyday phenomena, such as the color of the heating elements on an electric stovetop, or the color of coals in a fire.

Front 25

What color will an interstellar gas cloud composed of hydrogen glow, and why?

Back 25

One will observe the Red emission from the H-alpha line. This is generally due to a hot star or group of stars illuminating the hydrogen gas with UV. The UV is absorbed, and the H-alpha line is then prominent.

Front 26

What is an element?

List of the names of five diff elements and briefly explain what makes them different from each other.

Back 26

An element is an atom with a specified number of protons. For example, Hydrogen has atomic number 1, Helium has atomic number 2, Carbon has atomic number 6, and so on. Atoms with the same number of protons but different number of neutrons are called isotopes.

Here’s a list of 5 elements that have very different properties:

H: Hydrogen, chemically very active as positive ion (missing electron), normally found in gaseous state.

He: Helium, chemically inert, that is, forms no molecules.

C: Carbon, normally found as a non-magnetic solid. Semiconductor.

Cl: Highly chemically active as a negative ion (extra electron)

Fe: Iron normally solid metal, magnetic. Conductor.

Front 27

How are the three isotopes of hydrogen different from each other?

Back 27

Isotopes of an element have the same number of protons, but different numbers of

NEUTRONS. In the case of Hydrogen isotopes, all have 1 proton. Ordinary Hydrogen (H) has zero neutrons, Deuterium (D) has one neutron, and Tritium (T) has 2 neutrons.

Front 28

Why do different elements have different patterns of lines in their spectra?

Back 28

They have different patterns of spectral lines because each element has a distinctive set of energy levels. The line spectra thus are useful “fingerprints” for elements, even in other galaxies.

Front 29

Most of the planets orbit the Sun on or close to

Back 29

The ecliptic plane

Front 30

Suppose a planet 10 times the mass as the Earth was orbiting the Sun at the same distance as the Earth (1 A.U.). The gravitational force on this planet due to the Sun would be

Back 30

10 times the force on the Earth due to the Sun because

Front 31

According to Newton's Law of Graviation the Earth exerts a force on you equal to your weight. According to Newton's 3rd Law

Back 31

you exert a force on the Earth = to your weight

Front 32

The shadow created by the Moon on the Earth during a solar eclipse consists of

Back 32

a dark inner part known as the Umbra, and a less shaded surrounding Penumbra.

Front 33

The distinction between mass and weight is

Back 33

that weight is the force of gravity on an object,

while mass is a measure of the inertia

Front 34

Suppose the distance between two massive objects is tripled. Then the mutual gravitational force exerted between the masses is

Back 34

Decreased by a factor of 9 because force goes down with distance inverse squared law

Front 35

According to Newton's Law of gravitation what exerts gravitational forces on each other?

Back 35

All human beings, and all objects exert gravitational forces on each other.

Front 36

Halley's Comet has a Period (P) of 76 years. According to Kepler's 3rd Law (P2 = A3) the semi-major axis of the comet's orbit is

Back 36

17.9 A.U.

Front 37

Suppose two masses are exterting a gravitational force on each other. If BOTH masses are doubled, then the mutual force of gravitation would

Back 37

Quadruple because 2+2=4

Front 38

The class of curves known as conic sections, which describe orbits, include what shapes?

Back 38

Circle, ellipse, parabola, and hyperbola

Front 39

As viewed from Earth, sometimes a planet seems to reverse direction and move backwards across the sky. What is this type of planetary motion known as?

Back 39

Retrograde motion

Front 40

Seasons on the Earth are due to

Back 40

The tilt of the Earth's axis of rotation with respect to the ecliptic plane.

Front 41

At the time of a Lunar Eclipse the Moon is

Back 41

FULL

Front 42

According to Kepler's 1st Law how do the planets orbit the sun?

Back 42

In elliptical orbits.

Front 43

According to Kepler's 2nd law ("law of equal areas") how does a comet move when near the sun?

Back 43

A comet moves fast when near the sun (perihelion), but slow when far away from the Sun (aphelion)

Front 44

On the Equinoxes what happens?

Back 44

People everywhere on the globe have 12 hours of day and night.

Front 45

The Celestial Sphere rotates about the Celestial North Pole, a point very near the North Star, also known as

Back 45

Polaris

Front 46

During an eclipse of the Sun, in which the Moon passes between the Earth and the Sun, the phase of the Moon is

Back 46

New

Front 47

What wavelengths do visible light range from?

Back 47

400 nm to 700 nm.

Front 48

With a typical glass lens, red light and violet light are focused at different points which leads to an undesirable effect known as what?

Back 48

Chromatic Abberation

Front 49

Spectral lines emitted by low pressure atomic vapors have what characteristics?

Back 49

They are unique to individual elements, like a fingerprint.

Front 50

How do radio waves differ from light waves?

Back 50

Light waves have much shorter wavelengths.

Front 51

What is the basic structure of an atom?

Back 51

Negatively charged particles (electrons) orbit a central positive nucleus.

Front 52

What things are classified as electromagnetic radiation?

Back 52

Radio waves, light waves, and gamma-rays

Front 53

Green light has a longer wavelength than Violet light, and so Green light has what?

Back 53

A lower frequency than Violet light. Longer wavelength = lower frequency.