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# Astronomy 1010- Ch. 2

Ch. 2
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What are constellations? How did they get their names?
People of nearly every culture gave names to patterns they saw in the sky.
-Technical def: a region of the sky with well-defined borders; the familiar patterns just help us locate the constellations. The names and borders of the 88 official constellations were chosen in 1928 by the IAU (but they chose names that can be traced back to the ancient Middle East [Northern] and the 17th century European explorers [Southern].
On a clear, dark night, the sky may appear to be "full" of stars. Does this appearance accurately reflect the way stars are distributed in space? Explain.
No, there are billions of stars in the sky and we can only see about 6,000 of them.
Define horizon, zenith, and meridian. How do we describe the location of an object in the local sky? What are right ascension and declination?
-Horizon: the boundary between Earth and the sky
-Meridian: an imaginary half-circle stretching from horizon due south, through zenith, to the horizon due north.
-We describe the location of an object in sky by stating its direction along the horizon (the azimuth=degrees clockwise from due north) and its altitude above the horizon.
-Right Ascention: like longitude
-Declination: like latitude
Explain why we can measure only angular sizes and angular distances for objects in the sky. What are arcminutes and arcseconds?
We can only measure angular sizes and distances because we have no way to judge the true sizes or separations of the objects we see in the sky. Angular size is the angle it appears to span in your field of view. Angular distance is the angle that appears to separate a pair of objects. Arcminutes (')are the 60 subdivided unites of each degree. And Arcseconds (") are the 60 subdivided units of arcminutes. So there are 3600 arcseconds in 1 degree.
What are circumpolar stars? Are more stars circumpolar at the North Pole or at Athens, Ga? Explain.
Circumpolar stars are stars that never rise or set but make the daily counterclockwise circle around the north celestial pole. They are perpetually above the horizon. You would see more of these at the North Pole than Athens, GA.
What are earth latitude and longitude? Does the sky you observe vary as you change latitude? Does it vary with longitude? Explain.
Earth latitude measures north-south points. (It is 0 at the equator, increasing to 90degreesN at the North Pole and 90degrees S). Lines of latitude are circles running parallel to the equator.
-Earth longitude measures east-west position. Lines of longitude are semicircles extending from North to South Pole. (It is 0 degrees at the prime meridian [Greenwich, England]).
You are living on a planet where the sidereal day is 4 minutes LONGER than the mean solar day. How can this be?
Earth's sidereal day is 4 minutes SHORTER than the mean solar day. Therefore, for a planet to have the sidereal day 4 minutes LONGER, the rotation of the planet would have to be opposite that of the Earth.
Suppose a star rose at 10pm today. How do you calculate when it will rise a certain number of days from now?
4 minutes earlier each day, 2 hours each month, and 24 hours a year.. Therefore, a star would rise that amount earlier by day. So for example, in three days it would rise at 9:48pm.
Describe the path that a star on the celestial equator follows from the time it rises until it sets for a person at the following locations. Tell how long the star is above the horizon. Start with where it rises and sets.
A. at the North Pole
B. at the South Pole
C. Equator
D. a latitude of 34 degrees N
E. Do the same [ABCD above] for a star at +34 degrees declination.
...
OMIT
OMIT
What is the zodiac, and why do we see different parts of it at different times of year?
The zodiac is the constellations that the sun goes through. Any constellation that the ecliptic goes through
We see different parts of it at different times of year b/c as we go around the sun moves into different parts of it over the year.
Explain why the Earth has seasons. Suggest a way that it would NOT have seasons.
Earth has season b/c of the 23 degree tilt of the axis of rotation. This also causes the reason for why season are opposite in the hemispheres. It's based on which hemisphere is most tipped towards the Sun (either Northern or Southern).. which is why we also have seasons like Fall and Spring when the sun is shining on both hemispheres equal amounts. Earth could not have seasons if our axis of rotation was not tilted.
Briefly describe what is special about the summer and winter solstices and the spring and fall equinoxes.
During summer and winter, the sun is shining on only one of the hemispheres. Summer is northern. Winter is southern. Spring and fall equinoxes are when the sun is shining equally on both hemispheres.
How does the Sun move with respect to the constellations over the year?
As we go around the orbit, the sun's path changes in relation to our latitude and its own declination. Therefore, over the year, the sun's celestial latitude changes.
How are Martian seasons different than Earth's? Why?
The martian seasonal changes are more extreme than the seasonal changes on Earth because of the high variation that Mars experiences in its distance from the Sun during its orbit in the Martian year. Mars' axil tilt is also slightly greater than Earth's.
What is precession, and how does it affect the sky that we see from Earth?
Precession changes the orientation of the axis in space BUT it also changes the locations in Earth's orbit when the season's happen. Basically, precession is like a spinning top. At first the axis of rotation is stable and spinning vertically, but over time, it loses its momentum and then the axis begins to tilt away from vertical.
How could you eliminate precession?
Precession occurs because of the gravitational influence of other planets and moon, so you could eliminate precession if these other planets and moon doesn't exist.
OR. Make earth a perfect, homogonous sphere.
OR. Make earth's tilt 0 degrees.
Briefly describe the Moon's cycle of phases. Can you ever see a full moon at noon from Athens, Georgia? Explain.
New, waxing crescent, first, waxing gibbous, full, waning gibbous, third, waning crescent. All happen b/c half the moon always faces the sun and other half doesn't. Also, your orbit makes you see different combinations of bright and dark sides. No, b/c the moon would be behind the Earth at that time.
Describe the phases of the moon in terms of the moon's position in the sky relative to the sun.
New Moon, Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Third Quarter, Waning Crescent... New moon happens when the Sun basically blocks the moon (at around noon) so you only see the sky, Then through three phases until midnight when you see only the bright half (appearing as a Full moon). So across the day from noon to midnight, you go from seeing no moon, to more and more moon until it appears full.
Many people incorrectly guess that the phases of the Moon are caused by Earth's shadow falling on the Moon. How would you go about convincing a friend that the phases of the Moon have nothing to do with Earth's shadow? Describe the observations you would use to show that Earth's shadow couldn?t be the cause of phases.
Earth's shadow cannot cause the phases of the Moon because the phases result from two basic facts:
-Half the ball always faces the Sun and is bright while the other half always faces away from the Sun and is dark
-As you look at the ball in different orbital positions, you see different combos of its bright and dark faces.
What do we mean when we say that the Moon exhibits synchronous rotation? What does this tell us about the Moon's periods of rotation and orbit?
Synchronous rotation means that the Moon rotates on its axis in the same amount of time it takes for it to orbit Earth. Means that it makes a half turn in half the orbit. Happens b/c of Earth's gravity affecting the moon
Describe the motion of the moon 1) With respect to the sky over one night 2) With respect to the sky over the entire month 3) With respect to the constellations over an entire month. Include the path and the rate of motion along the path
Moon over one night...Lunar orbit is 5 degrees tilted to the ecliptic. Over the month: constellations travel through 12 constellations in 29.5 days. 13 degrees every 24 hours or so.
Why don't we see an eclipse at every new and full moon? Describe the conditions needed for a solar or lunar eclipse.
We don't see an eclipse b/c the Earth, Moon, and the Sun aren't in line with each other on every new and full moon. Solar eclipse only occurs when the Moon is new and is close enough to the ecliptic that its shadow partially or completely reaches the Earth. Since the moon's orbit is slightly inclined to the ecliptic, it only aligns perfectly twice in its orbit of the Earth.
So to have an eclipse, the phase of the Moon must be full (for a lunar) or new (for a solar) and each must occur during one of the periods when the two points in each orbit where the Moon crosses the surface are aligned with the Sun and the Earth.
Describe the circumstance and appearance of all three types of lunar eclipse. A sketch might help. Choose one of the types and describe how it would look if you were on the moon during that eclipse.
-Total: Sun Earth and Moon are nearly perfectly aligned and moon passes through the umbra. Appears red b/c the earth's atmosphere bends the sun's light around it making the umbral shadow, but the atmosphere takes out all the blue light.
-Partial: somewhat aligned but only part of the full moon passes through the umbra. Appears that part of the moon is dark
-Penumbral: moon passes through earth's penumbra. The earth's shadow would fall on the moon and it would be completely dark.Hardly visible even with a telescope.. just a light shading of grey on part of the moon.
-In a total lunar eclipse, from the moon, it would be in darkness, but you would see an Earth with a red ring of light (b/c the sun is rising everywhere at the same time).
Describe the astronomical conditions necessary for the occurrence of a total solar and total lunar eclipse.
For a total solar eclipse, the moon passes BETWEEN the sun and the earth. So the moon is in the middle.
With a total lunar eclipse, the moon passes BEHIND the earth into its shadow.. so Earth is in the middle.
Describe the 3 types of solar eclipses and suggest two circumstances might result in more total solar eclipses.
Total- the Moon's umbra touches a small area of Earth's surface. Occurs when the Moon is relatively close to Earth in its orbit.
Partial- A much larger region falls within the Moon's penumbral shadow. Only part of the Sun is blocked from view
Annular- the small part of Earth behind the umbral shadow...ring of light surrounds the Moon. If the eclipse occurs when the Moon is far from Earth, the moon's angular size means its umbral shadow may not reach Earth.

We could have more total solar eclipses by: if the Moon was much larger than it is now. or if the Moon did not have orbital tilt it would pass through the ecliptic every time it orbited earth
How could you eliminate eclipses? How could we have more eclipses?
Further away= less eclipses
Add more eclipses if the Moon's orbit was perfectly circular --if the orbital tilt was reduced. OR If moon's orbit was closer to Earth, the apparent size of the moon would be bigger
Assume you live on the Moon near the center of the face that looks toward Earth.
Suppose you see a full earth in your sky. What phase of the Moon would people on Earth see? Explain.
Suppose people on Earth see a full moon. What phase would you see for Earth? Explain.
Suppose people on Earth see a waxing gibbous moon. What phase would you see for Earth? Explain.
Suppose people on Earth are viewing a total lunar eclipse. What would you see from your home on the Moon? Explain.
a. would see a new moon. b/c
b. would see a new earth
c. would see a waning crescent
d. would see darkness
If you lived on the moon how often would you see:
a. The sun rise and set
b. The earth rise and set
c. A solar eclipse?
a. 29.5 days b/c the Moon goes through a full set of phases in that amount of time. You get a rise to a rise in the same amount of time. Rise to set is 15 days.
b. Depends which side you're on.. Either Moon is always in Sky or never.
c. couple times a year
Suppose you lived on the Sun (and could ignore the heat). Would you still see the Moon go through phases as it orbits Earth? Why or why not?
It wouldn't have phases b/c you would always see the lit side of the moon.
What do we mean by the apparent retrograde motion of the planets? Why was it difficult for ancient astronomers to explain? How do we explain it today?
Apparent retrograde motion of the planets is when the planets occasionally reverse course, meaning they move westward through the zodiac. It lasts anywhere from a few weeks to a few months (depending on the planet).
Astronomers had a hard time explaining it because they thought Earth was the center of the universe.
We explain it today by thinking about two people walking in a circle around an object representing us on Earth and another planet. You'll see your friend appear to be going the opposite direction when you are near it in orbit.
Describe the motion of a superior planet with respect to the constellations.
Superior planets are outside our orbit. They move west to east in respect to the constellations. except when we are close to them in orbit when apparent retrograde happens (b/c we are moving faster than them[b/c we're closer to the Sun])
Which planets can never be seen at opposition? At inferior conjunction?
All can be seen in conjunction but inferior gets 2. **Anything inside the earth's orbit cannot be seen at opposition.
What is stellar parallax?
Stellar parallax is the way your two eyes view something different on each side of your nose (depending on how far away it is as well). Basically, because we view the stars from different places in our orbit at different times of year, nearby stars should appear to shift back and forth against the background of more distant stars.