Observing the sky Flashcards
Geocentric
Earth centered view
Heliocentric
Sun centered view
The point of sky/celestial sphere directly above the observer
zenith
zenith
The point of sky/celestial sphere directly above the observer
North and South Celestial poles
the points where this line intersects the celestial sphere from the north and south poles are called the north celestial pole and the south celestial pole
north circumpolar zone.
For this observer, stars within 38° of the North Pole can never set. They are always above the horizon, day and night. This
Polaris
the star that moves the least amount as the northern sky turns each day.
How does the sun change position each day
The sun changes position gradually on the celestial sphere, moving each day about 1° to the east relative to the stars.
the ecliptic
The path the Sun appears to take around the celestial sphere each year
How many minutes later does the sun rise each day?
Because of its motion on the ecliptic, the Sun rises about 4 minutes later each day with respect to the stars.
Does the elliptic lie on the celestial equator?
The ecliptic does not lie along the celestial equator but is inclined to it at an angle of about 23.5°.
Celestial equator
the projection into space of the earth’s equator; an imaginary circle equidistant from the celestial poles.
What’s weird about dark matter?
It interacts with light in a weird way
What energy that is driving the expansion of our universe?
No one knows
What makes scientific theories valid
If they serve to explain and predict the new observations and expriments
When did we learn about the size of the earth and the moon?
Greek philosophers figured it out around 200BC
How long does it take light to travel to the moon
~1 sec
How long does it take light to travel to Sirius
8 years
How long does it take light to travel to the Sun
8 minutes
Andromeda Galaxy is how many light years away?
2.5 million light years
Why can’t we see galaxy 15 billion light years away?
Because we would be looking at a time before the universe existed
How many stars in the milky way galaxy
Milky way galaxy has about 100 billion stars
The astronomers of which early civilizations new approximately how long it took the earth to go around the sun
Ancient Babylonian, Assyrian, and Egyptian astronomers
The Mayan culture in Mexico and Central America developed a sophisticated calendar based on which planet
Venus
cosmology
Our concept of the cosmos—its basic structure and origin—
Belief in a spherical Earth may have stemmed from the time of _______
pythagorus
Pythagoras
a philosopher and mathematician who lived 2500 years ago.
What did pythagoras believe?
He believed circles and spheres to be “perfect forms” and suggested that Earth should therefore be a sphere. As
Who tutored Alexander the Great?
aristotle
What do the ideas of aristotle summarize?
summarize many of the ideas of his day. They describe how the progression of the Moon’s phases—its apparent changing shape—results from our seeing different portions of the Moon’s sunlit hemisphere as the month goes by (see Earth, Moon, and Sky).
How did aristotle know that the moon was closer than the sun?
Aristotle also knew that the Sun has to be farther away from Earth than is the Moon because occasionally the Moon passed exactly between Earth and the Sun and hid the Sun temporarily from view. We call this a solar eclipse.
Aristotle cited convincing arguments that Earth must be round:
First is the fact that as the Moon enters or emerges from Earth’s shadow during an eclipse of the Moon, the shape of the shadow seen on the Moon is always round
As a second argument, Aristotle explained that travelers who go south a significant distance are able to observe stars that are not visible farther north. And the height of the North Star—the star nearest the north celestial pole—decreases as a traveler moves south. On a flat Earth, everyone would see the same stars overhead.
What did Aristarchus of Samos (310–230 suggest?
suggested that Earth was moving around the Sun,
Why did some greek philosphers reject the heliocentric model
One of the reasons for their conclusion was the thought that if Earth moved about the Sun, they would be observing the stars from different places along Earth’s orbit.
parallax
The apparent shift in the direction of an object as a result of the motion of the observer
What did the greek philosophers conclude when they realized there was no parralax
This meant either that Earth was not moving or that the stars had to be so tremendously far away that the parallax shift was immeasurably small.
How did the aincent astronomers measure earth’s size using light rays
The more distant an object, the more nearly parallel the rays of light coming from it.
How did Eratosthenes figure out the size of earth?
he measured the length of a shadow from a vertical stick of a known height in two cities on the same day. The ratio between the north-south distance between the two cities and the angles measured gave a ratio which allowed Eratosthenes to calculate the size of the Earth.
Perhaps the greatest astronomer of antiquity was…
Hipparchus
Hipparchus
he measured, as accurately as possible, the positions of objects in the sky, compiling a pioneering star catalog with about 850 entries. He designated celestial coordinates for each star, specifying its position in the sky, just as we specify the position of a point on Earth by giving its latitude and longitude. He also divided the stars into apparent magnitudes according to their apparent brightness. He called the brightest ones “stars of the first magnitude”; the next brightest group, “stars of the second magnitude”; and so forth. This rather arbitrary system, in modified form, still remains in use today
Hipparchus’s most amazing discovery
the position in the sky of the north celestial pole had altered over the previous century and a half. Hipparchus deduced correctly that this had happened not only during the period covered by his observations, but was in fact happening all the time:
why does the earth wobble like a top.
Because our planet is not an exact sphere, but bulges a bit at the equator, the pulls of the Sun and Moon cause it to wobble like a top.
The last great astronomer of the Roman era
Claudius Ptolemy
what was Ptolemy’s most important contribution
a geometric representation of the solar system that predicted the positions of the planets for any desired date and time.
What causes retrograde motion
The temporary apparent westward motion of a planet as Earth swings between it and the Sun
Why did Ptolemy had to construct his model using circles alone?
the Greeks believed that celestial motions had to be circles,
What prob did ptolemy solve?
Ptolemy solved the problem of explaining the observed motions of planets by having each planet revolve in a small orbit called an epicycle.
deferent
The center of the epicycle then revolved about Earth on a circle
what did ptolemy have to do In order to match the observed motions of the planets..
Ptolemy had to center the deferent circles, not on Earth, but at points some distance from Earth. In addition, he introduced uniform circular motion around yet another axis, called the equant point. All
What did many ancient cultures regard the planets, stars as
representatives or symbols of the gods or other supernatural forces that controlled their lives.
when and where did astology begim
Astrology began in Babylonia about two and half millennia ago.
Spread of belief in astrology
When the Babylonian culture was absorbed by the Greeks, astrology gradually came to influence the entire Western world and eventually spread to Asia as well.
How did the Greeks democratisize astrology
developing the idea that the planets influence every individual.
natal astrology
In particular, they believed that the configuration of the Sun, Moon, and planets at the moment of birth affected a person’s personality and fortune
As famous for his astrology as for his astronomy, Ptolemy compiled the ________, a treatise on astrology that remains the “bible” of the subject.
Tetrabiblos
The key to natal astrology
horoscope
Sun sign astrology
(which you read in the newspapers and many magazines) is a recent, simplified variant of natal astrology.
Does natal astrology hold up when tested?
no
Nicolaus Copernicus developed what?
developed the heliocentric model
Who wrote the book De Revolutionibus Orbium Coelestium
Nicolaus Copernicus
Copernicus wanted to develop an improved theory from which…
… to calculate planetary positions,
Copernicus life to death date
1473-1543
Tycho Brahe
- was a noble man, known to organize and build the best observatory for his time
- didn’t think the earth went around the sun but knew all the other planets do
- Hired Kepler
Johanness Kepler
- trying to fit the planets into the circular model but it didn’t work
- found 8 arcminutes
horoscope
a chart used by astrologers that shows the positions along the zodiac and in the sky of the Sun, Moon, and planets at some given instant and as seen from a particular place on Earth—usually corresponding to the time and place of a person’s birth
parallax
the apparent displacement of a nearby star that results from the motion of Earth around the Sun
planet
today, any of the larger objects revolving about the Sun or any similar objects that orbit other stars; in ancient times, any object that moved regularly among the fixed stars
precession (of Earth)
the slow, conical motion of Earth’s axis of rotation caused principally by the gravitational pull of the Moon and Sun on Earth’s equatorial bulge
retrograde motion
the apparent westward motion of a planet on the celestial sphere or with respect to the stars
year
the period of revolution of Earth around the Sun
zenith
the point on the celestial sphere opposite the direction of gravity; point directly above the observer
zodiac
a belt around the sky about 18° wide centered on the ecliptic
