# revolutions of earth

## the heliocentric universe

Ptolemys geocentric model worked, but it was complicated and occasionally made errors in predicting the movement of planets. At the beginning of the 16th century A.D., Nicolaus Copernicus proposed that Earth and all the other planets orbit the Sun. With the Sun at the center, this model is called the heliocentric model, or "sun-centered" model. Although Copernicus model was simpler - it didnt need epicycles and deferents - it still did not perfectly describe the motion of the planets. Johannes Kepler solved the problem a short time later when he determined that the planets moved around the Sun in ellipses (ovals), not circles (Figure 1.2). Keplers model matched observations perfectly. The heliocentric model did not catch on right away. When Galileo Galilei first turned a telescope to the heavens in 1610, he made several striking discoveries. Galileo discovered that the planet Jupiter has moons orbiting around it. This provided the first evidence that objects could orbit something besides Earth. Galileo also discovered that Venus has phases like the Moon (Figure 1.3), which provides direct evidence that Venus orbits the Sun. Galileos discoveries caused many more people to accept the heliocentric model of the universe, although Galileo himself was found guilty of heresy. The shift from an Earth-centered view to a Sun-centered view of the universe is referred to as the Copernican Revolution. In their elliptical orbits, each planet is sometimes farther away from the Sun than at other times. This movement is called revolution. At the same time, Earth spins on its axis. Earths axis is an imaginary line passing through the Keplers model showed the planets moving around the Sun in ellipses. The phases of Venus. planets center that goes through both the North Pole and the South Pole. This spinning movement is called Earths rotation.

## earths revolution

Copernicus, Galileo, and Kepler were all right: Earth and the other planets travel in an elliptical orbit around the Sun. The gravitational pull of the Sun keeps the planets in orbit. This ellipse is barely elliptical; its very close to being a circle. The closest Earth gets to the Sun each year is at perihelion (147 million km) on about January 3rd, and the furthest is at aphelion (152 million km) on July 4th. The shape of Earths orbit has nothing to do with Earths seasons. Earth and the other planets in the solar system make elliptical orbits around the Sun. For Earth to make one complete revolution around the Sun takes 365.24 days. This amount of time is the definition of one year. Earth has one large moon, which orbits Earth once every 29.5 days, a period known as a month. Click image to the left or use the URL below. URL:

## earth orbits a star

Certainly no one today doubts that Earth orbits a star, the Sun. Photos taken from space, observations made by astronauts, and the fact that there has been so much successful space exploration that depends on understanding the structure of the solar system all confirm it. But in the early 17th century saying that Earth orbited the Sun rather than the reverse could get you tried for heresy, as it did Galileo. Lets explore the evolution of the idea that Earth orbits the Sun.

## the geocentric universe

To an observer, Earth appears to be the center of the universe. That is what the ancient Greeks believed. This view is called the geocentric model, or "Earth-centered" model, of the universe. In the geocentric model, the sky, or heavens, are a set of spheres layered on top of one another. Each object in the sky is attached to a sphere and moves around Earth as that sphere rotates. From Earth outward, these spheres contain the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. An outer sphere holds all the stars. Since the planets appear to move much faster than the stars, the Greeks placed them closer to Earth. The geocentric model explained why all the stars appear to rotate around Earth once per day. The model also explained why the planets move differently from the stars and from each other. One problem with the geocentric model is that some planets seem to move backwards (in retrograde) instead of in their usual forward motion around Earth. Around 150 A.D. the astronomer Ptolemy resolved this problem by using a system of circles to describe the motion of planets (Figure 1.1). In Ptolemys system, a planet moves in a small circle, called an epicycle. This circle moves around Earth in a larger circle, called a deferent. Ptolemys version of the geocentric model worked so well that it remained the accepted model of the universe for more than a thousand years.

## instructional diagrams

The diagram shows different imaginary lines around the earth. At the very north is the north pole and at the very south is the south pole of the earth. An imaginary line around the earth near the north pole is the arctic circle. It is located at 66.5 north of equator. An imaginary line around the earth near the south pole is the Antarctic circle. It is located at 66.5 south of equator. Equator is an imaginary line that goes round the Earth and divides it into two halves. The northern half is called northern hemisphere and the southern half is called southern hemisphere. Tropic of cancer and tropic of Capricorn are the two imaginary lines around the Earth on either side of the equator. The Tropic of Cancer is 23 26 north of it and the Tropic of Capricorn is 23 26 south of it.

This Diagram shows the Earth's rotation. Which is the amount of time that it takes to rotate once on its axis. This is, apparently, accomplished once a day every 24 hours. However, there are actually two different kinds of rotation that need to be considered here. For one, there's the amount of time it takes for the Earth to turn once on its axis so that it returns to the same orientation compared to the rest of the Universe. Then there's how long it takes for the Earth to turn so that the Sun returns to the same spot in the sky. Earth's rotation is slowing slightly with time; thus, a day was shorter in the past. This is due to the tidal effects the Moon has on Earth's rotation. Atomic clocks show that a modern-day is longer by about 1.7 milliseconds than a century ago, slowly increasing the rate at which UTC is adjusted by leap seconds.

## questions

the planets in our solar system revolve around

``````-->  a) the sun

b) the moon

c) saturn

d) earth
``````

this 17th century scientist was persecuted for saying the earth orbits around the sun.

``````a) newton

-->  b) galileo

c) ptolomy

d) wegner
``````

the suns gravitational pull keeps the planets in orbit.

``````-->  a) true

b) false
``````

in the geocentric model of the universe, everything in the heavens revolves around

``````a) the moon

b) saturn

c) the sun

-->  d) earth
``````

the planets appear to move slower than the stars.

``````a) true

-->  b) false
``````

ptolomys system worked so well that no one questioned it until the 20th century.

``````a) true

-->  b) false
``````

ptolemys system to explain the motions of the planets

``````a) had the planets orbiting earth.

-->  b) had the planets orbiting earth but also traveling in a small circle.

c) had the planets orbiting the sun.

d) had the planets orbiting the sun also with a retrograde motion.
``````

keplers solar system model

``````a) has the sun in the center.

b) has the planets moving in elliptical orbits.

c) matches observations perfectly.

-->  d) all of the above
``````

copernicus proposed that the planets orbit the sun, the heliocentric model.

``````-->  a) true

b) false
``````

through the first-ever telescope, galileo discovered

``````a) the elliptical orbits of the planets.

b) mercury has phases like the moon.

-->  c) jupiter is orbited by moons.

d) all of these.
``````

## diagram questions

Identify in the picture where the sun never sets

``````a. D

-->  b. S

c. H

d. A
``````

Which letter indicates short days and long nights?

``````-->  a. H

b. A

c. C

d. S
``````

Which label refers to the Arctic Circle?

``````a. U

b. E

c. W

-->  d. N
``````

What is E in this image of Earth poles?

``````a. TROPIC OF CAPRICORN

-->  b. TROPIC OF CANCER

c. EQUATOR

d. ARCTIC CIRCLE
``````

Imaginary line in between the Tropic of Capricorn and Tropic of Cancer

``````-->  a. F

b. A

c. U

d. E
``````

Which label shows the Tropic of Cancer?

``````a. X

-->  b. D

c. M

d. Y
``````

Which letter represents the Tropic of cancer?

``````a. Y

b. J

-->  c. D

d. M
``````

Which letter represents the north pole?

``````a. R

b. E

-->  c. F

d. S
``````

Which label refers to the equator?

``````-->  a. D

b. T

c. A

d. U
``````

Which label shows the equator?

``````a. N

b. A

-->  c. D

d. R
``````

By what letter is the equator represented in the diagram?

``````a. D

b. C

c. U

-->  d. T
``````

Which letter represents the Axis?

``````a. C

b. T

-->  c. D

d. U
``````

What is the name of the dividing line labeled T?

``````a. EQUAL AXIS

b. EQUESTRIAN

-->  c. EQUATOR

d. EQUILATERAL
``````

Where is the equator?

``````a. R

b. C

-->  c. T

d. D
``````

What lines cross the Equator?

``````a. Tropic of Cancer

-->  b. Magnetic Meridian, Geographical Meridian

c. South Pole

d. Tropic of Capricorn
``````

Which meridian runs directly through the south pole?

``````-->  a. geographical meridian

b. magnetic meridian

c. tropic of cancer

d. equator
``````

What lines crosses the equator?

``````-->  a. magnetic meridian, geographical meridian

b. north pole

c. tropic of equator

d. tropic of cancer
``````

How many poles has the Earth?

``````a. 4

b. 1

-->  c. 2

d. 3
``````

Which does not describe the day?

``````a. sun never sets

-->  b. sun never rises

c. long days

d. short days
``````

How many kinds of day length do we have based on the Earth's rotation axis?

``````a. 4

b. 6

-->  c. 5

d. 3
``````

How many magnetic poles does the Earth have?

``````a. 3

b. 4

-->  c. 2

d. 1
``````

In this diagram of planet earth, when the north pole is pointing towards the sun, where is the south pole facing?

``````a. Towards long days

b. Towards the equator

c. Towards the equal days and nights

-->  d. Away from the sun
``````

Where does the sun never set?

``````a. Close to the South axis

-->  b. Close to the North axis

c. Middle of the Earth

d. Nowhere
``````

How many poles does the earth have?

``````-->  a. 2

b. 4

c. 1

d. 3
``````

The earth's axis cuts through which of the following?

``````a. The equator and observer

b. The equator and the south pole

c. The north pole and the equator

-->  d. The north pole and south pole
``````

What is positioned in the middle of the earth?

``````a. south pole

b. north pole

c. observer

-->  d. equator
``````

What usually refers to an imaginary line on the Earth's surface equidistant from the North Pole and South Pole, dividing the Earth into the Northern Hemisphere and Southern Hemisphere?

``````-->  a. Equator

b. South Pole

c. Observer

d. North Pole
``````

What is the line in between the Tropic of Cancer and the Tropic of Capricorn?

``````-->  a. Equator

b. Antarctic Circle

c. Tropic of Cancer

d. Arctic Circle
``````

What is the imaginary line that crosses the Earth right in the middle?

``````a. Tropic of Capricorn

b. Tropic of Cancer

-->  c. Equator

d. Artic Circle
``````

What is the northern most circle of latitude?

``````a. Tropic of Capricorn

b. Equator

-->  c. Arctic Circle

d. Tropic of Cancer
``````

What imaginary line lies between the equator and the Antarctic circle?

``````a. Meridian

b. Tropic of Cancer

-->  c. Tropic of Capricorn

d. Arctic Circle
``````

What is the Southernmost part of the Earth?

``````-->  a. Antarctic Circle

b. Arctic Circle

c. Tropic of Cancer

d. Equator
``````

How many imaginary lines are there on Earth?

``````a. 3

b. 2

c. 7

-->  d. 5
``````

If the arctic circle faces the highest amount of sunlight, when would it face the least amount of sunlight?

``````-->  a. After 6 months

b. When the antarctic circle faces least sunlight

c. At night

d. after one year
``````

Which line represents the equator?

``````a. Top line

b. 4th line

c. Bottom line

-->  d. Middle line
``````

What do you call the imaginary line between the Tropic of Cancer and the Tropic of Capricorn?

``````a. Tropic of Capricorn

b. Arctic Circle

c. Antarctic Circle

-->  d. Equator
``````

Which continent is the closest to the Equator?

``````a. Europe

-->  b. South America

c. Oceania

d. North America
``````

New York City is between which parallel latitudes?

``````a. Tropic of Capricon and Antarctic Circle

-->  b. Arctic Circle and Tropic of Cancer

c. Equator and Tropic of Capricon

d. Tropic of Cancer and Equator
``````

It refers to an imaginary line on the Earth's surface equidistant from the North Pole and South Pole, dividing the Earth into the Northern Hemisphere and Southern Hemisphere.

``````-->  a. Equator

b. Tropic of Cancer

c. Tropic of Capricorn

d. Arctic Circle
``````

How many parts are between the north and south pole?

``````a. 3

b. 6

-->  c. 5

d. 4
``````

How many points of latitude are at 90 degrees?

``````a. 1

-->  b. 2

c. 4

d. 11
``````

In this diagram of planet earth, what is located just below the equator?

``````a. Aortic circle

b. North pole

-->  c. Tropic of capricorn

d. Tropic of cancer
``````

At what longitude is the tropic of cancer?

``````a. 66.5 degrees North

-->  b. 23.5 degrees North

c. 23.5 degrees South

d. 66.5 degrees South
``````

How many degrees is the arctic circle located at?

``````-->  a. 66.5 N

b. 65 W

c. 55 W

d. 67S
``````

How many degrees north is the Arctic Circle?

``````a. 55.5

b. 90

-->  c. 66.5

d. 25.5
``````

How many divisions does the earth have in this diagram?

``````a. 1

b. 2

c. 4

-->  d. 5
``````

Which parallel lies at 23.5 degrees N?

``````a. Equator

b. Arctic Circle

c. Tropic of Capricorn

-->  d. Tropic of Cancer
``````

What is the name of the imaginary line that divides the earth?

``````-->  a. Equator

b. Northern hemisphere

c. South Pole

d. North Pole
``````

What lies between northern and southern hemisphere?

``````a. axis

-->  b. equator

c. south pole

d. north pole
``````

How many hemispheres are there?

``````a. 3

b. 1

-->  c. 2

d. 4
``````

In this diagram of planet earth, which hemisphere is the south pole located in?

``````a. Equator

b. Northern Hemisphere

-->  c. Southern Hemisphere

d. North Pole
``````

What is the center of earth called?

``````a. northern hemisphere

b. southern hemisphere

c. eastern hemisphere

-->  d. equator
``````

At what longitude does the Tropic of Cancer lie?

``````-->  a. 23.5 degrees

b. 66.5 degrees

c. 30 degrees

d. 0 degrees
``````

Identify the arctic circle

``````-->  a. N

b. W

c. E

d. F
``````