Completed ePortfolio Assignment

Sample assignment

ePortfolio for Chapter 2 Observing the Sky: The Birth of Astronomy

What questions do I have about this topic?

How much did people in the past know about the sky? How did we figure out the Earth is round?

2.1 The Sky Above

What are the main features of the celestial sphere?

The celestial sphere is an imaginary sphere that surrounds Earth. Objects on the sky can be denoted by their locations on this sphere. The main features of the celestial sphere are the north celestial pole (NCP), the south celestial pole (SCP), the celestial equator and the ecliptic.

What is the system astronomers use to describe the sky?

Astronomers use the conceptual model of the celestial sphere.

How do the stars appear to move as seen from Earth? The motion of the stars will depend on your latitude. From the equator, stars appear to rise in the east and set in the west. From the north/south pole, stars appear to rotate counterclockwise/clockwise around the NCP/SCP. For an observer at either of the poles, all stars are circumpolar (never setting). As an observer moves south or north of the poles to middle latitudes the number of circumpolar stars decreases until observer reaches the equator where there are no circumpolar stars.

How do the Sun, Moon, and planets appear to move as seen from Earth?

They appear to rise in the east and set in the west.

What is the modern meaning of the term constellation?

A constellation is defined as one of the 88 sectors that the sky is divided into.

Section 2.2 Ancient Astronomy

What are some early examples of astronomy around the world?

Ancient Babylonian, Assyrian, and Egyptian astronomers knew the approximate length of the year. The Chinese also had a working calendar; they determined the length of the year at about the same time as the Egyptians. The Chinese also recorded comets, bright meteors, and dark spots on the Sun. (Many types of astronomical objects were introduced in Science and the Universe: A Brief Tour.

The Mayan culture in Mexico and Central America developed a sophisticated calendar based on the planet Venus, and they made astronomical observations from sites dedicated to this purpose a thousand years ago. The Polynesians learned to navigate by the stars over hundreds of kilometers of ocean.

In Britain, before the widespread use of writing, ancient people used stones to keep track of the motions of the Sun and Moon. We still find some of the great stone circles they built for this purpose, dating from as far back as 2800 BCE.

How did Greek astronomers deduce that Earth is spherical?

Aristotle used two arguments to deduce 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. Only a spherical object always produces a round shadow. His second argument was 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 decreases as a traveler moves south. On a flat Earth, everyone would see the same stars overhead. The only possible explanation is that the traveler must have moved over a curved surface on Earth, showing stars from a different angle.

How did Greek astronomers calculate Earth’s size?

Eratosthenes used a geometric method to determine Earth’s size. He knew that the Sun is so distant from us that all the light rays that strike our planet approach us along essentially parallel lines. The Sun is not infinitely far away, but light rays striking Earth from a point on the Sun diverge from one another by an angle far too small to be observed with the unaided eye.

Eratosthenes was told that on the first day of summer at Syene, Egypt, sunlight struck the bottom of a vertical well at noon. This indicated that the Sun was directly over the well—meaning that Syene was on a direct line from the center of Earth to the Sun. At the corresponding time and date in Alexandria, Eratosthenes observed the shadow a column made and saw that the Sun was not directly overhead, but was slightly south of the zenith, so that its rays made an angle with the vertical equal to about 1/50 of a circle (7°). Because the Sun’s rays striking the two cities are parallel to one another, therefore the two rays make the same angle with Earth’s surface. Eratosthenes reasoned that the curvature of the round Earth meant that “straight up” was not the same in the two cities. And the measurement of the angle in Alexandria, he realized, allowed him to figure out the size of Earth.

What is the motion of Earth called precession?

Precession is the slow (26,000 year) wobble of Earth due to the gravitational tug of the Sun and Moon.

What was Ptolemy’s geocentric system of planetary motion?

Ptolemy had an Earth-centered system were the planets orbited with “epicycles”, small circles within their orbits that attempted to explain apparent retrograde motion.

2.4 The Birth of Modern Astronomy

How did Copernicus develop the heliocentric model of the solar system?

Copernicus developed the heliocentric model of the universe by beginning with several assumptions that were common in his time, such as the idea that the motions of the heavenly bodies must be made up of combinations of uniform circular motions. Unlike many of his contemporaries, he did not assume that Earth had to be in the center of the universe, and he presented a defense of the heliocentric system that was elegant and persuasive.

What is the Copernican model of planetary motion what evidence or arguments are there in favor of it?

The Copernican model places the Sun at the center of the universe (solar system) and the planets orbiting in perfect circles, and in the correct order in distance from the Sun. The Copernican model also explains retrograde motions of the planets without epicycles.

What were Galileo’s discoveries concerning the study of motion and forces?

Galileo’s experiments showed that an object at rest stays at rest unless acted on by an outside force. Galileo also studied acceleration, and the acceleration due to gravity.

How did Galileo’s discoveries tilt the balance of evidence in favor of the Copernican model?

Galileo observed four satellites (moons) orbiting around the planet Jupiter. This indicated to him that not all bodies orbit the Earth, as proposed by the geocentric model. Galileo also observed phases of Venus, something that would not be possible in a geocentric system.

My own questions

At the beginning of the assignment I was curious about the how ancient people used astronomy and how humans were able to measure the size of Earth. The material in section 2.2 answered my questions, although I am enthusiastic about learning more for myself about Greek astronomy.