How has the amount of daylight we are

How has the amount of daylight we are receiving changed over the last two weeks?

What can you tell me about how the stars move in the sky during an evening?

What is the current phase of the moon? How has this changed over the last three weeks?

What can you tell me about how the stars move in the sky over the period of a year?

Ancient cultures studied the sky and developed calendars based upon the patterns they saw Motion of the sun n Motion of the moon n Motion of the stars Ancient culture based their calendars on repeating cycles they observed in the sky. n

What is a day? What they saw: n The time it takes the sun to cross the sky and return to the same point What we know: n Time required for the Earth to rotate once on its axis

What is a month? What they saw: n The amount of time required for the moon to move through a complete set of phases What we know: n The amount of time it takes the moon to revolve around the Earth

What is a year? What they saw n The time it takes for a group of stars (constellation) to return to the same part of the sky at a certain time of day What we know: n Time required for the Earth to revolve around the sun.

Early Observers The Beginnings of Astronomy Scientists have found evidence for ancient astronomical activities from all over the world.

Early Humans recognized the cycles of celestial objects in the sky

Nabta n n Southern Egypt – Some stones are positioned such that they would have lined up with the sun during the summer solstice 6, 000 years ago. The summer solstice occurs on the longest day of the year. (Day with most amount of sunlight)

Stonehenge n n Another ancient site that was probably used to make observations of the sky. Stones are arranged primarily in circles which are aligned with the sunrise during the summer and winter solstices. Built over a period of 1, 500 years. Built for ceremony and ritual.

Babylonians n n n Ancient empire located in present-day Iraq 700 B. C. to A. D. 50, Babylonians precisely tracked the positions of the planets and moon. Skilled at forecasting the movements of these celestial bodies, which enabled them to make an accurate calendar.

Ancient Chinese n n n 1, 000 B. C. could predict eclipses. The Chinese named 800 stars by 350 B. C. The Chinese skillfully tracked and predicted the same motions in the sky as the civilizations that influenced Western astronomy.

Ancient Greeks n n n Learned to observe the sky to keep track of time. Helped make astronomy a true science. Greek philosophers had tried to understand the place of Earth and humans in the universe (Logic & Geometry) Aristotle (Greek philosopher) successfully explained the phases of the moon Correctly stated that Earth was a sphere (an idea that was not popular at the time)

Native Americans n n Skilled in observing the sky Maya – 1, 000 years ago in the Yucatan Had complex systems of mathematics and astronomy. Many buildings were aligned with celestial bodies during certain astronomical events

Ancient Arabs n n n Followed Greeks, Romans, early Christians. Used Greeks’ knowledge of astronomy to develop astronomy into a science while Europe fell into the “Dark Ages” Many stars have Arabic names because of this period. Invented Astrolabe, algebra, modern number system.

Earliest Astronomers Summary n n n The ancient people had no history to learn from. Almost everything they knew about the universe came from what they could discover with their own eyes and minds. They thought that the universe was made up of the sun, moon, planets with all the stars somewhere towards the edge of the universe.

Early Models of the Universe

Pythagoreans (500 B. C. ) Believed the stars, planets, sun, and moon were attached to crystalline spheres which rotated around the earth. n (geocentric) n

Aristotle (350 B. C. ) Believed the Earth is motionless at the center of the universe and all the stars and planets rotated around it. n Stars and planets moved in circular paths. (geocentric) n

Ptolemy (140 A. D. ) n n n Expanded Aristotle’s theories to try to account for “retrograde motion” of the planets (Ptolemaic Theory) Planets traveled in smaller circular paths as they traveled around the Earth (epicycles and deferents) Popular model of universe for 1, 500 years.

Copernicus (1543) Sun at center of universe and the planets orbit the sun (Heliocentric) n Solved the problem of “retrograde motion” n Theory did not immediately replace Ptolemy’s theory. n

Tycho Brahe (1600) n n n Favored an Earth-centered universe different from Ptolemy’s theory Thought that other planets revolved around the sun, and that the sun and moon revolved around the Earth. Theory incorrect, but made many precise observations of planets and stars.

Johannes Kepler (1609) n n Mathematician Tycho’s assistant Used Tycho’s observational data to develop laws of planetary motion Kepler’s 3 Laws of Planetary Motion n Law of Ellipses Law of Equal Areas Law of Periods

Galileo Galilei (1609) First person to use telescope for astronomical observations n Discoveries * Galilean Satellites (Jupiter’s moons) * Craters on the moon * Sunspots on the Sun * Phases of Venus Favored Copernicus’s theory over Ptolemy’s

Isaac Newton n n 1687 Explained why planets orbit the Sun, and why moons orbit planets Newton’s Law of Motion Newton’s Law of Gravitation Completed the work of Copernicus, Tycho, Kepler, and Galileo