What is astronomy? Astronomy is the science that studies matter in outer space, especially the positions, motion, and composition. What is astrology? Astrology is the study of the movements and relative positions of celestial bodies interpreted as having an influence on human affairs. Until the ancient Greeks, astronomy was the same as astrology.

Early History of Astronomy AstronomerDatesNationalityAchievements/Ideas/Models

Babylonian Astronomers 1200 BC - 60 BC Compiled star catalogs First to divide circle into 360 degrees Made calculations of daylength changes, planet motions and lunar eclipses

Chinese Astronomers 600 BC onward Compiled star catalogs Observed and predicted comets and eclipses First to record a supernova

Greek Astronomy 4 th century BC - Greeks treated astronomy as a branch of mathematics and developed geometric models to explain the motion of the known planets—Mercury, Venus, Mars and Jupiter. Some Greek astronomers believed that the sun, moon, and the known planets followed a geocentric (earth center) model, others a heliocentric (sun center) model.

One of the objections to the heliocentric model was that, if the Earth was moving, why didn’t it leave the air and the moon behind?

Aristotle Greek mathematician and philosopher 384 BC – 322 BC He said that the universe was a system of 55 spheres with Earth at the center

Aristarchus of Stamos Greek astronomer and mathematician 310 BC BC First heliocentric model of the solar system

Eratosthenes Greek mathematician 276 BC – 195 BC Calculated the circumference of Earth Also measured the tilt of Earth’s axis

Eratosthenes (cont.) Measuring the sun’s angle at noon on the solstice at Alexandria and knowing that the sun’s angle at Syene was 0°, he calculated that this angle as 1/50 th of a circle (Greeks knew Earth was a sphere). Knowing the distance between Alexandria and Syene, he multiplied by 50 to get the Earth’s circumference!

Hipparchus of Greece c. 190 BC – c. 120 BC Mathematician Discovered the precession of Earth’s axis He calculated the distance to the moon and a method for predicting solar eclipses

Ptolemy Greek mathematician, astronomer c. AD 90 to c. AD 168 Using Aristotle’s model, he developed an epicycle model of a geocentric solar system.

Ptolemy’s Model Geocentric (earth center) Each planet moved in a small circle (epicycle) around the Earth

Retrograde Motion retrograde motion of a planet, when it appears to move backward. This apparent motion is caused by the different orbital speeds of Earth and the other planet.

Let’s take a break: Turn to your neighbor on your left or your right and answer these questions. Be prepared to share your answers. What is the difference between astrology and astronomy? Why does Mars appear to have retrograde motion?

Western Europe in the 4 th through 12 th centuries These were the Dark Ages when scientific studies declined. By the 10 th Century) the Arabic world had knowledge about astronomy. Western European scholars traveled to Spain and Sicily and translated the writings.

Aristotle’s Model Aristotle’s and Ptolemy’s model of the universe was reintroduced to scholars. 1. The stationary Earth is at the center of the universe. 2. The planets and other celestial bodies travel in perfect circles around it. 3. The heavens are made of a perfect, unchanging substance different from substances on Earth.

The Birth of Modern Astronomy In the 16 th Century, scientists began to develop more modern ideas about the universe. They investigated and discovered the natural laws that govern the universe. Scientists who expanded astronomy were Copernicus, Brahe, Kepler, Galileo, and Isaac Newton.

Nicolaus Copernicus Polish astronomer Earth was a planet Circular orbits of planets about the sun Understood that the stars are very, very far away

Tycho Brahe Danish astronomer Made extremely accurate observations about the solar system, esp. Mars Proposed a model where sun and moon orbited Earth but the other planets orbited the sun His observation of a supernova in 1572 gave evidence that the heavens could change

He kept detailed records Convinced that progress in astronomy meant more accurate observations, Brahe designed many sighting instruments like this wall quadrant

Johannes Kepler German astronomer Father of modern astrophysics) Originally an assistant to Brahe Developed 3 laws of planetary motion using observations of Brahe Supported the heliocentric model but with elliptical orbits instead of circles!

Kepler’s Laws of Planetary Motion First Law: The orbit of every planet is an ellipse with the Sun at one focus. An ellipse is an OVAL.

Kepler’s Laws of Planetary Motion Second Law: A line joining a planet and the Sun sweeps out equal areas during equal intervals of time. Planets revolve around the sun at a speed that varies depending on where it is in its orbit.

Kepler’s Laws of Planetary Motion Third Law: There is a proportional relationship between a planet’s orbital period and its distance to the sun. Orbital period is the time it takes to make one full orbit around the sun.

Astronomical Unit A convenient way of measuring distances in the solar system is relative to the Earth-sun distance. An astronomical unit (AU) is the average distance between Earth and the sun; it is about 150 million kilometers.

Galileo Galilei Italian physicist, astronomer, mathematician Father of modern observational astronomy Refined the refracting telescope

Galileo Galilei’s Observations 1. Saw that planets were disks rather than points of light 2. Saw craters on the moon 3. Discovered 4 moons of Jupiter 4. Observed sunspots 5. Saw the Milky Way is made of stars not just nebulous (made of gas)

Galileo observed that the phases of Venus as observed from Earth supported Copernicus’ heliocentric model. At long last, data to disprove one of the two conflicting models, geocentric and heliocentric! Geocentric modelHeliocentric model

Sir Isaac Newton English physicist First to understand that planetary motion is due to gravity Proposed the Universal Law of Gravitation

Let’s fire a cannonball from a cannon that is parallel to the ground (and neglect air drag). What happens if we increase the muzzle speed? Is there a speed that would have the cannonball NOT fall to the ground but stay in orbit? Yes, that speed is the minimum orbital speed at that particular height for the cannonball to achieve orbit around the Earth and stay there.

As it orbits, a planet feels a pull directly toward the sun (blue arrow) while it is travelling along its path. If somehow the sun could be turned off in an instant, the planet would fly off in the direction it had been going (red arrow) at that instant. This means the orbiting planet continually falls towards the Sun but also continually misses! Planetary Motion

The Gravity between two bodies depends on both masses and the distance between them. This means that the more massive an object is and the closer it is to another object, there is more gravity between them. OR The smaller an object is and the further away they are from each other, there is less Gravity between them