Origin of modern astronomy Earth Science Ch. 22 Origin of modern astronomy

22.1 Early Astronomy Ancient Greek Contributions Aristotle Concluded the Earth is round because it always casts a curved shadow on the moon Eratosthenes First successful attempt to establish the size of the Earth by observing the angles of the sun Aristarchus Calculated the distances from the Earth to the sun and moon First to provide evidence that the sun was the center of the universe and not the Earth

Ptolemy Stated that the planets move around the earth in circular orbits. Noticed that the planets seemed to at times move backwards which he called retrograde motion. Stated that planets move in small circles while in orbit called epicycles.

Geocentric Model of the Universe The ancient Greeks believed in a geocentric model of the universe According to this model the Earth is at the center of the universe with the sun, moon, and all the planets revolving around, or orbiting, the Earth Beyond the known planets was a transparent hollow sphere that contained all of the stars. This sphere was called a celestial sphere. Heliocentric Model of the Universe In the heliocentric model, the sun is at the center and the Earth and other planets orbit the sun.

The Birth of Modern Astronomy Copernicus Concluded that the Earth is a planet Proposed a model of the solar system where the sun is at the center Used circles as the paths of orbits but found that seemed to stray away from the predicted paths Brahe Measured positions of the moon, planets, and stars Observations were far more precise than any made previously

Kepler Discovered the 3 laws of planetary motion The path of each planet around the sun is an ellipse Each planets sweeps over equal areas in equal time intervals A planet travels more rapidly when it is near the sun and more slowly when it is farther from the sun

1st Law 2nd Law

(Orbital period)2 = (mean distance from the sun)3 Orbital period is measured in earth years and is the time it takes for a planet to travel around the sun The distance is measured in astronomical units (AU). 1 AU is the average distance between the earth and the sun T2 = d3

Galileo Most important discoveries were descriptions of the behavior of moving objects by using telescopes Discovered that there were 4 moons that orbit Jupiter which proved that the Earth was not the only center of motion Showed that planets are circular disks which showed that the planets were not stars

Discovered that Venus has phases which means that it must orbit the sun Discovered that the surface of the moon was not smooth which proved that objects in the sky were not smooth Discovered that the sun had sunspots which showed that the sun was not perfect.

Newton Was the first to formulate and test the law of universal gravitation Described the force that keeps a planet in orbit around the sun Gravitational force decreases with distance

The greater the mass of an object the greater the gravitational force Proved that the force of gravity combined with the concept of inertia results in elliptical orbits Force = (GM1M2)/r2 G (gravity), M (mass), r (distance)

22.2 The Earth-Moon-Sun System Motions of the Earth Rotation Spinning of the earth on its axis Causes days and nights Causes the apparent change of the location of stars An earth day can be measured in 2 ways: Solar : with respect to the sun (24 hours) Sidereal: with respect to the stars (23 hours, 56 minutes)

Why is there a difference? As the earth is rotating, it is also revolving. From our perspective, the stars revolve around the earth in only 23 hours and 56 minutes. A star rises and sets about 4 minutes earlier each day.

Revolution Movement along an orbit The earth has an elliptical orbit around the sun Perihelion: Earth is closest to the sun Aphelion: Earth is farthest from the sun

Earths axis of rotation is tilted about 23.5° The tilt of the earth’s axis is responsible for the seasons Precession Very slow motion of the earth where the earth’s axis points in a different location (26,000 years)

Motions of the Earth-Moon System It takes about one month for the moon to orbit the earth The orbit of the moon around the earth is elliptical Perigee: moon is closest to the earth Apogee: moon is farthest from the earth

Phases of the moon Crescent phase: less than half of the moon is visible Gibbous phase: more than half the moon is visible Waxing phase: the lighted portion of the moon increases Waning: the lighted portion of the moon decreases Lunar phases are caused by changes in how much of the sunlit side of the moon faces the earth.

The cycle from new moon to full moon and back takes 29. 5 days The cycle from new moon to full moon and back takes 29.5 days. This is called a synodic month. The true period of the moon is only 27.3 days. This is called a sidereal month. The reason for the difference is because as the moon is revolving around the earth, the moon-earth system is revolving around the sun.

The same side of the moon always faces the earth because it takes the moon the same amount of time to rotate as it does to revolve. Due to the rotation speed of the moon, it experiences 2 weeks of daylight followed by 2 weeks of darkness. Due to the slow rotation of the moon, the temperature differences are extreme.

A solar eclipse occurs when the moon is directly between the earth and sun. Only occurs during new moon phases A lunar eclipse occurs when the earth is directly between the moon and sun. Only occurs during full moon phases The reason why solar and lunar eclipses do not occur during every full and new moon is because the orbit of the moon is inclined 5° The usual number of eclipses each year is about 4 because only 4 times a year are the earth, moon, and sun in a straight path.

These locations view a total solar eclipse Penumbra Parts of a solar eclipse Umbra Location on earth during a solar eclipse completely in the shadow of the moon These locations view a total solar eclipse Penumbra Locations on earth in the light part of the shadow of the moon These locations view a partial eclipse

22.3 Earth’s Moon Facts about the moon The earth’s only natural satellite Believed to be about the same age as the Earth When compared to other moons, Earth’s moon is large when compared to the size of Earth. Other planets moons are much smaller when compared to their planet. The density of the moon is much less than that of Earth

The Lunar surface The moon has dark lowlands and bright highlands The dark regions of the moon are called maria There is no water or atmosphere on the moon Erosion does not occur on the moon because there are no erosional forces The only erosion that does occur on the moon is caused by meteoroids. Craters are found on the moon which are large depressions caused by meteoroids.

By contrast, the earth does not have many craters because most are burned up in the earth’s atmosphere The highlands of the moon are composed of mountainous areas. It is believed that the mountains were formed by asteroid collisions The dark smooth regions of the moon’s surface are called maria The maria regions are composed of basaltic lava Rilles are regions associated with maria. They are the remnants of ancient lava flows.

The upper most layer of the moon is composed of regolith. Regolith covers all lunar terrains Regolith is composed of igneous rocks, beads, and fine dust.

Lunar history The most widely accepted model for the formation of the moon is that when the solar system was forming a body the size of Mars impacted the earth. Material that was ejected into space entered into an orbit of earth where it combined to form the moon.

It is believed that the molten material that is found on the surface came from impacts and radioactive decay. Maria formed from lava flows The craters on the moon formed by impacts with space debris and meteoroids.