Lecture 3: Phases of the Moon, Lecture notes of Astronomy

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Phases

of the Moon

LECTURE 3

I. THE MOON MOON CYCLES Different phases of the Moon as it orbit the Earth. Moon between Earth and Sun. Moon appears completely dark. NEW MOON WAXING CRESCENT MOON WANING CRESCENT MOON FIRST QUARTER HALF MOON WAXING GIBBOS MOON FULL MOON WANING GIBBOS MOON LAST QUARTER HALF MOON A partial cresent-shaped of the Moon starts to become visible as it moves away from the Sun. Light crescent on the left side is visible. The left half side of moon’s surface is illuminated. More than half of the Moon on theleft side is visible, not yet full. The entire round circle of the Moon is illuminated by the Sun and fully visible from Earth. Moon’s bright part that is visible starts to shrink. More than half is visible but decreasing. Only half of the Moon is visible but decreasing. Only a small part of the Moon is visible. Illuminated shape gets smaller and smaller until it’s gone and back to New Moon. This Moon cycle only happens every 29.5 days.

Approximately 2,500 years ago, the Greeks discovered

Earth as a sphere and cannot be flat because ships were

getting smaller and “sinking” as it gets farther from land

(this cannot happen of Earth is flat).

II. IMPORTANT 150 BC A Greek astronomer Found interesting things about the orbits of the Sun and Moon around Earth In his perspective, Earth is a sphere, located at the center of the universe where other planets/stars (Sun, Jupiter, Mars, Moon, Saturn, etc) revolves around Earth. Also discovered something about Earth’s axis: => Position of North Star is changing => Precession of Earth’s axis ( Axial Precession ): position of stars shifts slowly overtime due Earth’s axis not perfectly upright (Ex. Gyroscope). => The Earth’s axis precession takes roughly 26,000 years to complete a full cycle. H I P P A R C H U S : ASTRONOMERS

Greek astronomer Measured Earth’s diameter around 200 BC He estimated Earth’s size by measuring the angle of Sun’s rays at two different locations. After, he created a map that divided Earth’s size into degrees of latitude and longitude to understand how apart different places are on Earth. Helped contribute to the study of how Earth correspond in the universe and modern geography. II. IMPORTANT ASTRONOMERS E R A T O S T H E N E S : P T O L E M Y : Around 140 AD Created the Geocentric Model where the universe is seen as a set of epicycles (multiple transparent spheres) where the Earth is at the epicenter while the Sun and other planets revolve around it. Retrograde Motion: when planets orbit around Earth switch positions.