Guiding Questions Why does the Moon go through phases? Is there such a thing as the “dark side of the Moon”? What is the difference between a lunar eclipse and a solar eclipse? How often do lunar eclipses happen? When one is taking place, where do you have to be to see it? How often do solar eclipses happen? Why are they visible only from certain special locations on Earth? How did ancient astronomers deduce the sizes of the Earth, the Moon, and the Sun?

Phases of the Moon new moon waxing Crescent first quarter waxing gibbous full waning gibbous third quarter waning crescent

The phases of the Moon are caused by its orbital motion and repeat every 29½ days.

The phases of the Moon are caused by its orbital motion and repeat every 29½ days.

Although the Moon will orbit Earth completely in 27 Although the Moon will orbit Earth completely in 27.32 days (sidereal month), it takes a 29 ½ days (synodic month) to become a new moon again because Earth moves around the Sun.

The Moon’s rotation always keeps the same face toward the Earth.

Synchronous Rotation: the moon takes exactly the same amount of time to orbit Earth as it does to spin on its lunar axis. Subsequently, there is no permanent “dark side” of the Moon, each location experiences about 14 days of daylight and 14 days of night.

Solar Eclipse When the new moon blocks the view of the Sun as seen from a particular location on Earth for several minutes.

Solar eclipses can be either total, partial, or annular, depending on the alignment of the Sun, Earth, and Moon. Total eclipse: where the Sun appears to be completely covered by the Moon. Partial eclipse: where part of the Sun appears to be covered by the Moon. Annular eclipse: where the Moon is too close in its orbit around Earth to completely cover the Sun.

Lunar Eclipse When Earth obscures most of the sunlight from illuminating the full moon for several hours.

Lunar eclipses can be either total, partial, or annular, depending on the alignment of the Sun, Earth, and Moon.

Eclipses occur only when the Sun and Moon are both on the “line of nodes.” Where the Moon’s orbital plane intersects with Earth’s orbital plane.

Lunar eclipses can be either total, partial, or penumbral, depending on the alignment of the Sun, Earth, and Moon

Ancient astronomers measured the size of the Earth Around 200 BC, Greek astronomer Eratosthenes used the observation that the Sun is 7º south of the zenith in Alexandria on the same day it was directly overhead in Syene (near Aswan). From this he deduced that Alexandria must be 7/360th of the way around a spherical Earth. Knowing that these two cities are separated by about 800 km, he calculated the Earth to have a circumference of about 40,000 km which is quite close to the actual size.

Ancient astronomers attempted to determine distances to the Sun and Moon Around 280 BC, Greek astronomer Aristarchus used the fact that the Sun, Moon, and Earth form a right triangle at first quarter and the Pythagorean trigonometric rules to estimate the distance to the Sun relative to the Earth-Moon distance.

Estimating the size of the Sun Aristarchus pointed out that if the Sun and Moon appear nearly the same size in the sky, then their actual diameters must be in the same proportion as their distances. e.g., If the Sun is 400 times farther than the Moon, then the Sun must be 400 times bigger.

Guiding Questions Why does the Moon go through phases? Is there such a thing as the “dark side of the Moon”? What is the difference between a lunar eclipse and a solar eclipse? How often do lunar eclipses happen? When one is taking place, where do you have to be to see it? How often do solar eclipses happen? Why are they visible only from certain special locations on Earth? How did ancient astronomers deduce the sizes of the Earth, the Moon, and the Sun?