Guiding Questions
1. Why does the Moon go through phases?
2. Is there such a thing as the “dark side of the Moon”?
3. What is the difference between a lunar eclipse and a
solar eclipse?
4. How often do lunar eclipses happen? When one is
taking place, where do you have to be to see it?
5. How often do solar eclipses happen? Why are they
visible only from certain special locations on Earth?
6. How did ancient astronomers deduce the sizes of the
Earth, the Moon, and the Sun?
Phases of the Moon
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new moon
waxing Crescent
first quarter
waxing gibbous
full
waning gibbous
third quarter
waning crescent
new moon
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
27.32 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
1. Why does the Moon go through phases?
2. Is there such a thing as the “dark side of the Moon”?
3. What is the difference between a lunar eclipse and a
solar eclipse?
4. How often do lunar eclipses happen? When one is
taking place, where do you have to be to see it?
5. How often do solar eclipses happen? Why are they
visible only from certain special locations on Earth?
6. How did ancient astronomers deduce the sizes of the
Earth, the Moon, and the Sun?