Science, Technology, and Society LET REVIEWER
General Education
Lesson 21: Earth’s Moon and Sun
What is the Moon?
The Moon, also called Luna, is
Earth’s only natural satellite. It is a ball of rock that travels around our
planet and is the brightest object in our night sky. The Moon is about one
quarter (¼) the size of Earth, and its gravity is only about one sixth (¹/₆) as strong, so you would feel
much lighter there. The Moon has almost no air, no liquid water, and no
weather, so its surface is dry, dusty, and covered with craters made by space
rocks long ago.
- Earth’s only
natural satellite
- About ¼
Earth’s size and ¹/₆ Earth’s gravity
- Rocky,
airless, dusty surface with many craters
Inside, the Moon has layers similar to Earth’s. It has a
small metal core at the center, a thick rocky mantle around
that, and a solid crust on the outside. The light‑colored
highlands are older, rough areas, and the darker patches, called maria,
are huge plains of frozen lava from ancient volcanic activity. Because there is
no wind or rain, footprints and tracks can stay on the surface for millions of
years.
- Interior
with core, mantle, crust
- Highlands (bright,
rough) and maria (dark lava plains)
- No
weather, so footprints last a very long time
The
Moon is a small, rocky world that orbits Earth, with weak gravity and a dusty
surface full of craters.
Moons rotation, revolution, and tidal lock
Image from Nasa Science
The Moon moves in two main ways at the same time. First,
it rotates, meaning it spins around an invisible line through its
center called its axis. Second, it revolves, meaning it
travels around Earth along an orbit. The Moon takes about 27⅓
days to complete one rotation and about 27⅓ days to
complete one revolution around Earth.
- Rotation:
Moon spins on its axis
- Revolution:
Moon orbits Earth
- Both
take about 27⅓ days
Because the rotation time and revolution time are almost
exactly the same, the same side of the Moon always faces Earth. This is
called tidal lock or synchronous rotation. We
always see the “near side” of the Moon from Earth, while the “far side” stays
turned away, even though it also gets sunlight.
- Tidal
lock = same side of Moon always faces Earth
- We
see the near side, not the far side
- Far side also has day and night
The
Moon spins and orbits at matching speeds, so the same face is always turned
toward us.
Lunar month and the phases of the Moon
Image from ThoughtCo
As the Moon orbits Earth, the Sun always lights up half of
the Moon, but from Earth we see different amounts of that lit half. This
changing view is what we call the phases of the Moon. It takes
about 29.5 days for the Moon to go from one new Moon back
to the next new Moon; this is called a lunar month or synodic
month.
- Moon’s
phases come from seeing different amounts of its lit half
- Lunar
month ≈ 29.5 days (new Moon to new
Moon)
- Called
the synodic month
The lunar month is longer than the 27⅓‑day orbit because
Earth is also moving around the Sun. While the Moon goes around Earth, Earth
moves along its own path, so the Moon must travel a little farther to reach the
same alignment with Earth and Sun again.
- Earth’s
motion around the Sun makes the phase cycle longer
- Orbit
= 27⅓ days, phase cycle = 29.5 days
- Extra motion needed to line up the same way again
The
Moon’s shape seems to change because we see different parts of its sunlit side,
and the full pattern repeats every 29.5 days.
The 8 main phases of the Moon
Image from Time and Date
During a lunar month, the Moon goes through eight
main phases in a fixed order. When the bright part is growing, we say
the Moon is waxing; when it is shrinking, the Moon is waning.
- Waxing =
lit part grows
- Waning =
lit part shrinks
- 8
main phases in a repeating cycle
The cycle begins with the new Moon. At this
time the Moon is between Earth and the Sun, and the sunlit side faces away from
us, so we cannot see the Moon in the night sky. As the Moon moves along its
orbit, a thin slice of light appears; this is the waxing crescent,
where about one quarter of the Moon’s face looks lit. When half of the Moon’s
disk is lit, we see the first quarter phase, which looks like
a “half Moon” even though the Moon is one quarter of the way around its orbit.
- New
Moon: Moon between Sun and Earth, dark to us
- Waxing
crescent: thin, increasing slice
of light (about ¼)
- First
quarter: half the face lit, Moon
¼ around its orbit
After first quarter, more than half of the Moon becomes
bright and we see the waxing gibbous phase, where about three
quarters is lit. When Earth lies between the Sun and Moon, we see the full
Moon, with the entire sunlit half facing us. Then the lit part begins to
shrink into waning gibbous, where it is still more than half but
slowly decreasing. Later we reach the last (third) quarter, where
the opposite half of the Moon is lit, and finally the waning crescent,
a thin decreasing curve of light, before returning to new Moon.
- Waxing
gibbous: more than half lit, still growing
- Full
Moon: whole sunlit side visible
- Waning
gibbous → last/third quarter → waning
crescent back to new
The
Moon goes from new, to crescent, to half, to gibbous, to full, then back again
in reverse as the bright part shrinks.
Eclipses
Image from Britannica
Sometimes the Sun, Earth, and Moon line up in a straight
line and their shadows create eclipses. In a lunar eclipse,
Earth moves between the Sun and the Moon, so Earth’s shadow falls on the
Moon. The order in space is Sun – Earth – Moon (S–E–M), and
this can only happen at full Moon. The Moon may look dark red
during a total lunar eclipse because some sunlight bends through Earth’s
atmosphere and reaches the Moon.
- Lunar
eclipse: Earth between Sun and Moon (S–E–M)
- Happens
only at full Moon
- Moon
passes through Earth’s shado
In a solar eclipse, the Moon passes between the
Sun and Earth, so the Moon’s shadow falls on Earth. The order
is Sun – Moon – Earth (S–M–E), and this can only happen at new
Moon. People in the narrow path of the darkest shadow can see the Sun
partly or almost completely covered for a short time, but they must protect
their eyes.
- Solar
eclipse: Moon between Sun and Earth (S–M–E)
- Happens
only at new Moon
- Moon’s
shadow makes the Sun look partly or fully covered
In
a lunar eclipse Earth blocks the sunlight from reaching the Moon; in a solar
eclipse the Moon blocks sunlight from reaching part of Earth.
Tides, spring tides, and neap tides
Image from Britannica
The Moon’s gravity pulls on Earth’s oceans and causes tides,
the regular rise and fall of sea level. When the Moon and Sun pull in the same
line, during new Moon and full Moon, their forces combine to make spring
tides. These tides have especially high high tides and low
low tides.
- Tides
are caused mainly by the Moon’s gravity
- Spring
tides during new and full
Moon
- Spring
tides give highest highs and lowest lows
When the Sun and Moon pull at right angles to
each other, during first quarter and last (third) quarter, their forces partly
cancel. This produces neap tides, which have lower high
tides and higher low tides, so the difference between high
and low water is smaller.
- Neap
tides during first and last/third
quarter
- Smaller
difference between high and low tides
- High
tides are not as high; low tides not as low
Spring
tides are extra‑strong tides when Sun and Moon line up; neap tides are weaker
tides when they pull at right angles.
Perigee, apogee, and “supermoons”
Image from Angelrls
The Moon’s orbit is not a perfect circle but an ellipse,
so its distance from Earth changes. When the Moon is at the point where it
is closest to Earth, this point is called perigee. A
full Moon near perigee appears slightly larger and brighter in the sky and is
often called a supermoon.
- Moon’s
orbit is elliptical (oval)
- Perigee =
Moon closest to Earth
- Full
Moon near perigee → supermoon (looks a bit bigger)
When the Moon is at the point where it is farthest
from Earth, we call that apogee. A full Moon near apogee looks
a little smaller and dimmer and is sometimes called a micromoon.
- Apogee =
Moon farthest from Earth
- Full
Moon near apogee → micromoon (looks a bit smaller)
The
Moon’s distance changes slightly; closer full Moons look larger (supermoon),
and farther full Moons look smaller (micromoon).
Apollo 11
Image from Syracuse
Humans have sent many spacecraft to the Moon, but the most
famous mission is Apollo 11, the first successful crewed landing.
Three astronauts traveled from Earth to lunar orbit. Neil Armstrong became
the first person to step onto the Moon’s surface, followed by Buzz
Aldrin as the second, while Michael Collins stayed in
the command module orbiting above. They collected rocks, set up experiments,
and then returned safely to Earth, showing that people could travel to another
world and come back.
- Apollo
11 = first crewed Moon landing
- Neil
Armstrong and Buzz Aldrin walked
on the Moon
- Michael
Collins remained in orbit in the command
module
Apollo
11 proved that humans could land on the Moon, explore it, and safely return
home.
What is the Sun?
The Sun is the star at the center of our
solar system and the main source of light and heat for Earth. It is a medium‑sized
star made mostly of the gases hydrogen and helium.
The Sun looks yellow from Earth because our atmosphere scatters blue light, but
from space it is closer to white. Its enormous mass gives it strong gravity
that holds Earth and the other planets in their orbits.
- Star at
the center of the solar system
- Made
mostly of hydrogen and helium
- Gravity
keeps planets in orbit
Deep inside the Sun, in the core, the
temperature and pressure are extremely high. Here, hydrogen nuclei are squeezed
together so tightly that they fuse to form helium. This
process is called nuclear fusion, and it releases huge amounts of
energy as light and heat. That energy travels outward and then across space to
reach Earth, where it warms the surface, drives weather, and supports life
through photosynthesis in plants.
- Energy
comes from nuclear fusion in the core
- H
+ H → He + energy (hydrogen to helium)
- Sunlight
warms Earth and makes life possible
The
Sun is a giant ball of hot gas that shines because hydrogen atoms fuse into
helium, releasing energy.
Earth’s orbit: perihelion and aphelion
Image from INTOSAI Community Portal
Earth travels around the Sun in an elliptical orbit,
not a perfect circle. Because of this, Earth is not always the same distance
from the Sun. When Earth is at the point where it is closest to the Sun,
that point is called perihelion. When Earth is at the point where
it is farthest from the Sun, we call it aphelion. The
change in distance is real but small compared to the total distance, so it does
not cause the seasons; the seasons are mainly caused by the tilt of Earth’s
axis.
- Earth’s
orbit is elliptical
- Perihelion =
Earth closest to the Sun
- Aphelion =
Earth farthest from the Sun
- Distance
change is not the main cause of seasons
Earth
is a bit closer to the Sun at perihelion and a bit farther at aphelion, but the
tilt of Earth’s axis is what truly causes the seasons.
Earth's Moon and Sun Quiz
