Astronomy: Earth, Moon, and Sun
*The Earth revolves around the Sun.
Earth in Space:
Astronomy – the study of the moon, stars, and other objects in space.
*Earth moves through space (around the Sun) in TWO major ways: Rotation and Revolution
Rotation - refers to the spinning of the Earth on its axis. This movement cause Day
and Night. One complete rotation takes 24 hours.
Revolution – refers to the movement of one object around another. A complete
revolution of Earth around the sun is called a year. This takes 365 days.
Axis – an imaginary line that passes through the Earth’s center from the North and South Poles.
Orbit – refers to a path of movement. The Earth’s orbit is around the sun and is
shaped like a slightly elongated circle or ellipse. (It is almost a perfect circle).
*Note: Planets closer to the sun have a shorter orbit; those farther away have a longer orbit.
Seasons – we have seasons because of the tilt of Earth’s axis (which is at 23.5
degrees) and the way that the Sun’s light comes in contact with Earth’s surface
(direct and indirect light). Seasons are not caused by the Earth’s distance from the Sun.
Solstice – is an event whereby the sun reaches its greatest distance north and
south of the equator twice each year:
June Solstice (the north end of the Earth’s axis is tilted toward the sun).
December Solstice (the south end of Earth’s axis is tilted toward the sun).
Equinox – is an event that occurs twice each year whereby neither end of Earth’s
axis is tilted toward the sun. Equinox means “equal night.” During this time
(March: Vernal (spring) & September: Autumnal (fall) both hemispheres receive
the same amount of light: day and night are each about 12 hours long everywhere
Gravity and Motion:
Force – is a push or a pull.
Gravity – is a force that attracts all objects toward each other. *Note: the
strength of gravity decreases as the distance between two objects increases. For
example, if the distance between two objects were doubled, the force of gravity
between them would decrease to one fourth of its original value.
Law of Universal Gravitation – states that every object in the universe attracts
every other object.
*Note: the strength of the force of gravity between two objects depends on two
factors: the masses of the objects and the distance between them.
Mass – is the amount of matter in an object. Example: The Earth is so massive it
exerts a gravitational force on the moon, large enough to keep the moon in orbit.
Also, the force of gravity decreases rapidly as the distance between two objects
Weight – is the force of gravity on an object. *An objects weight can change
depending on its location.
Inertia – is the tendency of an object to resist a change in motion. *The more
mass an object has, the greater the inertia.
*Note: Mass and Inertia keep the Moon in orbit around the Earth.
Newton’s First Law of Motion – states that an object at rest will stay at rest and
an object in motion will stay in motion with a constant speed and direction unless
acted on by a force.
Phases, Eclipses, and Tides
*Note: The changing relative positions of the Moon, Earth, and Sun cause the phases of the
Moon, eclipses, and tides.
Phases – are the set of different shapes of the Moon as it completes one revolution around
Earth. The phase of the Moon we see depends on how much of the sunlit side of the Moon faces Earth.
Solar Eclipse -
Lunar Eclipse -
Spring Tide -
Neap Tide -