Earth, Moon, and SunEarth, Moon, and Sun

Astronomy

• The study of the moon, starts, and other objects in space.

• The ancient Egyptians were among the first people to study the stars.– They watched the sky to predict floods, and to be

prepared to plow and plant their fields

How Earth Moves

• Earth moves through space in two major ways: rotation and revolution.

• The sun and moon seem to move across the sky each day because Earth is rotating on its own axis.

Rotation• The imaginary line that passes through the

Earth’s center and the North and South poles is Earth’s axis.

• The spinning of Earth on its axis is called rotation– This causes day and night. As Earth rotates, the part of it facing the sun experiences day, and the part of it on the opposite side experiences night.– It takes Earth about 24 hours to rotate once, hence one day being 24 hours long.

Revolution• In addition to rotating on its axis, Earth also

travels around the sun.• Revolution is the movement of one object

around another. – One complete revolution of the Earth around the sun is called one year.– Earth’s orbit, or path, is not quite circular, it’s an ellipse.

The Seasons of Earth

• Most places on Earth (besides the poles and tropics) have 4 seasons: winter, spring, summer, and autumn. The temperatures differ from place to place, though.

• Sunlight hits the Earth’s surface most directly at the Equator, which is why it’s warmest there.– Sunlight hits the poles at a steeper angle, causing it

to spread out over a greater area, which is why the poles are cooler.

Earth’s Tilted Axis and Seasons• Earth has seasons because its axis is tilted as it

revolves around the sun.– Otherwise, temperatures would remain the same

all year long.– Earth’s axis is always tilted at an angle of 23.5° from

vertical.– As it rotates, the North pole faces the sun for part

of the year, and away from the sun for part.– Distance from the sun does not cause seasons.http://www.phschool.com/webcodes10/index.cfm?wcprefix=cfp&wcsuffix=5012&area=view&x=0&y=0

Earth in June

• North end of axis is tilted toward the sun• In the Northern Hemisphere, there are more

hours of daylight than darkness.• It is summer because of direct sunlight and

more hours of it, which heats the Earth’s surface more than any other time.

Earth in December

• People in the Southern Hemisphere receive the most direct sunlight.

• Sun’s rays in the Northern Hemisphere are more slanted and there are fewer hours of sunlight. It is winter.

Solstices• The days when the sun is farthest north or

south of the equator.– Summer solstice for us is when the sun is farthest

north of the equator, around June 21st. • Longest day of the year for our hemisphere.

– Winter solstice for us is when the sun is farthest south of the equator, around December 21st.• Shortest day of the year for our hemisphere.

Equinoxes• Twice a year when day and night are each 12

hours long (light and dark) everywhere on Earth. Equinox means “equal night.” – Vernal (spring) Equinox – around March 21st, marks

the beginning of spring in the northern hemisphere– Autumnal Equinox – around September 22nd, marks

the beginning of fall in the northern hemisphere

Gravity and Motion

In 1666, when an apple fell from a tree, Isaac Newton first thought about the moon’s orbit and theorized that there must be a force between the Earth and moon to keep the moon in orbit.

Gravity• Force – a push or a pull– Most everyday forces require objects to be in

contact. Gravity is different.• Gravity – a force that attracts all objects toward

each other.• Law of Universal Gravitation – Newton’s law

which states that every object in the universe attracts every other object.

• The strength of the force of gravity between two objects depends on two factors: the masses of the objects and the distance between them.

Gravity, Mass, and Weight

• Why don’t you feel objects pulling you?• Mass – the amount of matter in an object– does not change

• Weight – the force of gravity on an object– can change depending on its location• On the moon, you would weight about 1/6 of your

weight on Earth. This is because the moon is much less massive than Earth, therefore the moon’s gravity on you is far less than that of Earth’s gravity.

Gravity and Distance

• The strength of gravity is affected by the distance between two objects as well as their masses.

• The force of gravity decreases rapidly as distance increases.– If the distance between two objects were

doubled, the force of gravity between them would decrease to ¼ of its original value.

Inertia and Orbital Motion• Inertia – the tendency of an object to resist a

change in motion.– The more mass an object has, the greater its

inertia.– An object with greater inertia is more difficult to

start or stop.• Newton’s First Law of Motion – 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 upon by a force.

Orbital Motion

• Newton concluded that two factors – inertia and gravity – combine to keep Earth in orbit around the sun and the moon in orbit around Earth.

Motions of the Moon• The “near side” of the moon always faces

the Earth, the “far side” always faces away.• As the moon revolves around Earth, the

relative positions of the moon, Earth, and sun change. – The changing relative positions of the moon,

Earth, and sun cause the phases of the moon, eclipses, and tides.

Phases of the Moon• The moon does not produce light – it reflects

light from the sun. This is why we can see it.• The different shapes of the moon you see from

Earth are called phases.– The moon goes through its whole set of phases each

time it makes a complete revolution around Earth.– Phases are caused by changes in the relative

positions of the moon, Earth, and sun.*the half of the moon that faces Earth is not always

the half that is sunlit*

Phases of the Moon, cont’d.• The phase of the moon you see depends on how

much of the sunlit side of the moon faces Earth.• About 29.5 days after the last new moon, the

cycle is complete, and a new moon occurs again.

http://phschool.com/atschool/phsciexp/active_art/moon_phases_and_eclipses/index.html

Eclipses

• When the moon’s shadow hits Earth or Earth’s shadow hits the moon, an eclipse occurs.– Most months, this does not happen!

• Eclipse – occurs when an object in space comes between the sun and a third object, casting a shadow on that object

Solar Eclipses

• Solar Eclipse – occurs when the moon passes directly between Earth and the sun, blocking sunlight from Earth. A new moon blocks our view of the sun

Total Solar Eclipse•Umbra – the very darkest part of the moon’s shadow, which is cone-shaped.– From any point in the umbra, light from the sun is

completely blocked by the moon.– The point of the umbra reaches only a small part of

Earth’s surface. People within the umbra experience a total solar eclipse.• For a short time, the sky turns dark and eerie as night –

even in the middle of a clear day – and the air cools. You can see the stars and the solar corona, the faint outer atmosphere of the sun.

Partial Solar Eclipse• Penumbra – another shadow cast

by the moon that is less dark than the umbra, and much larger. – In the penumbra, part of the sun is visible from Earth.– In a solar eclipse, people in the penumbra see only a

partial eclipse. An extremely bright part of the sun remains visible, making it dangerous to look at.

http://phschool.com/atschool/phsciexp/active_art/moon_phases_and_eclipses/index.html

Lunar Eclipses

• Lunar Eclipse – occurs at a full moon when Earth is directly between the moon and sun. Earth blocks sunlight from reading the moon.

Total Lunar Eclipse

• Like the moon’s shadow in a solar eclipse, the Earth also has an umbra and penumbra.

• When the moon is in Earth’s umbra, you see a total lunar eclipse.– Can be seen anywhere on Earth

that the moon is visible.– The edge of Earth’s shadow is

visible before and after a lunar eclipse.

Partial Lunar Eclipse

• Occurs when the moon passes partly into the umbra of Earth’s shadow.

• The edge of the umbra appears blurry, and you can watch it pass across the moon for 2 or 3 hours.

Tides• Tides – the rise and fall of ocean water that

occurs every 12.5 hours or so.– Water rises for about six hours, then falls for

about six hours, in a regular cycle.– The force of gravity pulls the moon and Earth

(including the water on Earth’s surface) toward each other.

– Tides are caused mainly by differences in how much the moon’s gravity pulls on different parts of Earth.

The Tide Cycle

• The moon’s gravity causes high tide on the side closest to the moon

• The moon’s gravity pulls on Earth more strongly than it does on the water on Earth’s opposite side, therefore causing high tide on the opposite side of Earth, as well.

• Between the areas of high tides, water flows away, causing low tides.

*see Tides handout, it’s more clear there!*

Spring Tides

• The gravity of the sun and the moon pull in the same direction. Their combined forces produce a tide with the greatest difference between consecutive low and high tides, called a spring tide.

Spring Tides

• The sun’s gravity also pulls on the Earth’s waters.

• During a new moon, the Earth, moon, and sun are nearly in line.– The gravities of the sun and moon pull in the same

direction, producing the highest difference between high and low tides, called a spring tide.

– Occurs twice per month, at a new and full moon.

Neap Tide

• During the moon’s first-quarter and third-quarter phases, the sun and moon are at right angles (90°) to Earth.

• The sun and moon pull at right angles, causing a neap tide, which has the least difference between high and low tides. – These occur twice per month.

Earth’s Moon

• Until 1609, no one knew anything about the moon. They could see the shapes on it, but couldn’t explain them.

• Telescope - a device build to observe distant objects by making them appear closer– 1609, Galileo Galilei made his own telescope and

was able to see much more detail about the moon than anyone had seen before. He was that the moon had an irregular surface, not smooth as the Greeks thought previously.

The Moon’s Surface• We know much more now than Galileo knew!• Features on the moon’s surface include:

–Maria

– Craters

– Highlands

Maria

• Galileo named the dark, flat areas of the moon’s surface maria, the Latin word for “sea,” after incorrectly thinking they were oceans.

• Maria are actually hardened rock formed from huge lava flows between 3 and 4 billion years ago.

Craters• Galileo saw that the moon’s surface

is marked by large round pits, or craters.– Some craters are hundreds of kilometers across!– Scientists mistakenly thought that craters were made

by volcanoes – they weren’t.– The craters were caused by the impacts of

meteoroids, chunks of rock or dust from space.• Maria don’t have as many craters as the rest of

the moon’s surface, suggesting the craters formed before the maria did.

Highlands

• Some of the light-colored features on the moon’s surface are highlands, or mountains.

• The lunar highlands cover much of the moon’s surface.

http://www.seasky.org/solar-system/earth-moon.html

Characteristics of the Moon• Although Earth’s closest neighbor is the moon,

it’s quite different from Earth.• The moon is dry and airless, small compared

to Earth, and has large variations in its surface temperature.

• Size and Density – The moon’s diameter is a little less than the

distance across the U.S.– The moon has 1/8 as much mass as Earth.• The moon has a dense core, but less dense outer layers.

Characteristics of the Moon cont’d.• Temperature and Atmosphere– Temperatures range from 130°C (266°F) in direct

sunlight to -180°C (-292°F) at night.• The temperatures range so much because the moon has no

atmosphere and the gravity is so weak that gases easily escape into space.

• Water– There is no liquid water on the moon, but there may

be large patches of ice near the moon’s poles.– Some areas are shielded from sunlight by crater walls.

Temps in these areas are so low that ice would remain frozen.

The Origin of the Moon• The best-fitting theory of the moon’s origin is

called the collision-ring theory.– About 4.5 billion years ago, the solar system was full

of rocky debris, some the size of small planets.– Scientists theorize that a planet-sized object collided

with Earth to form the moon.– Material from the object and Earth’s outer layers was

ejected into orbit around Earth, where it formed a ring. Gravity caused the material to combine to form the moon.

http://science.discovery.com/video-topics/space-videos/space-school.htm