Astronomy

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

Why Do We Have Seasons?• Because the Earth’s axis is tilted as it revolves

around the sun

Earth’s axis is always tilted 23.5 o

from the vertical

• As Earth revolves around the sun, the north (top) end of its axis is tilted away from the sun part of the year, and toward the sun for part of the year

• Our seasons ARE NOT caused by changes in Earth’s distance from the sun!

Solstices• The sun reaches its greatest distance north or

south of the equator 2 times each year. These days are called solstices– Summer Solstice in Northern Hemisphere = day

when sun is farthest north of the equator• Around June 21; longest day of year in that hemisphere

– Winter Solstice in Northern Hemisphere = day when sun is farthest south of the equator• Around December 21; shortest day of year in that

hemisphere

– Exactly opposite for Southern Hemisphere

Equinoxes

• Neither hemisphere is tilted toward or away from the sun 2 times per year– Halfway between the solstices– Equinoxes (equal nights)– Day and night are about 12 hrs long all across

Earth– Vernal Equinox (Spring Equinox) around March 21– Autumnal Equinox around September 22

Gravity

• Isaac Newton realized that there is a force between the Earth and the moon that holds the moon in orbit. It’s called gravity.

• Newton’s Law of Universal Gravitation: every object in the universe attracts every other object

Strength of the Force of Gravity

• The strength of the force of gravity between two objects depends on 2 things:– The masses of the objects (amount of matter), and– The distance between them

• Force of gravity as mass

• Force of gravity as distance

• The force of gravity on an object is called weight.– Mass never changes but weight does, depending on where you

are.

Why doesn’t the moon crash into Earth?

• Because of inertia– The tendency of an object to resist a change in

motion– Newton’s 1st Law of Motion says: an object at rest

will stay at rest and an object in motion will stay in motion with a constant speed and direction unless it is acted on by a force (a push or a pull)• Think of a soccer ball in a car that has to stop short

So,• Gravity and inertia work together to keep

Earth orbiting around the sun and the moon orbiting around the Earth.

Motions of the Moon

• The same side of the moon always faces the Earth because 1 day on the moon = 1 year on the moon

• The changing positions of the moon, Earth, and the 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

• The shapes of the moon we see are called phases– The moon goes through a full set of phases each

time it makes a complete revolution around Earth.

• The phase of the moon you see depends on how much of the sunlit side of the moon faces Earth

– The sun lights the moon so ½ moon is almost always in sunlight

– The moon revolves around Earth so you see the moon from different angles

– The half of the moon that faces Earth isn’t always the half that is sunlit

• During the New Moon, the side of the moon facing Earth is not lit because the sun is behind the moon

• As the moon revolves around Earth, you see more of the lighted side every day until you see it is fully lit (Full Moon)

• As the moon continues to revolve, you see less and less of the lighted side. After 29.5 days, the cycle is done and a new moon occurs again

• Waxing means growing• Waning means shrinking• Crescent shape is

Eclipses

• Eclipses happens when either the moon’s shadow hits Earth or when the Earth’s shadow hits the moon.

• When an object in space comes between the sun and another object, the first object causes a shadow on the second object, causing an eclipse.

2 Types

• Solar Eclipse – when the moon passes between the sun and Earth, blocking the sunlight from Earth

• Lunar Eclipse – when the Earth passes between the moon and the sun, blocking the sunlight from the moon

Total Eclipse versus Partial Eclipse• Darkest part of shadow is cone-shaped shadow,

& called the Umbra. Only people in the Umbra experience a total eclipse.

• A larger part of the shadow is less dark, and is called the Penumbra. In the penumbra, part of the sun is still visible.

• You are much more likely to see a total lunar eclipse than a total solar eclipse because a total lunar eclipse can be seen from anywhere on Earth

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

occurs every 12 ½ hrs or so. The water rises for about 6 hours, then falls for about 6 hrs.

What Causes Tides?

• Caused by differences in how much the moon’s gravity pulls on different parts of the Earth

• At any time, there are 2 places on Earth with high tides and 2 places on Earth with low tides.

High Tides and Low Tides

Spring Tides and Neap Tides• Spring Tide – the tide with the greatest

difference between high tides and low tides– Produced by the combined gravitational forces of

the sun and the moon on Earth– Occur 2x/month: at new moon and full moon

• Neap Tide – the tide with the least difference between high tides and low tides– Produced by the sun’s gravitational pull at right

angles to the moon’s gravitational pull– Occur 2x/month: at first quarter moon and at

third quarter moon phases

About Earth’s Moon

• Surface Features– Maria• Dark flat areas of hardened rock from huge ancient lava

flows

– Craters• Large round pits from being hit by meteoroids

– Highlands• Light colored features; mountains; cover much of moon’s

surface

Moon’s Characteristics

• Size/Density– Small compared to Earth; Diameter about ¼

Earth’s diameter– Moon’s density is like the density of Earth’s outer

layers• Temperature– Much bigger differences in highs and lows than on

Earth because no atmosphere on moon– Higher highs and lower lows than Earth

More Moon’s Characteristics

• Atmosphere– None; weak gravity lets gases escape into space

• Water– No liquid water– Signs that there may be large patches of ice near

the moon’s poles

Origin• Collision Ring Theory – current theory – About 4 ½ billion years ago when Earth was young,

lots of rocky debris floating in space-some as big as planets

– A planet-sized object hit Earth – Material from Earth and from the object got shot

into Earth’s orbit and formed a ring– Earth’s gravity pulled the pieces in the ring

together to form the moon

• Video: Teachers' Domain: The Origin of the Moon