Chapter 1:

The Cycles of the Sky

Constellationshelp

astronomersknow the positions

of celestialobjects

Zodiac signs

These coordinates can be used to describe the position

of stars locally

zenith

Altitude

Azimuth

Angular distances of stars are measured in degrees

An imaginary Celestial Sphere

surrounding Earth aids in navigating

the sky

What is wrong with this cartoon?

Rotation vs. Revolution

• Rotation represents the Earth’s spin on its axis.

• This defines the length of 1 day.

• 1 day= 24hours

• Revolution represents the Earth’s orbit around the Sun.

• This defines the length of 1 year

• 1 year= 365.25 days

Earth’s counter-

clockwise rotation

causes the Sun and stars to

appear to move from

East to West

Motion of Stars at the North Pole

Motion of Stars at the Equator

Precession causes the

position of the North Celestial Pole to change over a period of 26,000 years.

UNLEARN

Common Misconception

The seasons occur due to the changing distance of the Earth from the Sun. When we are in our part of the orbit

that is closer to the Sun, it is Summer, and when we are farther away from

the Sun, it is winter

This statement is NOT correct!

LEARN

Some Facts

• When the Earth is physically closer to the Sun, it is winter time in the Northern Hemisphere, NOT summer

• When the Earth is physically farther from the Sun, it is summer time in the Northern hemisphere, NOT winter

• The Northern hemisphere and Southern hemisphere have opposite seasons from each other.

Thus, Earth’s seasons are NOT caused by its changing distance from the Sun!!!

Birds in the Northern Hemisphere fly south for the Winter...

That is when it is winter in the Northern Hemisphere, it is summer in the Southern

Hemisphere!

...which means that the Earth’s revolution around the Sun can not be the only explanation for

seasons!

The correct reason for the seasons:

1)the tilt of the Earth’s axis of rotation and

2) Earth’s revolution around the Sun

Earth is tilted!

Equinox positionSummer solstice

Winter solstice

The “altitude” of the Sun impacts how much energy is received on Earth.

Concept Question

What would be different if the Earth had no tilt?

UNLEARN

Common Misconception The phases of the Moon occurs because as the Moon orbits the Earth, the Earth casts a shadow on the Moon and blocks some or all of the sunlight from reaching the the Moon causing the amount of sunlight reflecting off the Moon to change.

This statement is NOT correct!

LEARN

The correct reason for the phases of the

Moon: Although half of

the Moon is always lit by the

Sun, we can only see the side of the

Moon that is facing the Earth and only on that

side does the amount of

illumination change

as the Moon orbits the Earth.

Concept Question

If the moon was between the Earth and theSun, what phase do you think it will be?

A) fullB) newC) first quarterD) third quarter

In other words ...

• The Earth’s rotation on its axis determines the length of the day.

• The moon’s orbit around the Earth determines the length of a month.

• The Earth’s orbit around the Sun (revolution) determines the length of the year.

Lunar eclipses occur only when the Moon crosses the ecliptic during the full phase

Lunar Eclipses

Lunar eclipse only happenswhen the moon travels through the Earth’s shadow

Darkest part of the shadow =

Intermediate part of the shadow =

Full

Partial

Partial

Lunar Eclipses

The moon is not completely dark during a total lunar eclipse

Lunar Eclipses

The moon is not completely dark during a total lunar eclipse sincesmall amount of light is bent around the edge of the Earth’s atmosphere!!!

Solar eclipses occur only when the Moon crosses the ecliptic during the new phase

Solar Eclipses

The moon has the same angular size as the Sun!

Moon = 400 X smaller thanthe Sun, but 400X closer

Solar Eclipse from Space

Solar Eclipse

Annular Eclipse

Partial Eclipse

Total Eclipse

Solar EclipsesNext total solar eclipse visible in the U.S. is August 21, 2017!Last annular eclipse visible in the U.S. was May 20, 2012!