Lecture 4

Lecture 4

ASTR 111 – Section 002

Terms

• Apogee/Perigee• Subtend• Parsec, light-year, AU• Parallax• Solar and Sidereal time• Small angle formula• Ecliptic• Zenith• Tropic of Cancer, Capricorn, Artic and Antarctic Circle• Equinox, Solstice• Zodiac• Accuracy, Precision, and Bias

While you are waiting for class to start, try to define these terms

First Exam

• Slated for 9/29

• Based on lecture notes, problems worked in lecture, and quizzes.

• Approximately 50 questions

• In the Testing and Tutoring Center (using Blackboard system).

Tuesday September 15th• Class is canceled.

• Quiz is still due at 9 am on the 15th.

• I will be available via email.

Outline

1. Quiz Discussion

2. Rotation – review generally

3. The Seasons – review generally

4. The Moon in its orbit

5. Math review

Quiz Discussion

Outline

1. Quiz Discussion

2. Rotation – review generally

3. The Seasons – review generally

4. The Moon in its orbit

5. Math review

Rotation

Outline

1. Quiz Discussion

2. Rotation – review generally

3. The Seasons – review generally

4. The Moon in its orbit

5. Math review

The Seasons

Outline

1. Quiz Discussion

2. Rotation – review generally

3. The Seasons – review generally

4. The Moon in its orbit

5. Math review

• From http://www.dslreports.com/forum/remark,16822681• Uses color saturation http://

en.wikipedia.org/wiki/Saturation_(color_theory)

The Moon in its orbit

Eventually we want to be able to explain …

A simple model• Moon executes circular orbit

• Moon orbit is in Earth’s ecliptic plane

Looking down on North Pole

Can you “see” white ball if you are at Equator?

NO! You would need to see through Earth!

What is wrong with this picture?

Looking down on North Pole

Can you “see” white ball if you are at Equator?

Looking down on North Pole

If you walk around along the equator, where will you be able to see the white ball?

Looking down on North Pole

Can you “see” white ball if you are at Equator? NO! You would need to see through Earth!

NO!

Looking down on North Pole

Can you “see” white ball if you are at Equator? NO! You would need to see through Earth!

NO!

Looking down on North Pole

Closer to scale

• Draw a diagram that explains when you will first be able to see the first quarter moon if you live on the equator.

• Stated another way, at what time will you see the 1st quarter moon rise from Earth?

A

B

C

D

E

1. Fill in the dark and light parts of the Moon for A-D (from this perspective)

2. From the perspective of someone on Earth what position of A-E best fits the Moon view in the lower-left-hand corner?

3. In the blank boxes below, sketch how the Moon would appear from Earth from the four Moon positions that you did not choose for Question 2. Label each box with a letter.

Earth

View of Moon from Earth at one of thepositions (A-E) above.

Sun

’s r

ays

G

H

I

F

4. Shade in the part of the Moon that is not illuminated by the sun when it is at positions F-I.

5. Which Moon position (F-I) best corresponds with the Moon phase shown in the lower-left corner?

6. How much of the Moon’s surface is illuminated by the sun during this phase?

7. How much of the Moon’s illuminated surface is visible from Earth for this phase of the Moon?

Earth

View of Moon from Earth from one of the positions (F-I) above.

Sun

’s r

ays

A simple model• Moon executes circular orbit

• Moon orbit is in Earth’s ecliptic plane

Model can explain the phases of the Moon

• The phases of the Moon occur because light from the Moon is actually reflected sunlight

• As the relative positions of the Earth, the Moon, and the Sun change, we see more or less of the illuminated half of the Moon.

What does the Earth look like from the Moon at

• Full Moon

• New Moon

• First Quarter

• Third Quarter

What are 2 observations simple model does not predict?

1.

2.

What are 2 observations simple model does not predict?

1. Why there are not eclipses every month

2. Why there are “annular” and “total” eclipses

Eclipses occur only when the Sun and Moonare both on the line of nodes

What are 2 observations simple model does not predict?

1. Why there are not eclipses every month

2. Why there are “annular” and “total” eclipses of the sun

Solar eclipses can be either total, partial, orannular, depending on the alignment of

the Sun,Earth, and Moon

Eventually we want to be able to explain …

Lunar eclipses can be either total, partial, orpenumbral, depending on the alignment of

the Sun, Earth, and Moon

Question• If you were looking at Earth from the side

of the Moon that faces Earth, what would you see during– A total lunar eclipse?– A total solar eclipse?

The Moon’s rotation always keeps the same face toward the Earth due to synchronous rotation

• Two types of months are used in describing the motion of the Moon.

• With respect to the stars, the Moon completes one orbit around the Earth in a sidereal month, averaging 27.32 days.

• The Moon completes one cycle of phases (one orbit around the Earth with respect to the Sun) in a synodic month, averaging 29.53 days.

Time and the Moon

• sidereal month, averaging 27.32 days.

• sidereal day – 23 hr 56 min

• synodic (lunar) month, averaging 29.53 days.

• solar day – 24 hr

Question• On a certain date the Moon is in the

direction of the constellation Gemini as seen from Earth. When will the Moon next be in the direction of Gemini?

1. One year later?

2. 366.2425 days later?

3. One sidereal month later?

4. One synodic month later?

Outline

1. Quiz Discussion

2. Rotation – review generally

3. The Seasons – review generally

4. The Moon in its orbit

5. Math review