Eclipses - University of Texas at Austinwesteros.as.utexas.edu/AST301/Lecture 5 - Eclipses.pdf ·...

September 12th, 2013

Eclipses

Who was the favorite Star Wars character of the class?

✤ A) Obi-Wan

✤ B) Jar Jar

✤ C) Luke Skywalker

✤ D) Yoda

News!

✤ Dark matter

http://mcdonaldobservatory.org/news/releases/2013/09/10

✤ New black holes

http://www.space.com/22736-monster-black-holes-nasa-discovery.html

Review - Seasons

✤ Question 22: What are the two things most directly responsible for making summer warm and winter cold?

✤ Which of these has the strongest impact on the seasons? The least?

✤ A) Varying distance between Earth and Sun

✤ B) One hemisphere being slightly closer to the Sun than the other

✤ C) The Earth’s axial tilt

✤ What axial tilt would cause the most extreme seasons? The least extreme?

Review from last time

✤ Every 27.3 days, the Moon completes one orbit around the Earth.✤ Every 29.5 days, the Moon returns to its same position relative to the

Sun in the sky.✤ The 29.5 day cycle of illumination patterns are called the phases of the

Moon.✤ The rotation period of the Moon (its “day”) is also 27.3 days.

✤ This is because the Moon is tidally locked, so we always see the same side.

✤ During a crescent Moon, we can see the rest of the Moon illuminated faintly due to Earth-shine.

Do you know your phases?!?

NewWaxing crescent

Waxing gibbous

First quarter

Full

Waninggibbous

Third quarter

Waning crescent

Question

What phase is this?A) Waxing crescentB) Waning CrescentC) NewD) Waxing gibbous

Lets check out an animation.

Copyright © 2005 Center for Astronomy Education (CAE) University of Arizona

Astronomy Ranking Task: Phases of the Moon

Exercise #2 Description: The figure below shows a “top view” of the Sun, Earth and six different positions (A - F) of the Moon during one orbit of Earth. Note that the distances shown for the Sun to Earth and for Earth to the Moon are not drawn to scale.

Ranking Instructions: Rank (from greatest to least) the amount of the Moon’s illuminated surface that is visible from Earth at each of the six positions (A – F) shown. Ranking Order: Greatest 1 ___ 2 ____ 3 ____ 4 ____ 5 ____ 6 ____ Least Or, the amount of the Moon’s illuminated surface visible from Earth is the same in all positions. _____ (indicate with check mark). Carefully explain your reasoning for ranking this way: _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________

• Which position of the Moon best corresponds with the moon phase that shown at the upper right?

Tutorial: Cause of Moon Phases - p. 81

✤ Work with a group of 2-3 people.✤ Read the instructions and questions carefully✤ Talk to each other and discuss your answers with each another✤ Come to a consensus answer you both agree on✤ If you get stuck or are not sure of your answer ask another group✤ If you get really stuck or don’t understand what the Lecture Tutorial

is asking as one of us for help.✤ We will discuss as a class when done.

✤ The Earth and Moon cast shadows.

✤ When either passes through the other's shadow, we have an eclipse.

What causes eclipses?

Types of eclipses

✤ A lunar eclipse occurs when the Earth is directly between the Sun and the Moon.

✤ The Earth’s shadow falls on the Moon.

✤ A solar eclipse occurs when the Moon lies directly between the Sun and the Earth.

✤ The Moon’s shadow falls on the Earth.

Question

At which position is a Solar eclipse possible? At which position is a Lunar eclipse possible?

A

B

C

D

QuestionWhich of these statements is correct:

a) Both solar and lunar eclipses occur during new moon.b) Both solar and lunar eclipses occur during full moon.c) Solar eclipses occur during new moon, and lunar eclipses occur during full moon.d) Lunar eclipses occur during new moon, and solar eclipses occur during full moon.

Why don’t we have an eclipse every month?✤ If the Moon’s orbital plane was exactly aligned with the ecliptic, we

should have a solar eclipse every new moon, and a lunar eclipse every full moon.✤ However, the Moon’s orbital plane is off the ecliptic by 5 degrees.

✤ To have an eclipse, the new or full moon has to occur at one of the nodes of the Moon’s orbit - the point where the orbit crosses the ecliptic.

Lunar eclipse

✤ When the Earth blocks the sunlight from hitting the Moon for several hours, occurs when the Moon is in the full phase.

✤ The Moon does not go completely dark - rather, it turns a reddish color, due to sunlight hitting the moon after going through the Earth’s atmosphere.

✤ Its reflecting the light from all of the sunrises and sunsets going on at that time.

There are a few types of solar eclipses.

✤ The Moon’s orbit isn’t perfectly circular, so we only see a total eclipse when the new moon occurs at a node at the closest point it its orbit.✤ An annular eclipse occurs when the Moon is at a farther point in its

orbit, so its angular size is not large enough to block out the entire Sun.✤ A partial eclipse occurs if you are at a spot on the Earth that does not

get completely covered by the Moon’s shadow.

Annular Eclipse

Partial Eclipse

Total Eclipse

Solar Eclipse

Image of eclipse I took from Kyoto, Japan in May 2012. What kind of eclipse is this?

A) LunarB) Annular SolarC) Partial SolarD) Total Solar

✤ Planets known in ancient times:

✤ Mercury

✤ difficult to see; always close to Sun in sky

✤ Venus

✤ very bright when visible; morning or evening "star"

✤ Mars

✤ noticeably red

✤ Jupiter

✤ very bright

✤ Saturn

✤ moderately bright

Motions of the Planets

✤ Planets usually move slightly eastward from night to night relative to the stars.

✤ But sometimes they go westward relative to the stars for a few weeks: apparent retrograde motion.

What was once so mysterious about planetary motion in our sky?

We see apparent retrograde motion when we pass by a planet in its orbit.

Animation

✤ Easy for us to explain: occurs when we "lap" another planet (or when Mercury or Venus laps us).

✤ But very difficult to explain if you think that Earth is the center of the universe!

• In fact, ancients considered but rejected the correct explanation.

• More about this next time.

Explaining Apparent Retrograde Motion

Homework

✤ Don’t forget to turn in your sheet of paper!

✤ Read text: Section 3.1, 3.2, 3.3

✤ Mastering Astronomy Homework #4

✤ Due by 7:00am Tuesday 9/17/13.