# Presentation: 465438

## Cycles in the Sky

**Solar**** ****Eclipses**
**Through Space **
**and Time**

**Cycles in the Sky**
**Lou Mayo, NASA/GSFC**

**Notes:**
In this presentation, you will learn about:
What a solar eclipse is and what it looks like
History of solar eclipses
How and why solar eclipses happen
Types of solar eclipses
Eclipse cycles
Scientific research performed during solar eclipses
The March 29th  2006 Solar Eclipse
Eclipses and transits elsewhere in our solar system and beyond
An eclipse activity involving calculation of angular diameters

## Slide 2

**Notes:**
Image of a total solar eclipse. The corona is visible here. The lines in the corona coming out radially from the sun are plasmas captured by the sun’s magnetic field and flowing along these magnetic field lines.

## Slide 3

**Notes:**
Total solar eclipse just prior to totality. Reddish prominences can be seen off the sun’s limb.

## Slide 4

**Notes:**
Solar eclipse from Africa. This picture was shot with just a 35mm camera.

## Slide 5

**Notes:**
Time series of a total solar eclipse – put together by Fred Espenak (Mr. Eclipse).

## Slide 6

**Notes:**
Solar eclipse animation. These real images of an eclipse show what happens as the moon blocks out the solar disk or “photosphere”. The corona glows brightly. It is difficult to see the corona if even a little bit of the photosphere is visible.  Some structure can be seen in the corona, again due to the sun’s magnetic field.

## Slide 7

**Notes:**
Images of the hybrid solar eclipse visible from the East Coast of the US. These images were taken from Gaithersburg, MD where the moon eclipsed only about 5-10% of the sun. Bottom image – A “SunSpotter” provides very safe solar viewing.

## Slide 8

**Notes:**
Public eclipse event in Gaithersburg, MD. Notice the solar filter on the front end of the telescope.

## Chinese astrologers wrote of an eclipse occurring over 4000 years ago. 

**Historians and astronomers believe that this was an eclipse that happened on 22 October 2134 B.C. **
**Two astrologers at the time, Hsi and Ho, had apparently failed to predict this eclipse, and so were beheaded. **

**Notes:**
Eclipses had mystical origins for thousands of years. The Chinese believed the sun was being eaten by dragons. They could predict eclipses through careful observations of the sun and moon. Failure to predict an eclipse was often met with death.

## "Nothing can be sworn impossible since Zeus made night during mid-day, hiding the light of the shining Sun." 

*- Archilochus 648 BC*

## Ancient Meaning

**A celestial being, usually a monster, attempts to destroy the Sun **
**The Sun fights with its lover the Moon **
**The Sun and the Moon make love and discreetly hide themselves in darkness **
**The Sun grows angry, sad, sick, or neglectful **
- 	***Littmann and Willcox, “Totality”***

**Norse mythology: the wolflike giant Sköll follows the Sun hoping to devour it. **
**Ancient Egypt: the evil god Set was thought to have leapt into the eye of the Sun god, Horus. **
**Ancient China: A heavenly dog ate the Sun. **
**Chippewa Indians shot flaming arrows at the Sun hoping to rekindle the flames. **

**Ancient Meaning**

## BASIC ECLIPSE GEOMETRY

**BASIC ECLIPSE GEOMETRY**

**Notes:**
The moon orbits the Earth once every 29.5 days. This is called a synodic lunar month and is defined as the time between new moon and the next new moon. This is slightly longer than a “sidereal” lunar month (27.3 days) which is the time it takes the moon to go 360 degrees around the Earth, because the Earth will have advanced in its orbit by about 27 degrees . To get back to a new moon, the moon then has to travel a little further each sidereal month to get back into a straight line with the sun and Earth. Eclipses do not, however, occur every 29.5 days because the moon’s orbit around the Earth is tilted with respect to the Earrth’s orbit by about 5 degrees, enough to make the moon’s shadow miss too high or too low most of the time. Eclipses can occur only when the sun, moon, and Earth line up along the line of nodes which is the intersection of the Earth’s orbital plane with the moon’s orbital plane.

## Slide 13

**Notes:**
This graphic illustrates how the shadow of the moon falls on the Earth during a total solar eclipse. Notice how small and narrow the path of totality is. The darker central shadow is called the umbral shadow. Viewers inside this small shadow area will see a total eclipse. The lighter shadow is called the penumbral shadow. Observers within the penumbral shadow will see a partial eclipse. The umbral shadow advances across the face of the Earth at a little over 1000 miles per hour. This motion is due largely to the Earth’s rotation on its axis.

## Slide 14

**Notes:**
Total solar eclipse geometry showing path of umbral shadow (or totality).

## Slide 15

**Notes:**
Annular eclipses occur because the moon’s orbit around the Earth is not a perfect circle but is slightly elliptical. Its average distance from Earth is 384,400 km but it can get as close as 363,300 km and as far as 405,500 km. So its apparent size can vary by around 13%. When it is near apogee (its furthest point from the Earth), it appears too small to fully cover the sun. In these cases, we have an annular eclipse and a thin ring of light from the sun’s photosphere peaks out all around the moon. You can not see the corona during an annual eclipse.

## Perigee

**Perigee**

**Apogee**

**Notes:**
Visual appearance of the moon at its closest and furthest points from Earth.

## Slide 17

**Notes:**
Annular eclipse.

## When do we get an eclipse?

**Whenever the Sun is within 18.5° of a node. **
**The Sun travels along the ecliptic at about 1° per day**
**It takes about 37 days to cross through the eclipse zone centered on each node.  **
**A New Moon occurs every 29.5 days and therefore at least one solar eclipse must occur during each of the Sun's node crossings. **

## Saros Cycle

- “Saros” : Greek meaning “Repetition”
- 1 Saros = 18 years, 11 1/3 days
- Line of nodes drifts westward at 19 deg / year
- Eclipses repeat because the moon and the nodes return to the same place wrt the sun
- The 1/3 day means you must go through 3 Saros to have an eclipse at the same location on the Earth (54 years, 1 month)

**Notes:**
A “Saros Cycle” occurs because of  the resonance between three lunar orbital months:
Synodic Month	New Moon to New Moon		29.53d
Draconic Month	Node to Node		27.21d
Anomalistic Month	Perigee to Perigee		13.54d

Eclipses that happen 1 Saros apart (or 18 years, 11 1/3 days apart) will have the same geometry and so the path of totality will be the same but displaced by 1/3 of the circumference of the Earth. For more information on Saros Cycles, go to:

## Slide 20

**Notes:**
Diagram of the Earth looking pole-on, showing the paths of totality for many solar eclipses. Notice that they appear to occur in three apparently identical sets. This is the 1/3 day of the Saros Cycle making itself known.

## Fun Eclipse Facts

- The moon’s shadow moves at 1700 km/hour (1,048 mi/hr) .
- Maximum totality is ~7 1⁄2 minutes.
- Every place on Earth will see a total solar eclipse about every 400 years.
- Solar Eclipses occur more frequently than lunar eclipses ( by 5:3).
- There must be at least two solar eclipses every year.
- There can be two solar eclipses in back to back months with a total lunar eclipse in between.
- This triple eclipse can occur twice during an eclipse year (1935, 2160).
- Seven eclipses is the maximum - 4 solar, 3 lunar (1982, 2485).

## Will we always have total solar eclipses?

**D(sun) = 870,000 mi (1.4M km)**
**	**** ****	**** ****(32.7’ to 31.6’)**
**D(moon) = 2,160 mi (3,476 km)**
**	****	**** ****(33.5’ to 29.4’)**
**The moon is receding from the Earth by 3.8 cm / year.**
**When it has drifted another 12,552 mi (20,200 km), it will always be smaller than the sun (~1/2 billion years)**
**Earth’s day lengthens by 0.0016s / century**

## ECLIPSE SCIENCE

**May 29, 1919: General relativity is verified**
**Total solar eclipses provide opportunity to study composition of corona. **
**Accurate timings allow calculation of solar dimensions.**
**Studies of ancient records reveal 0.001s slowing of Earth’s rotation**

**ECLIPSE SCIENCE**

**Notes:**
Helium was discovered first on the sun by observing the sun’s chromosphere during a total solar eclipse.

The May 29th 1919 solar eclipse provided an opportunity to see if starlight was bent by the gravity of the sun. It was!

## 1919 Solar Eclipse – Proving General Relativity

**1919 Solar Eclipse – Proving General Relativity**

**Notes:**
On May 29, 1919, British astronomer, Sir Arthur Eddington headed two British Expeditions to Sobral, Brazil and Principe, West Africa, to confirm Einstein's predictions from his general theory of relativity that the path of a light ray would be bent in the presence of a strong gravitational field. The goal was to measure the positions of stars just off the limb of the sun (impossible to do under normal sunny day conditions) and to see if their apparent positions would indeed be altered by the sun’s enormous gravitational field (fig e). Five stars were selected and photographed. Their positions were compared with pictures taken of the same star field before the eclipse. The results agreed with Einstein’s predictions both qualitatively (yes, the stars positions did change) and quantitatively (they changed by the predicted amount). 

## Slide 25

**Notes:**
GOES images of the Earth showing the path of the moon’s shadow during the August 11, 1999 total solar eclipse.

## Slide 26

**Notes:**
A closer look at the 1999 eclipse.

## A Perfect Day

**Notes:**
August 11 1999 eclipse in Soisson, France. It is raining and completely overcast minutes before the eclipse.

## Slide 28

## Solar Eclipse

- August 11, 1999
- Soissons, France

**Notes:**
...but the clouds parted just enough to catch these photos of the partial phase. During totality, the sky was overcast but you could tell totality had started because the light level dropped very quickly.

## March 29, 2006

**March 29, 2006**

## Slide 31

**Notes:**
Close up of the path of totality in Brazil. Here the eclipse begins at sunrise.

## Slide 32

**Notes:**
Map of the Earth showing the path of totality for all eclipses between 2001 and 2025. Notice that there are two eclipses (2017 and 2024) that can be seen in totality over the United States.

## Slide 33

**Notes:**
A solar eclipse involving Jupiter’s moon, Io as seen by the Voyager spacecraft.

## Slide 34

**Notes:**
Lots of things in the sky move in front of lots of other things. Here, Saturn slowly disappears behind the limb of the moon in this video sequence.

## 1882 Transit

**1882 Transit**
**(USNO)**

**Notes:**
The 1882 transit of the planet Venus across the sun was viewed by millions of professional and amateur astronomers. This image was taken at the US Naval Observatory.

## Slide 36

**Notes:**
Animated sequence of images taken from the surface of Mars. Here we see the moon, Deimos moving in front of the sun.

## Application to Exo Planet Studies

- HD 209458 (mv = +7.7) in Pegasus

**Notes:**
Transits of planets around distant stars are one way astronomers can detect these extra solar planets. As the planet moves in front of the star, a small percentage of the light from that the star is blocked and the light curve (shown at right) dips. When the planet moves off the limb of the star, the light curve recovers. This method is only effective when the plane of the orbit of the planet is in our line of sight. 

This viewgraph depicts real data of a planetary transit around the star HD209458 in the constellation of Pegasus.

## Solar Eclipse Activity

**Solar Eclipse Activity**

**GOALS: ****   To simulate a solar eclipse ****   To understand the concept of angular size ****   To make estimates of absolute and**
**       ****relative size **
**MATERIALS: ****   Yard or meter stick (don't **
**       ****confuse your units!) ****   Construction paper ****   Tape ****   Scissors ****   CD-ROM ****   Pencil ****   Black and yellow markers**** **

## PROCEDURE: 
   	1. MAKE THE SUN: Lay the CD on the construction paper and trace around its outer edge. Then trace around the center hole. 
   	2. Draw two lines (a tab) down from the CD and fanning out so the CD circle and tab look like the picture on this slide. The tab will be used to mount the CD circle on the yard/meter stick. 
3. Cut out the large CD circle and connected tab. This will represent the sun. The small circle in the center will represent the size of the moon (of course, this is not to scale). 
   	4. Color the CD circle yellow (for the sun) and the small center circle black. 
5. MAKE THE MOON: Now, on a different piece of construction paper, trace just the center hole in the CD. Make the same kind of tab for this circle as you did for the sun circle. Make the tab a bit longer than the sun's tab. Color the moon black and cut it out. 
   	6. ASSEMBLE: Bend the sun and moon back 90 degrees from their tabs at the BASE of the tab. Wrap the fanned out portion of the tabs around the yard/meter stick and tape the ends together. The sun should be near the end of the stick and the moon should be near the front. The sun and moon should now be able to slide up and down the stick. 

**   ****	****1. MAKE THE SUN: Lay the CD on the construction paper and trace around its outer edge. Then trace around the center hole. **
**   ****	****2. Draw two lines (a tab) down from the CD and fanning out so the CD circle and tab look like the picture on this slide. The tab will be used to mount the CD circle on the yard/meter stick. **
**3. Cut out the large CD circle and connected tab. This will represent the sun. The small circle in the center will represent the size of the moon (of course, this is not to scale). **
**   ****	****4. Color the CD circle yellow (for the sun) and the small center circle black. **
**5. MAKE THE MOON: Now, on a different piece of construction paper, trace just the center hole in the CD. Make the same kind of tab for this circle as you did for the sun circle. Make the tab a bit longer than the sun's tab. Color the moon black and cut it out. **
**   ****	****6. ASSEMBLE: Bend the sun and moon back 90 degrees from their tabs at the BASE of the tab. Wrap the fanned out portion of the tabs around the yard/meter stick and tape the ends together. The sun should be near the end of the stick and the moon should be near the front. The sun and moon should now be able to slide up and down the stick. **

## Moon

**Moon**

**Sun**

## Now, holding the yard/meter stick against your cheek, sight down the stick. The smaller moon circle will cover some portion of the sun circle. Slide the moon back and forth to a place where it just covers the sun.

Looking at the yard/meter stick, note the distance (in inches or cm) of the moon. Then note the distance of the sun. Finally, measure the diameter of the moon. You can now create similar triangles that will help you answer the following questions:

**Looking at the yard/meter stick, note the distance (in inches or cm) of the moon. Then note the distance of the sun. Finally, measure the diameter of the moon. You can now create similar triangles that will help you answer the following questions:**

**On the yard/meter stick, how much further away is the sun than the moon?**
**2.  Given the diameter of the moon, can you predict the diameter of the sun?**
**In space, our real moon has a diameter of 3,476 km and is on average 384,400 km from Earth. The sun is about 149,600,000 km from the Earth. How many times further is the sun then the moon? What would you estimate to be the diameter of the sun?**
**What is the angular size of the sun? moon?**
**     ****(hint: construct right triangles and use trig)**

**Notes:**
Answers for teachers:
(For a clean copy of this math activity, download the file “slide 41 eclipse math activity”)
       3,476km              X
              ________  =  _______
              384,400km   149,598,000km (definition of the A.U.)

                (3,476km) (149,598,000km) = 384,400km X

        (3,476km) (149,598,000km)
         ______________________    =  X;             X = 1,352,764 km ~ Diameter of the sun **

                       384,400km

** The reported solar diameter is 1,391,785 km. Your derived value is a bit small because we are using the AVERAGE distance to the moon. In order for the moon to completely cover the sun, it would be a bit closer to the Earth than its average distance.

4. Compute angular diameter of the sun:

	Given a right triangle ABC,
	Where side AB = distance to the sun ~ 150,000,000 km
	            side BC = radius of the sun = 678,000 km
                    angle ABC is a right angle
	            point A is the observer on the Earth
	First find the value for angle A  (angular semi-diameter of the sun)
 	a. You can use: tan(A) = BC / AB  so  A = tan-1 (BC/AB) = tan-1 (678,000/150,000,000) = 0.259 degrees
	or  you can use the small angle formula: A = BC / AB ( in radians) and then just multiply by 360 / 2*pi to get degrees. 

	Then multiply your answer by 2 to get the angular diameter of the sun (answer: 0.52 degrees)

	b. To get the angular diameter of the moon, do the same calculation but using the distance (384,400km) and radius (1,738km) of the moon instead of the sun (answer: 0.259 * 2 = 0.52 degrees)