SolarEclipses Through Space and Time Cycles in the Sky Lou Mayo, NASA/GSFC
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.
"Nothing can be sworn impossible since Zeus made night during mid-day, hiding the light of the shining Sun." - Archilochus 648 BC
Ancient Meaning • Solar eclipse have been generally explained in one of four ways: • 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.
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)
Fun Eclipse Facts • The moon’s shadow moves at 1700 km/hour (1,048 mi/hr) . • Maximum totality is ~7 ½ 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 August 16, 1868: Helium is discovered in solar corona. 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
1919 Solar Eclipse – Proving General Relativity Oh leave the Wise our measures to collateOne thing at least is certain, LIGHT has WEIGHTOne thing is certain, and the rest debate --Light-rays, when near the Sun, DO NOT GO STRAIGHT.- Arthur S. Eddington (1920)
Solar Eclipse August 11, 1999 Soissons, France
1882 Transit (USNO)
Application to Exo Planet Studies HD 209458 (mv = +7.7) in Pegasus
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.
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: • 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)