SOLAR ECLIPSE Solar Eclipses – Short Video • http://www.youtube.com/watch?v=HrloqdXrz N4 • This video touches on the various types of solar eclipses. An Eclipse from Another World • Mars’ Phobos eclipses the sun as seen from Curiosity Rover. • http://www.youtube.com/watch?v=OyZoD7B RTtg Solar Eclipse • A solar eclipse occurs when the moon moves between Earth and the sun. – There are total solar eclipses – the moon covers the disk of the sun completely. – Partial solar eclipses – less dramatic, when the moon only covers part of the solar disk. A total eclipse Partial Eclipse Angular Diameter • Solar eclipses are spectacular because the moon and sun have nearly the same angular diameter. – This means, simply, that the moon appears big enough that it covers almost exactly the sun’s disk. • Angular diameter of an object is the angle formed by lines extending toward you from the opposite edges of the object and meeting at your eye. Angular Diameter • This is the figure from your book – Figure 3-7. Angular Diameter • The formula for angular diameter is: • Angular diameter/206.265 = linear diameter/distance • 206.265 is the number of arc seconds in a radian. This allows us to, when we divide, to convert the angle from arc seconds to radians. – A radian is a unit of angular measure. Angular Diameter • Angular diameter/206.265 = 3476 km/384,000 km. – This gives us an angular diameter 1,870 arc seconds. When we divide 1,870/60 we get 31 arc minutes. An when we divide that by 60, 31/60, we get degrees. – The angular diameter of the moon is approximately .5 degrees. Angular Diameter • Using this same equation, you can find the angular diameter of the sun. – AD/206.265 = 1.39 X 106km/1.5 X 108km – AD = 1,900 arc seconds – This, after dividing by 3600, gives as approximately 0.5 degrees. – In other words, the sun is actually a touch bigger but not by much Angular Diameter • As luck would have it, our moon is the right size to almost exactly cover up the sun – leaving the sun’s atmosphere, its corona, visible. Why is this Important? • At a total solar eclipse observed from the ground, we can see part of the corona that is not accessible in any other way. – Though we do now have space crafts that are able to send us pictures of the corona, eclipses were, pre-space age, the only opportunities to observe the corona. – It was especially important for measuring the temperature of the solar corona. Why is this Important? • The corona has a temperature of one to three million degrees Kelvin (water boils at 373 degrees Kelvin). – This is significantly hotter than the sun’s surface which is only 5,800 degrees Kelvin. • We also use total solar eclipses to study how sun’s magnetic field shapes the corona. Sun’s Magnetic Field Photographed During an Eclipse Why is this Important? • Most importantly of all, it was observation of a total solar eclipse in 1919 (along with observation involving the precession of Mercury) which provided significant support for Einstein’s general theory of relativity (specifically the bending of space time) and the overthrow of Newtonian gravitational theory. – Einstein’s conception of gravity became accepted, though Newton’s still is useful. The First Evidence for General Relativity • In May 1919, a team led by the British astronomer Arthur Stanley Eddington claimed to have confirmed Einstein's prediction of gravitational deflection of starlight by the Sun while photographing the solar eclipse with dual expeditions in Sobral, northern Brazil, and Príncipe, a west African island. The Moon’s Shadow • To see a solar eclipse you must be in the moon’s shadow. • Like in a lunar eclipse, there is a penumbra and an umbra. The moon’s umbral shadow produces a spot of darkness about 170 miles in diameter on the Earth’s surface. The picture at right is of the umbral shadow over Alaska. The Moon’s Shadow • Due to the orbital motion of the moon and the rotation of Earth, the moon’s shadow moves across Earth at high speeds (1,600 mph). – This called its “path of totality.” – People in the path of totality see a total eclipse. – Outside the umbral shadow, there is a penumbral shadow, which is a partial eclipse. Eclipse Depends on Distance to Earth • At apogee, the moon is at its farthest point form Earth and its angular diameter is 6% less than average. At apogee, the moon’s disk may not cover the sun’s. This produces an annular eclipse – an eclipse in which a ring (an annulus) is visible around the moon’s disk. Next Total Solar Eclipse in US • From what I was able to find, August 21, 2017 will be the date for the next solar eclipse visible near us. • Takes place at noon CDT. Features of a Solar Eclipse • When there is a total eclipse, it is dark causing: birds to go to roost (bed) and automatic streetlights to come on. • The sun’s photosphere (the bright surface of the sun) is completely covered in a total eclipse. – This allows you to directly view the corona, the sun’s outer atmosphere. – At the point of totality, seeing only the corona, it is safe to look directly at an eclipse. Features of a Solar Eclipse • The corona is made up of hot, low-density gas, which has a wispy appearance due to the solar magnetic field. Why is the sun’s magnetic field important • Much solar activity appears to be directly connected with the properties of the sun’s magnetic field. • The sun has a magnetic field cycle that changes in polarity every 22 years, which is exactly twice the length of time of the sunspot cycle. Why is the sun’s magnetic field important • They are caused by intense magnetic activity, which inhibits convection currents, forming areas of reduced surface temperature. They usually appear as pairs, with each sunspot having the opposite magnetic pole to the other. • Most solar flares and coronal mass ejections originate in magnetically active regions around visible sunspot groupings. Why is the sun’s magnetic field important • The number of sunspots correlates with the intensity of solar radiation. Why is the sun’s magnetic field important • The increase in the sun’s ejections can be seen on Earth as the Aurora Borealis and the Aurora Australis. – Video: Aurora Borealis: https://www.youtube.com/watch?v=FcfWsj9OnsI Why is the sun’s magnetic field important • “The sudden increase [from the sun’s ejections] in power can damage sensitive electronic equipment. Power transformers can overload, causing long-lasting blackouts. Long metal structures like oil and gas pipelines can carry currents, which can enhance their corrosion over time and lead to devastating effects if proper safety measures are not in place. The resulting variations in the ionosophere can disrupt GPS signals, giving inaccurate readings.” • http://www.space.com/11506-space-weathersunspots-solar-flares-coronal-mass-ejections.html Questions • Why do solar eclipses happen only at new moon? – Why not every moon? • Why isn’t the corona visible during partial or annular solar eclipses? Features of an Eclipse • Just above the unseen photosphere, we are able to see the chromosphere during an eclipse. – The chromosphere is marked by eruptions on the solar surface – Astronomers call these eruptions prominences – glowing with a “clear pink color due to the high temperature of the gases involved. Totality is Short • It lasts no longer than 7.5 minutes, but is usually only 2 to 3 minutes long. – When totality ends, the corona and chemosphere vanish. A Misconception • Sunlight is not more dangerous during a solar eclipse. – It is always dangerous to look at the sun. – Eclipses are more dangerous only because it is when people are tempted to ignore common sense and look at the sun directly burning their eyes.