Why is the Solar Eclipse such a big deal? Only due to its rarity?

Approximately once every 18 months (on average) a total solar eclipse is visible from some place on the Earth’s surface.  That's two totalities for every three years. Since the distance to the moon varies, the width of the path of totality differs from one eclipse to another.  This width will change even during a single eclipse, because different parts of the Earth lie at different distances from the moon and also because of geometrical effects as the shadow falls at an oblique angle onto the Earth's surface.

But in any location on the Earth, a total eclipse will only occur once every 360 years. And the duration of any eclipse will be different.

Today’s eclipse is unique in the length of the eclipse over the Earth’s surface, the passing of the eclipse over land, the population with easy access to the path of totality, and the duration of the totality itself.

Explain why they occur and the reason for it to be so occasional.  Why is it sometimes partial and others total?

An eclipse takes place when one heavenly body such as a moon or planet moves into the shadow of another heavenly body. There are two types of eclipses on Earth: an eclipse of the moon and an eclipse of the sun. Sometimes when the moon orbits Earth, it moves between the sun and Earth. When this happens, the moon blocks the light of the sun from reaching Earth. This causes an eclipse of the sun, or solar eclipse. During a solar eclipse, the moon casts a shadow onto Earth.

There are three types of solar eclipses.

• The first is a total solar eclipse. A total solar eclipse is only visible from a small area on Earth. The people who see the total eclipse are in the center of the moon’s shadow when it hits Earth. The sky becomes very dark, as if it were night. For a total eclipse to take place, the sun, moon and Earth must be in a direct line.
• The second type of solar eclipse is a partial solar eclipse. This happens when the sun, moon and Earth are not exactly lined up. The sun appears to have a dark shadow on only a small part of its surface.
• The third type is an annular solar eclipse. An annular eclipse happens when the moon is farthest from Earth. Because the moon is farther away from Earth, it seems smaller. It does not block the entire view of the sun. The moon in front of the sun looks like a dark disk on top of a larger sun-colored disk. This creates what looks like a ring around the moon.

During a solar eclipse, the moon casts two shadows on Earth. The first shadow is called the umbra. This shadow gets smaller as it reaches Earth. It is the dark center of the moon’s shadow. The second shadow is called the penumbra. The penumbra gets larger as it reaches Earth. People standing in the penumbra will see a partial eclipse. People standing in the umbra will see a total eclipse.

Solar eclipses happen once every 18 months. The Moon does not orbit the Earth in a perfect circle, and its orbit has a slight angle from the equator of the Earth. Similarly, the Earth does not orbit the Sun in a perfect circle, and its orbit is not exactly aligned with the equator of the Sun. Therefore, every time the Moon passes between the Sun and the Earth, there are small differences in the positions and times that add up to the eclipse being short or long, partial or total.

How rare are they?

In an average town on Earth, a total eclipse will only occur once every 375 years. But the actual times are quite erratic. For example, in Spain there were 4 total eclipses in 63 years (1842, 1870,1900, 1905), while Jerusalem has a gap of 1108 years (1133, 2241)

Will the TSE 2017 in USA be visible in other countries?

The total eclipse will be visible only in the USA in a band stretching from the northwest coast to the southeast coast. However, a partial eclipse will be visible from many other locations. For example, a partial eclipse will be visible from Spain, and many other countries.

What are the usual effects of a TSE? For example, temperature drops.

During the totality, the sky will go dark. The temperature will drop, and cloud patterns may change as the heat of the sunlight into the atmosphere suddenly drops.

• 360-degree sunrise/sunset. While everyone’s attention will be focused on the sky, looking down can reveal another scene that not many have experienced. Spectators that look at the horizon during totality will witness the colors of sunrise and sunset around them in every direction.This 360-degree sunset effect is caused by the light from the sun in areas outside of the path of totality and only lasts as long as the face of the sun is covered by the moon.
• Stars and planets. As the moon causes day to turn to night, the darkness will reveal the stars in the sky as well as a few planets. The celestial alignment will also reveal the sun’s corona, the area of hot gas the surrounds the sun. A few lucky spectators may even be fortunate enough to see a meteor streak across the sky during the brief period of darkness.
• Temperature change. Not only will an eclipse cause the environment to appear different, but also feel different. “When sunlight fades at twilight, we always notice how things start to cool down. The same is true for the temporary dimming during a total solar eclipse,” NASA said. Depending on factors such as the time of year, cloud cover and the length of totality, the air temperature can drop more than 20 degrees F. A drop of about 10 degrees F is more likely.

How do you predict an eclipse?

In calculating a solar eclipse, one of the first steps is to determine the shadow's relation to the "fundamental plane," which passes through the Earth's center and is perpendicular to the moon-sun line. The path of the axis of the shadow across this plane is virtually a straight line. It is from this special geometry that the intersection of the moon's dark shadow cone with the rotating spheroid of our Earth must be worked out, using lengthy procedures in trigonometry. To say the least, these factors can make the calculations quite involved.

Are planets and stars visible during Solar Eclipses? How to identify them?

YES!

For instance, ,

• In Total Eclipse 2017, USA: four planets were visible to the unaided eye near the eclipsed sun. In order of brightness, these planets were Venus, Jupiter, Mars and Mercury. Mars is slightly brighter than Mercury, but you probably won’t notice a difference. About 15 to 30 minutes before totality, the planet Venus will gradually become clearly visible near the darkening sun. It will be to the west of the sun. About 30 seconds before and after totality, two other planets will appear. Close to the west side of our star will be Mars, appearing as an orange “star.” At a similar apparent distance, on the east side of the sun, you will see planet Mercury. Jupiter – the second-brightest planet in Earth’s sky – will also be very easy to spot, as the bright planet will be located farther to the southeast of the eclipsed sun.
• In Total Eclipse 2019, La Serena Chile: two planets will be visible, Mars and Mercury. Venus will be setting so difficult to see.

Why can’t we see the moon during a solar eclipse?

When we see the Moon, we are seeing sunlight reflected off of its surface. During the eclipse, the Moon is directly in front of the Sun, so we are seeing the dark side of the Moon.

Nevertheless, one can increase the exposure of images during totality and distinguish the Moon's topography on the reflected light from Earth's surface, like in this spectacular image. 

Why are shadows much sharper during the eclipse?

The change in lighting makes shadows look sharper on the ground, so it's possible to see individual hairs on your head in your shadow. When the Sun is a little more than half covered, the color of the sky will slowly become a duller shade of blue. Shadows on the ground will appear sharper as the apparent size of the Sun shrinks. If you look in the shadows of trees, the gaps in the shadows will begin to look weird. It may take a while for you to realize it, but there will be many crescent shapes in the shadows. The tiny gaps in the leaves will act like multiple pinhole cameras, projecting the Sun's image to the ground.

Umbra and Penumbra

• Umbra: the complete or perfect shadow of an opaque body, as a planet, where the direct light from the source of illumination is completely cut off.
• Penumbra: the partial or imperfect shadow outside the complete shadow of an opaque body, as a planet, where the light from the source of illumination is only partly cut off.

Why does the Eclipse move from West to East?

The Earth rotates counter-clockwise (as viewed looking down on the North Pole). The Moon also rotates around the Earth in this same, counter-clockwise direction. So the Moon moves between the Sun and the Earth from the West part of the sky, as seen from a spot on the Earth.

Shadow speed

Because of the geometry of the Earth’s shape, the shadow travels faster across its surface at the ends of the eclipse path, and slowest right in the middle.  The Moon’s shadow (also called the “Umbra”) is moving, for example (Total Solar Eclipse 2017, USA):

• 2410mph in Western Oregon
• 1747mph in central Nebraska
• 1462mph in Western Kentucky
• 1502mph near Charleston SC

Baily beads

• What is it? Why it occurs?

The Baily's beads effect is a feature of total solar eclipses. As the moon "grazes" by the Sun during a solar eclipse, the rugged lunar limb topography allows beads of sunlight to shine through in some places, and not in others. The name is in honor of Francis Baily who provided an exact explanation of the phenomenon in 1836. The diamond ring effect is seen when only one bead is left; a shining diamond set in a bright ring around the lunar silhouette. While Baily's beads are seen briefly for a few seconds at the center of the eclipse path, their duration is maximized near the edges of the path of the umbra, reaching 1–2 minutes.

• How tall are the mountains, valleys, etc. through which the light passes?

Lunar mountains can be as high as 5.5 kilometers. Lunar valleys can be as deep as 1 kilometer.

Shadow bands

• What are they?

Mysterious bands of shadow race across the landscape in the seconds before totality. Shadow bands are thin wavy lines of alternating light and dark that can be seen moving and undulating in parallel on plain-colored surfaces immediately before and after a total solar eclipse. Shadow bands have been noted throughout history. Scientists still don’t fully understand what they are. You can study them yourselves by taking measurements and photographs and come up with your own hypothesis.

Corona

• Why isn’t it visible normally?

The corona is not visible because it is not as bright as the main part of the Sun. The corona is 10−12 times as dense as the photosphere, and so produces about one-millionth as much visible light.

• What is it?

A corona (Latin for 'crown') is an aura of plasma that surrounds the sun and other stars. The Sun's corona extends millions of kilometers into space.  The sun's corona is much hotter (by a factor from 150 to 450) than the visible surface of the Sun: the photosphere's average temperature is 5800 kelvins compared to the corona's one to three million kelvins. The exact mechanism by which the corona is heated is still the subject of debate. The outer edges of the Sun's corona are constantly being transported away and hence generating the solar wind.

• What is the polarization of corona?

The Hanle effect is the modification of the linear polarization of a spectral line by a local magnetic field. Theoretically, direct determination of the magnetic field in the solar corona could be achieved through linear polarization of spectral lines with suitable sensitivity to the Hanle effect.

• Why is it white?

The innermost part of the corona is nearly white with small shift to green especially in active regions which is due to Fe XIV radiation. The color shifts to slightly reddish-white with increasing distance but yellowish-white areas also exist as the yellow color is the mixture of green and red color. Finally the corona disappears in the blue color of the sky in its outer part. In the universe out of the Earth's atmosphere, the corona would continuously extend as zodiacal light which is of a slightly reddish color.
Zodiacal light is a faint, diffuse, and roughly triangular white glow visible in the night sky that appears to extend from the vicinity of the Sun along the ecliptic or zodiac. It is caused by sunlight scattered by space dust in the zodiacal cloud around the Sun. It is best seen during twilight after sunset in spring and before sunrise in autumn, when the zodiac is at a steep angle to the horizon. However, the glow is so faint that moonlight and/or light pollution outshine it, rendering it invisible.

Chromosphere

• What is it?

The chromosphere (literally, "sphere of color") is the second of the three main layers in the Sun's atmosphere and is roughly 3,000 to 5,000 kilometers deep. The chromosphere's rosy red color is only apparent during eclipses. The chromosphere sits just above the photosphere and below the solar transition region.
The density of the chromosphere is only 10−4 times that of the photosphere, the layer beneath, and 10−8 times that of the atmosphere of Earth at sea level. This makes the chromosphere normally invisible and it can be seen only during a total eclipse, where its reddish color is revealed. The color hues are anywhere between pink and red. Without special equipment, the chromosphere cannot normally be seen due to the overwhelming brightness of the photosphere beneath.

• Why is it red?

The chromosphere emits a reddish glow as super-heated hydrogen burns off.

• Why is it interesting to see the solar spectrograph during the eclipse and the corona elements (H, Fe, Mg…)?

By analyzing the spectrum of the chromosphere, it was found that the temperature of this layer of the solar atmosphere increases with increasing height in the chromosphere itself. The temperature at the top of photosphere is only about 4,400 K, while at the top of chromosphere, some 2,000 km higher, it reaches 25,000 K.