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The Sun, Transits and Eclipses: Shadows of the Sun

About Our Sun

Our sun's average distance from Earth is 150 million km (93 million miles); the Earth's orbit is slightly elliptical, so that distance varies over the course of a year. The Earth is closest to the sun (it reaches perihelion) in January. Our sun is about 109 times wider than the Earth.

Like Earth, our sun has many different layers. Unlike Earth, our sun is made of plasma (simplistically called ionized gas). While approximately 60 different elements make up our sun's gases, hydrogen accounts for about 92 percent of the atoms (almost three-fourths of the mass) and helium makes up most of the rest (7.8 percent of the atoms).

The sun's energy is generated in its core, through nuclear fusion as hydrogen is gradually converted to helium. Gravitational pressures compress and heat the material in the core to over 15 million degrees Celsius or about 27 million degrees Fahrenheit!

Energy passes from the core into the radiative zone. Here the energy (radiation) moves randomly from atom to atom, with some of the energy moving toward the sun's surface. As energy moves up out of the radiative zone, it enters the convective zone. Here the atoms do not pass the energy from particle to particle; the atoms themselves move, carrying the heat with them. The hotter material near the radiative zone rises to the cooler surface of the convective zone. As it reaches the top of the convective zone, it cools and sinks.

Our sun has three layers in its atmosphere: the photosphere, the chromosphere and the corona. The photosphere (above the convective zone) is the "surface" of our sun; because our sun is made of gas, it does not have a solid surface. The photosphere has temperatures that reach about 5,525 degrees Celsius (9,975 degrees Fahrenheit) and is the layer that releases most of the light that reaches Earth.

The surface of our sun has continuously changing dark regions -- sunspots. The spots are dark because they are cooler than the surrounding gas (about 3,230 degrees Celsius or 5,850 degrees Fahrenheit). Sunspots can persist for an hour to several months. The number of sunspots increases and decreases in an 11-year cycle -- the solar cycle.

The chromosphere ("sphere of color") is a 2000-km-thick layer of gas above the photosphere. Most of the energy from the chromosphere is released as red light, which means that the chromosphere can be viewed with special telescopes that filter out the other wavelengths. The chromosphere is dynamic; convection cells swirl the surface, and material shoots off the surface as flame-like features.

The corona is the outer layer of our sun that is seen during a solar eclipse. The corona emits energy at many different wavelengths. Loops and arches of matter are often seen extending out from the corona along lines of the sun's magnetic field. This material flows away from our sun as the solar wind. Some of the particles reach Earth's atmosphere and interact with atmospheric particles to create the aurora.

For more information about the sun, go to SOHO and

For information about the sun's interaction with Earth's magnetic field, check out the YSS topic Magnetospheres.

All About Eclipses

Eclipses are only possible when, during its monthly revolution around the Earth, the Moon lines up with the Earth and the sun. The Moon's orbit around the Earth is titled by 5 degrees so that the Moon usually passes a little higher or a little lower, rather than lining up exactly with the Earth and sun. A solar eclipse happens at New Moon when the Moon is directly between the Earth and sun; a lunar eclipse happens at Full Moon when the Earth is directly between the sun and the Moon. There are at least 4 eclipses each year.

A partial solar eclipse is visible over a large portion of the Earth. However, a total solar eclipse can only be seen from within a narrow track known as the path of totality. This path is typically just 100 miles wide, so people often travel great distances just to see a total solar eclipse. During a lunar eclipse, anyone on the Earth facing the Moon can see the eclipse; a partial lunar eclipse is not particularly striking and often ignored by the public, while a total lunar eclipse is widely viewed.

More information about solar eclipses is at
More information about lunar eclipses is at

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Shadows of the Sun Videos

Titles and Introduction
Solar Eclipses
Lunar Eclipses
Transit of Venus and Mercury
Occultations Reveal Planets Around other Stars
Occultations and Transits Provide New Science
Questions and Answers
Education and Public Outreach

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Last Updated: 12 Sep 2014