| A | B |
|---|
| astronomical unit (AU) | A unit of astronomical distance equal to the earth's average distance form the sun. (93 million miles) |
| solar spectrum | The entire span of electromagnetic wavelengths that the sun emits. |
| nuclear fusion | A nuclear change where small atomic nuclei are forcefully combined to make larger nuclei, releasing tremendous amounts of energy as nuclear mass is converted to heat and light. |
| photosphere | The visible surface of the sun. |
| chromosphere | The lowest layer of the sun's atmosphere, starting at the photosphere and extending out to 1200 mi from the sun's surface. |
| corona | The hot outer atmosphere of the sun that extends far out into space to become solar wind. |
| prominence | A stream of solar plasma that rises into the corona from the chromosphere, then gradually falls back. |
| solar wind | The flow of mostly protons, electrons, and small atomic nuclei ejected from the sun outward into the solar system. |
| sunspot | A small, cool area on the sun's photosphere. |
| solar flare | A sudden explosion of the plasma from the sun's surface that emits a burst of energetic rays and particles into space; large flares are called coronal mass ejections. |
| ray | A bright streak that radiateds from some craters on the moon's surface |
| mare | A broad, lowland plain on the moon made of hardened lava. |
| rile | A long, narrow valley on the moon believed to have originated from various tectonic processes. |
| ecliptic | The sun's apparent path among the stars as observed from Earth; equivalent to the plane of the earth's orbit |
| revolution | Motion of an astronomical body around another astronomical body that is the center of an orbital system. |
| rotation | The spin of an object around an imaginary axis that passes through its center. |
| lunar phase | The regular, repeatedly changing appearance of the shape of the lighted face of the moon as seen from Earth. |
| terminator | The line or edge separating the light and dark sides of any astronomical body. |
| solar eclipse | An astronomical event that occurs when the sun, earth, and moon are aligned so that the moon blocks a portion of the sunlight falling on the earth. |
| lunar eclipse | Astronomical event that occurs when the sun, earth, and moon are aligned so that the earth blocks the sunlight falling on the moon; occurs only a full moon. |
