Astro Glossary

Solar Wind

A flow of atomic nuclei and electrons from the Sun that travels at an average speed of about two million miles per hour (400 km/s).

Solstices and Equinoxes

The solstice is an event in the Earth’s orbit during which the tilt of the Earth’s axis is pointed most directly towards or away from the Sun. The summer solstice for the northern hemisphere occurs within a few days of June 21 every year. It is on this day that the position of the Sun in the sky at noon is at its highest altitude of the year, and the position of the Sun at Sunrise and Sunset is farthest north for the year. The winter solstice is around December 21, marking the date on which the Sun is lowest in the sky at noon and rises and sets farthest south. The day of the summer solstice is the longest day of the year, and the day of the winter solstice is the shortest day of the year. The equinox is the time of year when the Sun appears in the sky at the intersection of the ecliptic and the celestial equator. Two such points exist: The vernal equinox (approx. March 21) marks the start of spring. The autumnal equinox (approx. Sept. 20) marks the start of autumn. At each of these points, the Sun rises due east and sets due west.

Sombrero Galaxy, M104

Messier 104 is one of the most photogenic galaxies. A lane of dark dust outlines its edge like the brim of a hat, giving M104 a distinctive nickname: the Sombrero. It is in the constellation Virgo, and is visible through small telescopes.

South Pole

Soviet/Russian Space Program

Space Exploration

Space Junk

Space junk is debris that orbits Earth, presenting hazards to operating spacecraft. The debris includes dead satellites and booster stages, pieces of exploded boosters, pieces of spacecraft, and even bits of equipment accidentally lost from manned space missions. The amount of space junk is increasing at a frenetic pace as companies launch thousands of new communication satellites into orbit. The United States Space Force tracks debris down to just a few centimeters (inches) across, and provides warnings to satellite operators when one of their craft is threatened. Debris is a special problem for the International Space Station, which is the largest craft ever placed in orbit. Astronauts aboard the station sometimes have to retreat to their return capsules when space debris is predicted to come especially close. Several space agencies are studying ways to reduce the amount of space junk, which could make space flight especially hazardous in the years to come.

Space Observatories

Space Shuttle

Space Stations