# Astro Glossary

• ## Astrometry

Measuring an object’s position in the sky.

• ## Astronauts and Cosmonauts

Astronauts and cosmonauts are trained personnel who travel in space. “Astronaut” refers to an American or western European space traveler and “cosmonaut” refers to a Soviet or Russian space traveler. China calls its space travelers “taikonauts.”

• ## Astronomers

Scientists who study the universe beyond Earth, from the planets and moons of the solar system to the birth and fate of the universe.

• ## Astronomical Distances

Distances in space are often measured in astronomical units, light-years, or parsecs.An astronomical unit is the average from Earth to the Sun, about 93 million miles (150 million km), and is used to measure relatively short distances, such as those between the Sun and its planets or between the stars in a binary system.A light-year is the distance light travels in one year, which is about 5.88 trillion miles or almost 800 times the diameter of the solar system. The nearest star is 4.2 light-years away, while the nearest spiral galaxy lies about 2.5 million light-years from Earth.A parsec is a unit of distance equal to 3.26 light-years. The name means “PARallax-SECond,” and it refers to a way to measure the distances to other stars. The most accurate way to measure the distances to close stars is to use basic geometry. Astronomers measure the position of a star in the sky at six-month intervals, when Earth is on opposite sides of the Sun. If the star is close, then it will appear to shift a bit compared to the background stars. It’s the same effect you see if you hold your finger in front of your face and look at it with first one eye, then the other: the finger appears to move against the background of objects. This effect is called parallax. If a star has a parallax of one second — in other words, if it appears to shift back and forth across the sky by exactly one second of arc (1/3600 degree) — then its distance is one parsec.

• ## Auriga, the Charioteer

A constellation that cruises high across the sky in January and February.

• ## Aurorae (Northern Lights)

Beautiful ribbons of light caused by the interaction of high-energy particles in the solar wind and Earth’s magnetic field. These are common in both extreme northern (aurora borealis or northern lights) and southern latitudes (aurora australis/southern lights), near Earth’s magnetic poles.

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