# 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.

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Discovering a one-time neighbor Thursday, May 7, 2015
A “runaway train” in the stars Wednesday, May 6, 2015
Measuring the girth of a big star Saturday, March 7, 2015
Pinning down a giant star Monday, February 23, 2015
Looking deep into the universe Saturday, December 20, 2014
Measuring the solar system with a ruler Wednesday, December 10, 2014
A 20-parsec jaunt across the galaxy Friday, December 5, 2014
Discovering the expanding universe Friday, November 21, 2014
The Seven Sisters move farther away Tuesday, October 28, 2014
Pacing off the distance to the North Star Thursday, October 23, 2014
Looking into the past of our home galaxy Tuesday, September 30, 2014
Taking a census of our stellar neighborhood Wednesday, August 6, 2014
Exploding stars with a difference Saturday, July 19, 2014
Two lights passing in the night sky Thursday, July 10, 2014
A close but wide-spread trio Saturday, July 5, 2014
The nearby shoulder of a centaur Monday, June 9, 2014
Racking up the frequent-flyer miles Thursday, April 24, 2014
The emptiness between the stars Monday, April 21, 2014
A journey to a stellar supergiant Friday, March 21, 2014
Getting close to bright Mars Tuesday, March 18, 2014
Pacing off the distance to a giant Sunday, January 5, 2014
Taking the measure of a giant star Saturday, November 23, 2013
A journey into the distant past Thursday, November 7, 2013
A crane strides across the southern sky Thursday, October 31, 2013
Mapping a billion stars Friday, October 4, 2013
Counting up the stars Thursday, October 3, 2013
Looking 67 years back in time Tuesday, August 27, 2013
A small but vast astronomical gap Tuesday, May 21, 2013
Taking the measure of the solar system Friday, May 3, 2013
Measuring astronomical distances Saturday, February 23, 2013
Poking into the life of a famous star Thursday, February 21, 2013
The Milky Way bulks up Saturday, February 9, 2013
A star by the numbers Thursday, January 17, 2013
Angling in on the stars Wednesday, December 5, 2012
The rhythmic beat of a star Tuesday, December 4, 2012
Taking the measure of a nearby star Sunday, October 28, 2012
Long pauses in long-distance calls Thursday, October 11, 2012
The flying horse’s bright nose Sunday, September 23, 2012
Bright and brighter in the dawn sky Friday, July 6, 2012
Taking the measure of a jumbo star Saturday, June 30, 2012
Turning on some far-away lamps Wednesday, June 27, 2012
Measuring out a solar-system ruler Monday, June 4, 2012
Taking a bead on the lion’s tail Sunday, March 18, 2012
Lining up some bright companions Sunday, December 18, 2011
Looking deep into the universe Tuesday, October 25, 2011
Taking a peek at a busy downtown Monday, September 26, 2011
A tiny measurement of a big distance Friday, July 8, 2011
Filtering out the starlight Monday, July 4, 2011
The brilliant death of a distant star Monday, June 20, 2011
Spreading out to watch a little black dot Monday, June 6, 2011
Looking for a "revolutionary" star Sunday, April 10, 2011
The long path to a nearby star Sunday, January 16, 2011
Taking the measure of a neighbor Sunday, October 3, 2010
A quasi-stellar mystery Sunday, September 26, 2010
An experiment that could use a dust cloth Saturday, July 31, 2010
Getting a handle on the Big Dipper Sunday, May 9, 2010
Padding after a faint celestial cat Saturday, February 13, 2010

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