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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Radio Programs

Different Explosions Exploding stars with a difference Saturday, July 19, 2014
Mars and Spica Two lights passing in the night sky Thursday, July 10, 2014
Moon, Mars, and Spica A close but wide-spread trio Saturday, July 5, 2014
Menkent The nearby shoulder of a centaur Monday, June 9, 2014
Moon and Venus Racking up the frequent-flyer miles Thursday, April 24, 2014
Emptiness The emptiness between the stars Monday, April 21, 2014
Moon and Antares A journey to a stellar supergiant Friday, March 21, 2014
More Moon and Companions Getting close to bright Mars Tuesday, March 18, 2014
Jupiter at Opposition Pacing off the distance to a giant Sunday, January 5, 2014
Uncertain Giant Taking the measure of a giant star Saturday, November 23, 2013
Andromeda Galaxy A journey into the distant past Thursday, November 7, 2013
The Crane A crane strides across the southern sky Thursday, October 31, 2013
Gaia II Mapping a billion stars Friday, October 4, 2013
Gaia Counting up the stars Thursday, October 3, 2013
Moon and Aldebaran Looking 67 years back in time Tuesday, August 27, 2013
Moon and Spica A small but vast astronomical gap Tuesday, May 21, 2013
Model Cosmos Taking the measure of the solar system Friday, May 3, 2013
Yardsticks Measuring astronomical distances Saturday, February 23, 2013
Polaris III Poking into the life of a famous star Thursday, February 21, 2013
Milky Way Matters The Milky Way bulks up Saturday, February 9, 2013
Epsilon Geminorum A star by the numbers Thursday, January 17, 2013
First Steps Angling in on the stars Wednesday, December 5, 2012
RR Lyrae The rhythmic beat of a star Tuesday, December 4, 2012
Aries II Taking the measure of a nearby star Sunday, October 28, 2012
Moon and Venus Long pauses in long-distance calls Thursday, October 11, 2012
The Horse’s Nose The flying horse’s bright nose Sunday, September 23, 2012
Venus and Aldebaran Bright and brighter in the dawn sky Friday, July 6, 2012
Moon and Antares Taking the measure of a jumbo star Saturday, June 30, 2012
More Moon and Companions Turning on some far-away lamps Wednesday, June 27, 2012
Venus Transit Measuring out a solar-system ruler Monday, June 4, 2012
The Right Angle Taking a bead on the lion’s tail Sunday, March 18, 2012
Moon and Companions Lining up some bright companions Sunday, December 18, 2011
Andromeda Galaxy Looking deep into the universe Tuesday, October 25, 2011
Milky Way’s Heart Taking a peek at a busy downtown Monday, September 26, 2011
Moon and Spica A tiny measurement of a big distance Friday, July 8, 2011
Moon and Regulus Filtering out the starlight Monday, July 4, 2011
Brightest X-Ray Burst The brilliant death of a distant star Monday, June 20, 2011
Transit of 1761 Spreading out to watch a little black dot Monday, June 6, 2011
Revolutionary Star Looking for a "revolutionary" star Sunday, April 10, 2011
Sirius The long path to a nearby star Sunday, January 16, 2011
Moon and Regulus Taking the measure of a neighbor Sunday, October 3, 2010
3C48 A quasi-stellar mystery Sunday, September 26, 2010
Moon and Jupiter An experiment that could use a dust cloth Saturday, July 31, 2010
Alkaid Getting a handle on the Big Dipper Sunday, May 9, 2010
Lynx Padding after a faint celestial cat Saturday, February 13, 2010
Moon and Spica A bright star gets closer Thursday, December 10, 2009
Moon, Mars, and Regulus Taking the measure of the night sky Sunday, December 6, 2009
Andromeda Galaxy Gazing into the far-distant past Friday, October 16, 2009
Eta Aquilae The rhythm of a pulsating star Thursday, September 10, 2009
Deneb A star loses some of its luster Tuesday, August 25, 2009
Hipparcos Taking the measure of the stars Saturday, August 8, 2009
Shooting the Moon Getting ready to shoot the Moon Friday, July 17, 2009
Moon and Antares Looking for a star that may not be there Friday, July 3, 2009
Distant Dubhe A light from the 19th century Saturday, June 13, 2009
Barnard's Star A star that could use a better agent Tuesday, June 2, 2009
Moon and Saturn Stocking up for a long roadtrip Sunday, May 3, 2009
Stellar Yardsticks Calibrating a cosmic yardstick Friday, March 27, 2009
Moon, Regulus, and Saturn A "light" journey through the night Tuesday, January 13, 2009
Distance to Sirius Remeasuring the distance to a star Thursday, January 8, 2009

Featured Images

Orion, the hunter, including his faint shield
Shielding the Hunter Friday, February 17, 2012
Venus's path across the Sun during the transit of 1761
Bright Passage Monday, June 6, 2011
Neighbors Tuesday, April 14, 2009

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