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

Light-Year Speeding across the universe Friday, November 20, 2015
Pleiades II Taking a census of a star cluster Tuesday, November 10, 2015
Moon and Companions Bright companions for the Moon Wednesday, October 7, 2015
Far, Far Away Mapping the distance to the galactic center Friday, August 14, 2015
Moon and Betelgeuse Tracking a big, brilliant star Sunday, August 9, 2015
Delta Cephei A surprise from a well-known star Monday, August 3, 2015
Former Neighbor Discovering a one-time neighbor Thursday, May 7, 2015
Fast Neighbor A “runaway train” in the stars Wednesday, May 6, 2015
Arcturus Measuring the girth of a big star Saturday, March 7, 2015
Canopus II Pinning down a giant star Monday, February 23, 2015
Deep Vision Looking deep into the universe Saturday, December 20, 2014
Moon and Jupiter Measuring the solar system with a ruler Wednesday, December 10, 2014
Moon and Aldebaran A 20-parsec jaunt across the galaxy Friday, December 5, 2014
More Edwin Hubble Discovering the expanding universe Friday, November 21, 2014
New Distance The Seven Sisters move farther away Tuesday, October 28, 2014
Polaris Pacing off the distance to the North Star Thursday, October 23, 2014
Distant Hub Looking into the past of our home galaxy Tuesday, September 30, 2014
RECONS Taking a census of our stellar neighborhood Wednesday, August 6, 2014
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

Featured Images

Artist's concept of Scholz's Star passing close to the solar system
Former Neighbor Thursday, May 7, 2015
The Andromeda galaxy, M31
Old News Saturday, December 20, 2014
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


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