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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.

Resources

Facts and Figures June 10, 2010

Astronomers and Observatories June 10, 2010

Radio Programs

Emptiness The emptiness between the stars July 23, 2017

Far, Far Away Mapping the distance to the galactic center July 22, 2017

Moon, Mars, and Aldebaran Angling in on a star’s distance April 28, 2017

M100 The grandeur of a spiral galaxy April 19, 2017

Moon, Jupiter, Spica A 3-D journey in the night sky February 14, 2017

Leading the Dog One dog leads another across the sky January 21, 2017

Birth of a Giant Locating the birthplace of a giant December 29, 2016

New Missions Stars and ripples for two spacecraft December 19, 2016

Moon and Regulus Lining up a pair of stellar lamps November 20, 2016

Big Neighbor A bright galaxy far, far away October 23, 2016

Barnard’s Star Discovering a close but faint neighbor June 6, 2016

Moon and Spica The end of a long journey February 26, 2016

Pollux Mapping out a nearby star February 20, 2016

Moon and Companions Bright companions for the Moon October 6, 2015

Far, Far Away Mapping the distance to the galactic center August 13, 2015

Moon and Betelgeuse Tracking a big, brilliant star August 8, 2015

Delta Cephei A surprise from a well-known star August 2, 2015

Former Neighbor Discovering a one-time neighbor May 6, 2015

Fast Neighbor A “runaway train” in the stars May 5, 2015

Arcturus Measuring the girth of a big star March 6, 2015

Canopus II Pinning down a giant star February 22, 2015

Deep Vision Looking deep into the universe December 19, 2014

Moon and Jupiter Measuring the solar system with a ruler December 9, 2014

Moon and Aldebaran A 20-parsec jaunt across the galaxy December 4, 2014

More Edwin Hubble Discovering the expanding universe November 20, 2014

New Distance The Seven Sisters move farther away October 27, 2014

Polaris Pacing off the distance to the North Star October 22, 2014

Distant Hub Looking into the past of our home galaxy September 29, 2014

RECONS Taking a census of our stellar neighborhood August 5, 2014

Different Explosions Exploding stars with a difference July 18, 2014

Mars and Spica Two lights passing in the night sky July 9, 2014

Moon, Mars, and Spica A close but wide-spread trio July 4, 2014

Menkent The nearby shoulder of a centaur June 8, 2014

Moon and Venus Racking up the frequent-flyer miles April 23, 2014

Emptiness The emptiness between the stars April 20, 2014

Moon and Antares A journey to a stellar supergiant March 20, 2014

More Moon and Companions Getting close to bright Mars March 17, 2014

Jupiter at Opposition Pacing off the distance to a giant January 4, 2014

Uncertain Giant Taking the measure of a giant star November 22, 2013

Andromeda Galaxy A journey into the distant past November 6, 2013

The Crane A crane strides across the southern sky October 30, 2013

Gaia II Mapping a billion stars October 3, 2013

Gaia Counting up the stars October 2, 2013

Moon and Aldebaran Looking 67 years back in time August 26, 2013

Moon and Spica A small but vast astronomical gap May 20, 2013

Model Cosmos Taking the measure of the solar system May 2, 2013

Yardsticks Measuring astronomical distances February 22, 2013

Polaris III Poking into the life of a famous star February 20, 2013

Milky Way Matters The Milky Way bulks up February 8, 2013

Epsilon Geminorum A star by the numbers January 16, 2013

First Steps Angling in on the stars December 4, 2012

RR Lyrae The rhythmic beat of a star December 3, 2012

Aries II Taking the measure of a nearby star October 27, 2012

Moon and Venus Long pauses in long-distance calls October 10, 2012

The Horse’s Nose The flying horse’s bright nose September 22, 2012

Venus and Aldebaran Bright and brighter in the dawn sky July 5, 2012

Moon and Antares Taking the measure of a jumbo star June 29, 2012

More Moon and Companions Turning on some far-away lamps June 26, 2012

Venus Transit Measuring out a solar-system ruler June 3, 2012

The Right Angle Taking a bead on the lion’s tail March 17, 2012

Featured Images

Artist's concept of Scholz's Star passing close to the solar system

Former Neighbor May 7, 2015
The Andromeda galaxy, M31

Old News December 20, 2014
Orion, the hunter, including his faint shield

Shielding the Hunter February 17, 2012
Venus's path across the Sun during the transit of 1761

Bright Passage June 6, 2011