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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

Radio Programs

Moon and Companions Changing distance, changing brightness February 7, 2018

More Moon and Companions Stepping toward the stars December 14, 2017

Close Encounters A dangerous encounter with a star November 1, 2017

Future Blast A future blast from a galactic neighbor October 27, 2017

Moon and Saturn Heading out for space-y adventures September 26, 2017

Milky Way Center Discovering the galaxy’s distant heart August 26, 2017

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

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