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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

Saturn at Opposition A bright but distant giant August 25, 2023

Moon and Spica A good “semi-quincentennial” star August 20, 2023

Earth at Aphelion An important astronomical yardstick July 6, 2023

Moon and Companions Lining up a celestial block party May 22, 2023

Moon, Venus, Aldebaran A time delay in the night sky April 22, 2023

Moon and Twins Some looong trips through the cosmos March 2, 2023

Close Neighbor A bright galaxy that’s far, far away October 22, 2022

Moon and Jupiter A slight delay in the night sky September 10, 2022

Puppis Looking across time and space March 14, 2022

Canopus Profiling a big, bright star March 6, 2022

Moon and Spica “Wiggle room” in the distance to a star February 19, 2022

Close Neighbors Invisible neighbors for Earth February 16, 2022

Camelopardalis A tall but faint giraffe November 26, 2021

Messier 31 Looking deep into the universe November 6, 2021

The Crane A crane strides across the southern sky November 1, 2021

Moon and Aldebaran A bright star slashes across the sky October 23, 2021

Venus and Spica A bright illusion in the evening sky September 1, 2021

Jupiter at Opposition “Wandering” across the night sky August 19, 2021

Moon and Zubenelgenubi Stepping back in time August 14, 2021

Mars, Venus, Moon Three neighbors huddle close together July 11, 2021

Moon and Spica Plotting the motions of a bright star June 19, 2021

Gliese 710 A possible future menace gets closer June 16, 2021

Mars and the Twins Mars drives into the sunset June 6, 2021

Farthest Stars A star at the edge of the galaxy May 15, 2021

Mile Markers Mile markers in the distant universe April 21, 2021

GN-z11 A trip to the early universe April 12, 2021

Puppis An impressive star in the poop deck March 31, 2021

Mystery Explosion An “in-between” stellar explosion March 6, 2021

Cursa “Blinking” the distance to a star February 12, 2021

Moon and Spica The end of a long, long journey February 1, 2021

Top Dog The long path to the Top Dog January 11, 2021

Bellatrix Pondering the fate of a giant star January 4, 2021

Betelgeuse Conflicting reasons for a big fade December 14, 2020

Speedy Altair A close neighbor that moves fast August 27, 2020

Brighter Vega A bright star with a brighter future July 28, 2020

Moon and Planets Measuring out the solar system July 5, 2020

Alnilam A star that’s really a superstar March 12, 2020

Emptiness The emptiness between the stars December 8, 2019

Andromeda Galaxy A journey into the distant past October 26, 2019

Moon and Companions A star that might not be there October 2, 2019

Moon and Spica Closing the distance to a star August 5, 2019

Vega A long trip to a nearby star July 5, 2019

Baade’s Window A window into the heart of the galaxy July 1, 2019

The Coathanger Hanging up a star cluster June 22, 2019

Moon and Regulus How time can stand still January 22, 2019

Moon and Regulus Shaving parsecs off the Kessel run November 1, 2018

Lacerta New distances to the lizard September 9, 2018

NGC 6397 Taking the measure of a star cluster August 12, 2018

61 Cygni Zeroing in on a nearby star August 4, 2018

Vindemiatrix Taking the measure of a giant July 29, 2018

Mars and Company Bright sights in the night sky July 24, 2018

Summer Milky Way The outline of our galactic home July 8, 2018

Far from the Sun Measuring the distance to the Sun July 6, 2018

Ancient Star Looking deep into the past June 17, 2018

Moon and Regulus The life expectancy of a light ray May 21, 2018

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

Featured Images

the observable universe

Everything There is to See August 2, 2022

Busy Neighbor October 26, 2019
Hubble view of NGC 6397, a globular star cluster

Stellar Lab August 12, 2018
Artist's concept of Scholz's Star passing close to the solar system

Former Neighbor May 7, 2015