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Q: astronomy ( Answered 5 out of 5 stars,   4 Comments )
Subject: astronomy
Category: Miscellaneous
Asked by: mongolia-ga
List Price: $20.00
Posted: 16 Nov 2003 05:57 PST
Expires: 16 Dec 2003 05:57 PST
Question ID: 276376
When distances are quoted to Astronomical Objects I would like to know what
 would be the approximate error in the distance give? I am obviously
not looking for an exact measurement but lets say it could a guide(
1cm , 1 metre,
1 kilometre, 100000 kilometre, one tenth of a light year, 1 light year, 100
 light years etc)

 Here are some specific objects wher I would like to know the accuracy
 of the distance measured:

 - Moon
 - Pluto
 - Neptune
 - Proxima Centauri
 - The star Rigel
 -  Globular Cluster in Hercules
 - Androemda Galaxy
 - A Galaxy in the Virgo group
 - The quasar 3c273


  For an extended object such as a star cluster or Galaxy , I would be 
  looking for the accuracy to a particular point or perhaps star within
  the object.

Request for Question Clarification by techtor-ga on 16 Nov 2003 07:00 PST
Hello Mongolia,
May I know if you mean distances of the objects you listed from the
earth, or from another point?

Clarification of Question by mongolia-ga on 19 Nov 2003 12:56 PST
To clarify I would say for stellar objects (e.g. a globular cluster) it
 could be to the central point of the sun. It could also be to the
centre of teh earth at a specific point in time. If we know the
distance to Proxima centauri for example to such an accuracy that the
earth's point in its orbit around the sun would make a difference
,then of course I would  talking about the distance at a specific
point in time. It if serves to simplfly the issue then perhaps it
should be to the cental point of the sun.

 Regarding the issue brought by the commentor , I would of course be
looking at the latest and what is considered the most accurate method.
If different methods of measuring a distance yield different answers,
this in of itself would determine the error in the measurement of the

  I fully accept that ther is a certain subjectivity in answering this question.


Clarification of Question by mongolia-ga on 22 Nov 2003 05:21 PST
any progress on this?

Request for Question Clarification by techtor-ga on 22 Nov 2003 08:42 PST
Hello Mongolia,
Your question proved to be more difficult that I thought. I haven't
found any web pages discussing accuracy of astronomical distances
since it does not seem to be a very popular topic for discussion these
days. I'll keep on looking though.

Request for Question Clarification by techtor-ga on 22 Nov 2003 08:58 PST
I had forgotten to mention, what makes this difficult for me (on the
premise that you are looking for the margin of error in the recorded
measurements) is that bodies in space do not have perfectly circular
orbits, so the distance of one body from another actually varies, and
there seems to be no definite standard to use in getting the margin of
error. I doubt if even average distance is hard to use as a standard
since the celestial bodies are in constant motion, so it seems futile
to test how accurate one's distance measurement is.

If you're looking for something about the accuracy of the methods used
to measure distance, I haven't found anything yet on it. If I can't,
perhaps someone else with a better knowledge of astronomical metrics
Subject: Re: astronomy
Answered By: hlabadie-ga on 23 Nov 2003 08:49 PST
Rated:5 out of 5 stars
The distance to the Moon has been measured by Lunar Laser Ranging,
utilizing lasers that are transmitted through telescopes on Earth to
reflectors that had been placed on the lunar surface by Apollo
astronauts and a Russian lunar lander. The distances have been
calculated so precisely that fractional differences in centimeters
have been measured. The Moon is receding from the Earth at a rate of
3.8 cm per year.

Gravitational and Relativistic Physics (GRP)

"Ranging has provided the most accurate technique available for
measuring the Moon's rotation and orbit. It has also determined that
the length of an Earth day has distinct small-scale variations. Also
observed are crustal plate drifts on Earth. Measurements show that the
Moon is moving away from the Earth at a rate of about 3.8 centimeters
per year. Laser ranging has also made possible a wealth of new
information about the dynamics and structure of the Moon."

ILRS Satellite Missions

Distances to the inner planets of the Solar System have been
determined by radar ranging, in a manner similar to that used in laser
ranging of the moon. The distances to the outer planets Neptune and
Pluto are such that traditional means of measurement by triangulation
are used. Observations from widely spaced Earth observatories provide
parallax measurements from which the distances to the most distant
planets can be calculated. Calculations are based upon tables for
planetary ephemerides, the most accurate being produced by the Jet
Propulsion Laboratory, which directs many interplanetary missions and
must have precise figures by which navigation of spacecraft can be
accomplished. The margin of error for the planets Neptune and Pluto
are such that respective errors of 4000 and 10000 km are normal.

The Solar System

"Pluto, normally the most remote planet, has an average distance of
5,913,520,000 km from the Sun."
"One of the most accurate ways to measure the distances to the planets
is by bouncing radar off them, or sending a spacecraft there, which
can send a radio signal back to the Earth that can be timed. Radar is
essentially microwave electromagnetic radiation (microwaves fall under
the radio spectrum). Since electromagnetic radiation, in all of its
forms, is light, we know that radar travels at the speed of light - 3
x 10^5 km/s. We know that a distance traveled is equal to the time it
takes to travel that distance, times the velocity used to travel that
distance (x=vt). If we bounce radar off a planet, and measure the time
it takes the signal to get there and back, we can use this information
to calculate the distance of the planet.

In the absence of spacecraft or radar, one can, in principle, use
triangulation to measure the distance to a planet. That is, measure
the celestial position of a planet from two different points on Earth
simultaneously; knowing the difference in the angles that were
measured, and the distance between the two points on Earth, the
distance to the planet can be calculated. Even with modern technology,
this is not trivial - even with a baseline of 10,000 km (about as long
a baseline that is possible on Earth), the angular offset at two sites
must be measured to be accurate to 4 seconds of arc (1/450th of the
diameter of a full moon) to achieve ~10% accuracy in the distance to
Venus, our nearest neighbor."


"This method has been used in one form or another to determine the
distances to all of the planets in our solar system (except Pluto,
which we have not visited)."

Description of JPL Solar System Ephemeris


"One method of describing the accuracy of the positions provided in
DE403 is to consider the angles between various ephemeris objects as
viewed from Earth.

If the two objects observed are taken from the list (Sun, Mercury,
Venus, Mars, Moon) the angles computed from the ephemeris positions
are accurate to one or two milli-arcsecond.(This assumes all
appropriate corrections are applied for light time, stellar
aberration, and relativistic effects to the ephemeris derived
positions.)  If you add the barycenter of Jupiter to this list,
uncertainty in ephemeris derived angles may grow to a few hundredths
of an arcsecond.  Adding Saturn, Uranus and Neptune will raise the
uncertainty level to approximately 0.1 arcseconds.  Finally, adding
Pluto to the list raises the observable errors to 0.3 arcseconds for
the present and increasing into the future.

Radial distances to the centers of objects follow a similar trend. The
radial distances between the inner objects of the solar system as
computed via the ephemeris are accurate to 1 to 2 km.  The distance
between the Earth and the Jupiter Barycenter is accurate to better
than 10 km.

The uncertainty in the distances to Saturn, Uranus and Neptune are
approximately 1000, 2000 and 4000 km respectively.  For Pluto, the
radial distance from earth may be in error up to 10000 km for the
present and growing into the future."

New Accuracy Levels for Solar System Ephemerides


"The JPL HORIZONS /On-Line/ Solar System Data and Ephemeris
Computation Service provides access to key solar system data and
flexible production of highly accurate ephemerides for solar system
objects (163000+ asteroids and comets, 128 natural satellites, 9
planets, the Sun, L1, L2, select spacecraft, and system barycenters)."

The Hipparcos Space Astrometry Mission

The Hipparcos and Tycho-1 Databases

The Tycho-2 Database

The Guide Star Catalog

Based upon the observations of the European Space Agency's (ESA) High
Precision Parallax Collecting Satellite (HIPPARCOS), within 30 light
years of Earth, the accuracy of distance measurements is about 1 per
cent. Within 150 ly, the accuracy is about 5 per cent. Within 300 ly,
the accuracy is about 10 per cent. Beyond those distances, however,
the error and be as high as 50 percent, the greater the measured
distance from our solar system.

Extremely large distances are calculated by measuring the Doppler
shift of spectra, and this requires an accurate value for the Hubble
constant. The different means by which a value for the Hubble constant
can be determined can yield differing numbers, and thus differing
results from the calculations for distance can result. The standard
means of getting a value is to find some distant object to which a
reasonably firm distance can be attached. Choosing such an object is,
however, contentious. One generally accepted means uses Cepheid
variable stars. This method is itself disputed, however, by some
astronomers, because of the underlying assumptions, themselves of
doubtful validity. A second method relies upon masers, and has fewer
assumptions. It gives a significantly different value. There is no
agreement on the better method. Therefore, even distances within the
Milky Way galaxy have a large margin of error built into them.

Distances to the Sun and Stars

"Hipparcos worked by observing stars through two telescopes aimed 58
degrees apart. The light from the two telescopes was merged into a
detector with a fine grid of wires. As the satellite rotated,
different stars passed through the field of view of each telescope and
blinked on and off as the stars passed across the grid of wires. These
observations allowed extremely accurate relative positions of the
stars to be determined. The relative positions of all the stars could
then be combined into an extremely accurate catalog of star positions
across the entire sky.

For 118,000 selected stars, Hipparcos measured their parallax accurate
to .001 second of arc. That's the apparent diameter of a quarter at a
distance of 5000 kilometers, or putting a quarter in New York and
viewing it from San Francisco. It's also the amount the hair on a
person a meter away appears to grow in one second. A secondary mission
named Tycho measured another million stars to an accuracy of "only"
0.01 second."
"Accuracy Level    Earth-Based Data           Hipparcos Data
1 percent       50 stars 10 light years     400 stars 30 light years
5 percent      100 stars 20 light years   7,000 stars 150 light years
10 percent    1000 stars 50 light years  28,000 stars 300 light years"
"When we say that the bright star Deneb is 1600 light years away, for
example, the distance is an estimate based on its brightness and
spectral type. The distance could easily be 25% larger or smaller.
Deneb is still too far away for even Hipparcos to measure accurately."
"Cepheid Variables

The cluster method works out to a thousand light years or so.
Fortunately, within that distance are stars that give us a yardstick
to distant galaxies. These are the Cepheid Variables. Cepheids are
named for a star in the constellation Cepheus, the first star of this
type discovered, but the most famous Cepheid, and also the nearest, is
Polaris, the Pole Star. Polaris is about 300 light years away and
varies in brightness too slightly to be obvious to the unaided eye.
Before Hipparcos, the distance to Cepheids had to be determined
indirectly by the cluster method, but now the distances to several
have been determined directly."

Cosmic Yardsticks: Pulsing Stars Unlock Universe's Secrets

"Astronomers calculate the Hubble Constant by observing the "Doppler
shift" in the color of light galaxies emit, determining how fast
galaxies are moving away from us. Then, they combine that with the
distances to those galaxies, as measured using Cepheid stars.

Existing methods have a 30-percent to 50-percent margin of error for
measuring the distance to a single Cepheid, although measuring
numerous Cepheids sharply reduces that margin, Lane said.

The new method now has a 15-percent maximum error margin for measuring
distance to one Cepheid, but that should drop to less than 3 percent
when the method is used with bigger, advanced interferometers, he

The Distance to the Galactic Center

"The distance between the Sun and the Galactic Center, referred to as
R_o , is an important one. Many of the measured parameters of galactic
objects such as distance, mass and luminosity are directly related to
R_o , which has an estimated value of 8.0 kilo parsecs (~26,000 light
years), with a standard error of about 0.5 kilo parsecs."
"Salim and Gould expect to be able to determine R_o to an accuracy
within 4% as early as the year 2002. While this is indeed an
improvement over the current standard of error, it is a slight one.
Nonetheless, the astronomical community awaits their results."

NASA's measure of the universe questioned

"A technique using a radio-telescope is a "golden ruler" for measuring
cosmic distances and calls into question the conclusions announced by
astronomers using the Hubble Space Telescope who were supported by the
National Aeronautics and Space Administration, a researcher said

"Ours is a direct measurement, using geometry, and is independent of
all other methods of determining cosmic distances," said Jim
Herrnstein of the National Radio Astronomy Observatory. "It is the
most precise distance ever measured to a remote galaxy.""
"He said the new technique shows a 15 to 20 percent margin of error in
the results announced last week by a NASA team led by Wendy Freedman
of the Carnegie Institute of Washington.

The NASA team using the space telescope said last week that it had
successfully achieved the goal of measuring within an uncertainty of
only 10 percent the speed at which the universe is expanding, a value
called the Hubble constant.

Herrnstein said this calculation is off by 15 to 20 percent. He based
his conclusion on the difference in calculated distance to a specific
galaxy using his technique compared with the calculation method used
by the NASA group."
"But Herrnstein said the technique using Cepheids is far less accurate
than his new method that directly measures the motion of gas around a
galaxy. NGC 4258 is surrounded by a rotating cloud of gas. Within this
gas cloud is water vapor, which tends to amplify radio signals. This
creates radio "hot spots" called masers.

The orbital speed of masers between NGC 4258 and Earth was measured in
1994, and again every few months over the following three years. By
determining the speed at which the masers were moving, the astronomers
created a triangle with the first maser position at one angle, the
latest position at a second angle, and the galaxy center at the third
angle. Measuring the angles gives the distance.

The distance to NGC 4258 was calculated at 23.5 million light years,
with an accuracy to within 4 percent. This distance has been
calculated using the Cepheid star technique at 27 to 29 million light

Herrnstein said that the distance measure using Cepheid stars is based
on a series of assumed values, such as the distance to the Large
Magellanic Cloud, a galaxy neighbor to the Milky Way. These assumed
values, said Herrnstein, have a greater margin of error than does the
direct measuring technique he is using."

Astronomers dispute NASA gauge of universe's age

Quasar 3C273

Thus, the margins of error:

Moon - fractional centimeters

Neptune and Pluto - thousands of kilometers

Proxima Centauri (4.22 ly) - ~ 1 per cent

The star Rigel (910 ly) - greater 10 per cent less 25  percent

Globular Cluster in Hercules (There are four globular clusters in
Hercules, the nearest, M13, is 13000 ly from the Sun: M93 is 25000 ly
away: 6229 is 102000 ly distant.) - margin of error greater than 25
per cent

Spiral Galaxy in Andromeda, M31 (~2 million ly) - greater than 25 per cent

A Galaxy in the Virgo group (3000 galaxies, ~65 million ly)  - greater
than 25 per cent

The Quasar 3C273 (~2.5 billion ly) - greater than 25 per cent,
possibly as much as 40 percent




Request for Answer Clarification by mongolia-ga on 12 Dec 2003 15:30 PST
I am very impressed with your comprehensive answer. (I will rate it shortly)
 However I must say I am surprised by the huge uncertainy for anything
beyond the moon. Regarding Neptune I would have thought it's distance
would have been known within a few tens of miles considering it has
been visited by a spaceprobe and that that Radar ranging should be

  When one considers a star like Rigel (which in our Backyard in cosmological
  terms) a error measurement between 10% and 25% sounds very large. 

  However again thank you for the effort you have put into my question



Clarification of Answer by hlabadie-ga on 12 Dec 2003 16:41 PST
As regards Neptune, it is at the outer limits of direct measurement
within the Solar System, and navigation to the planet naturally
entails numerous corrections in the flight path of spacecraft, both
for errors in measurement and for gravitational perturbance. An error
of only 4000 km at that distance on a first estimate is not excessive,
I think. It is an average, I believe.

With respect to Rigel, the problem is the minute differences
detectable in parallax. HIPPARCOS demonstrates that even a relatively
near stellar object is still a very great distance away, and the
difficulty in measuring distance with even a very precise instrument
above the atmosphere is challenging.


Clarification of Answer by hlabadie-ga on 13 Dec 2003 05:02 PST
Planet Neptune: diameter, distance from Sun
Information about the planet Neptune

To give the margin of error some perspective, 4000km is less than the
radius of the Earth and less than 1/12th of the diameter of Neptune.
In other words, if one aimed at the center of Neptune from Earth, one
could miss one's target by 4000 km and still hit Neptune by a
comfortable margin. Put a different way, 4000km is only 1125000th of
the average distance of Neptune from the Sun, a remarkably small

"Neptune's Diameter:   49,528 km, 3.9 Earth Diameters"
"Neptune Orbital parameters

Semimajor axis (10^6 km)           4,495.06
Sidereal orbit period (days)     60,189
Perihelion (10^6 km)               4,444.45
Aphelion (10^6 km)                 4,545.67"

mongolia-ga rated this answer:5 out of 5 stars and gave an additional tip of: $5.00

Subject: Re: astronomy
From: sublime1-ga on 16 Nov 2003 20:41 PST

Since you are asking for the accuracy of the measurements
it would also help to know "as measured by whom, and/or by
what method?", since different sources for these measurements
will no doubt reflect different degrees of error, possibly
based, in part, on different methods of measurement.
Subject: Re: astronomy
From: hlabadie-ga on 22 Nov 2003 11:27 PST
The distance to the moon has been measured so accurately by laser that
it is known that it is receding from the Earth at rate of 3.8 cm per
year. Measurements of the distance of the planets from the Earth are
made with radar and by triangulation. Naturally, those distances have
a greater margin of error. Distances to nearby stars (within 300 light
years) have been measured by satellites using parallax techniques.
Within 30 light years of Earth, the accuracy of distance measuresments
is about 1 per cent. Within 150 ly, the accuracy is about 5 per cent.
Within 300 ly, the accuracy is about 10 per cent. Beyond those
distances, the error can vary by 20-40 per cent.

Subject: Re: astronomy
From: hlabadie-ga on 13 Dec 2003 07:24 PST
Thank you for the tip and the rating.

Subject: Re: astronomy
From: neilzero-ga on 17 Mar 2004 16:52 PST
The distance to the moon has been measured (repeatedly) plus or minus
one part per billion. Using these baselines and computers, the
distance to the Moon can calculated for any date and time (in the last
100 years) to better accuracy than one part per million. Perhaps one
part per million is also available for Neptune and all the other
planets except Pluto. Error for Pluto may be about one percent.
Perhaps 2% for the three stars in the Centarii system. Probable error
increases with distance, but we all but surely have the decimal point
in the right place.   Neil

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