What is Parallax: Definition and 55 Discussions

Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. Due to foreshortening, nearby objects show a larger parallax than farther objects when observed from different positions, so parallax can be used to determine distances.
To measure large distances, such as the distance of a planet or a star from Earth, astronomers use the principle of parallax. Here, the term parallax is the semi-angle of inclination between two sight-lines to the star, as observed when Earth is on opposite sides of the Sun in its orbit. These distances form the lowest rung of what is called "the cosmic distance ladder", the first in a succession of methods by which astronomers determine the distances to celestial objects, serving as a basis for other distance measurements in astronomy forming the higher rungs of the ladder.
Parallax also affects optical instruments such as rifle scopes, binoculars, microscopes, and twin-lens reflex cameras that view objects from slightly different angles. Many animals, along with humans, have two eyes with overlapping visual fields that use parallax to gain depth perception; this process is known as stereopsis. In computer vision the effect is used for computer stereo vision, and there is a device called a parallax rangefinder that uses it to find range, and in some variations also altitude to a target.
A simple everyday example of parallax can be seen in the dashboard of motor vehicles that use a needle-style non-LCD speedometer gauge. When viewed from directly in front, the speed may show exactly 60, but when viewed from the passenger seat, the needle may appear to show a slightly different speed, due to the angle of viewing.

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  1. D

    Stargazing What is the maximum distance that parallax error can be used?

    using the Earths orbit, what is the maximum distance that can be measured using parallax error?
  2. yucheng

    I Issue with Stellarium: transit of Venus (find the parallax)

    These are the two snapshot (on Stellarium) of the Third Contact between Venus and the Sun at the same time at different locations on Earth. The top image is viewd from Quito, Ecuador, the bottom image is from Harrisburg. I am supposed to determine the parallax. The angles were calculated using...
  3. vibha_ganji

    B Definition of Stellar Parallax from Alonso and Finn Volume 1

    Hi! I read this definition of Stellar Parallax "It is expressed quantitatively by one-half the angle subtended by the Earth's diameter E1E2 perpendicular to the line joining the star and the sun (see Fig. 2-10)." (Source Alonso and Finn: Volume 1). But, I was unable to understand how they...
  4. J

    B Trivial questions regarding parallax and measurement of distances.

    While trying to understand parallax and its use in measuring distances here, I had a few questions. (1) Parallax is defined as the apparent movement of an object with reference to another object in the background when one views it from different angles. As in the movement of a finger relative...
  5. sbleuz

    I The use of 3D reconstruction techniques to build a galaxy model

    I'm learning about imaging techniques and projection/deprojection for images of galaxies at university. The big issue there is the fact that an image is a 2D representation of a 3D object (for example a galaxy), and to learn some properties of the object you need the 3D structure. What happens...
  6. nmsurobert

    B Parallax Limitations: Measuring Distances Miles Away

    I'm setting up a lab for my astro students relating to parallax. We will be using some pretty simple tools. Measuring tape and a protractor with a straw fastened to it. I know that calculating distances with tools like these work in the classroom but what if I wanted to calculate the distances...
  7. K

    B How do astronomers find the "parallax angle" of a star?

    https://www2.jpl.nasa.gov/teachers/attachments/parallax.html It's written that they find the distance by calculating from the parallax angle. But how do astronomers find the parallax angle?
  8. J

    I Calculate object distance by using just the parallax angles

    I have a simple paper with parallel grid lines and I know the height of the observer from the ground level. Now when I snap the picture with my camera, the effect i get is what is seen in the image below. Even if the lines are parallel, the perception is that they converge in the middle. When...
  9. G

    Stargazing Trig Parallax: Measurement & Apparent Shift of Nearby Stars

    Hi community, I get the concept of trig parallax and the apparent shift of nearby stars when viewed against a distant background, by viewing the star in say summer and then winter and it appears to move against the much further away distant background. I get what the angle p represents...
  10. M

    Using a set square to reduce parallax error

    Homework Statement In one of my revision guides, there is an experiment for calculating the acceleration of free fall using the electromagnet and trap door arrangement. The biggest error is in the measurement of the height over which the ball falls. The guide says: "You could reduce the error...
  11. NatFex

    B Usefulness of Stellar Parallax?

    This isn't a very technical series of questions, just some passing thoughts I had whilst looking over some notes I had on stellar parallax distancing. Stellar parallax is always described as a means of "measuring the distance to stars." But from where? What I mean is, the distance, or the side...
  12. Y

    Ability to measure distances with parallax versus size

    I would like to preface this by stating that I am not very well-versed in cosmology or astrophysics, but I've been thinking: I understand the idea of parallax, both in the sense of human eyes measuring distances to nearby objects and in the sense of telescopes on Earth (or in orbit) measuring...
  13. R

    What Can Parallax Error Tell Us About the Universe?

    Hello Forum, We have an assignment due tomorrow and I need help ASAP. (I asked other forums but no one replied) We had to relate Physics to Biology and thus we are doing something on the Parallax error. We found the basics but three slides with barely anything won't give us marks. Can you guys...
  14. R

    Venusian Angular Units (vau): Describe & Calculate Sun's Parallax

    Homework Statement Like humans, astronomers of Venus use the same method for definitions of parallaxes and of parsec but measure them in different (their own) units. For example, the distance to Sirius equals to 19.6 vpc (19.6 venusial parsec). - Describe the most evident system of angular...
  15. R

    Calculating heights from parallax effect

    I'm trying to calculate the heights of birds from a plane flying at a known altitude above them using the parallax effect. They're being recorded from the plane and the head and tail end coordinates marked on each frame they appear. The equation I'm using is from here...
  16. F

    Calculating the Distance and Parallax of a Star: A Practical Guide

    A star exhibits a parallax of 0.250 arcseconds when measured from Earth. a) What is its distance (pc)? I know this uses the formula d=1/p, so i can solve this very easily b) What would its parallax be if measured from Venus’s orbit (arcseconds)? (Look up the one parameter you need to...
  17. alemsalem

    Measuring the distance to stars using parallax

    how can you use the angle to determine the distance if you don't know the motion of the star itself. is it by observing the star from the same point in two different years and then subtracting off this effect?
  18. NanaToru

    How can knowing parallax of a farther object help with finding a nearer object?

    my professor kind of off-handedly said something along the lines of "Parallex can help you find the distance to a nearby object if you have a farther one as a reference." I think he means general, so I've been assuming that our farther object is so far that parallex shifts are ignored. I...
  19. F

    Calculating distance (and percent error) using parallax

    Hi all, This is a homework problem, but I don't think asking for help here presents any ethical issues. I'm permitted to consult my classmates and compare solutions with them as long as I do my own work, and I plan to ask my professor for help if I still feel unsure after reading your...
  20. C

    Measuring distance with parallax

    Homework Statement a) Two observers are separated by a baseline equal to Earth's diameter. If the differences in their measurements of Mars' position at opposition (i think this means when it is closest to earth) is 33.6" (arcsec). What is the distance between Earth and Mars at opposition...
  21. Philosophaie

    Horizontal Parallax Correction

    Parallax is a way of measuring the distance of one object to another. By taking the distance of an already known object, say the Sun, and drawing a line to the object. Then drawing another line between you and the object in question. The angle you get is the Horizontal Parallax angle...
  22. A

    Parallax during the earth's orbit?

    Hi, Would the difference in the distance between the Earth and the sun when the Earth is in perihelion and when it is in aphelion be considered a parallax? Thanks,
  23. strangerep

    SR Foundations: Parallax Methods? - Reference Request

    Standard treatments (e.g., Rindler) of the foundations of Special Relativity invoke use of a "rigid rod" or "rigid scale" by an observer to construct his inertial frame -- by (somewhat fictionally) transporting the rod (and also cloning copies of a standard clock at every point in the frame)...
  24. R

    Parallax formula derivation problem?

    parallax formula derivation problem!? can anyone tell me in detail how to derive the parallax formula of deriving the distance of stars ? if animated videos or movies are included it would be better!
  25. P

    Parallax and Perspective Explained

    I completely understand the effects of parallax as well as perspective...intuitively; but I can't seem to explain them to a particularly inquisitive child I'm tutoring. He wants to know, in conceptual terms, why objects in the distance like the moon seem to move in the same direction as you...
  26. C

    Solving Solar Parallax Distance Homework

    Homework Statement the solar parallax is 8.8 arc seconds.. calculate its distance from earth? Homework Equations distance(in parsec)=1/parallax in arc sec atempts to solution i know this formula but it provides me weird ans like distance=0.11pc while using this...
  27. J

    Understanding Parallax Angles in Astronomy: Explained with a Diagram

    Homework Statement Can someone please explain to me how parallax works? I don't understand how the angle of parallax is measured Question to help: Astronomers measuring the parallax angle to Planet X found that the angle measured after 6 months was 1.52 arcsec different from the initial...
  28. M

    Error propagation of distance modulus and parallax

    Hi! Here is my problem: there is a star, for which we know the distance, d=21.2 pc, the measurement error is delta_d=1.8 pc. The question is that how far should we put this star, so that the following equation would be true: d/delta_d = 3? The teacher told me to use two formulas...
  29. S

    Better Hubble Constant through Parallax?

    The above is taken from a paper by Wendy Freedman (http://www.pnas.org/content/96/20/11063.full.pdf) In 1999, she wrote that the Space Interferometry Mission (SIM), a project primarily focused on mapping the Milky Way and finding other Earth-like worlds, would provide parallax data that could...
  30. A

    Measuring Parallax Angle to the Method

    I have read the parallax method of determining large distances and i do understand it. I agree it is easy to find the distance of the basis(distance between two observation points) but how do they measure the parallax angle?
  31. S

    What is the trigonometric parallax of a binary system?

    Hi It is stated that: "for a visual binary system, the measured trigonometric parallax is 0.3749+/-0.0057 arcsec." I don't know this angle is parallax of what? I can guess it like this: 0.3749+0.0057 arcsec is the parallax of the whole system when they are at most distance of each other...
  32. P

    Callculating orbital radius with parallax and arc seconds

    Homework Statement I'm attempting to find the orbital radius for a binary system based on the parallax of the system and angular seperation: 1. A visual binary system that is 5 pc away is seen edge-on (i.e. we are in the plane of the orbit). The maximum angular separation of the two stars is...
  33. Y

    Finding Distance to Sirius Using Parallax Method

    how will you find the distance to the star sirius using parallax method using eath based telescpoes? i know the distances and angle in association with the equation...but i don't get how u accuatly get those data. The very first step in achieving those data. like for eg ...the distance is...
  34. E

    Measuring the distance to Mars with parallax.

    Homework Statement a) Two observers are separated by a baseline equal to Earth's diameter. If the differences in their measurements of Mars' position at opposition (i think this means when it is closest to earth) is 33.6" (arcsec). What is the distance between Earth and Mars at opposition...
  35. S

    Determining Parallax Accuracy for Stellar Distances

    Homework Statement Out to what distance in parsecs can we find the distances of stars to 10% accuracy or better; i.e., at what distance would the 1 mas accuracy of the measurements result in a 10% uncertainty in the distance? (HINT: To reasonable accuracy, the percentage uncertainty in the...
  36. K

    Astro: Uncertainty in distance using the spectroscopic parallax method.

    Homework Statement The uncertainty \DeltaM in the absolute magnitude for a given spectral type could be 0.25 magnitude. while the uncertainty in the apparent magnitude magnitude m, can be negligable. Give an indication of the relative uncertainty in the derived distance. Homework Equations m...
  37. K

    Astronomy - Parallax error calculation.

    1. Homework Statement Two stars belong to the same cluster, the parallax measured to one star is (2±1)x10^-3 arcseconds, towards the other the parallax is (4±2)x10^-3 arcseconds. Find the distance to both objects seperatly and also compute the distance when both measurements are...
  38. K

    Astronomy - Parallax error calculation.

    Homework Statement Two stars belong to the same cluster, the parallax measured to one star is (2±1)x10^-3 arcseconds, towards the other the parallax is (4±2)x10^-3 arcseconds. Find the distance to both objects seperatly and also compute the distance when both measurements are combined...
  39. M

    Parallax, angle to observer etc.

    Hi, I'm a mathematics analphabet, but still, sometimes I come across a problem, that I just can't keep away from. I hope someone finds this one worth a thought. Here it is: An observer is looking at a long object with marks spaced equally along its length (like a measuring rod or a...
  40. S

    Distance to stars from my house using the parallax technique?

    Is it possible to find the distance to stars from my house using the parallax technique? If I can how close does a star have to be to be measured in a month instead of six months?
  41. C

    Using parallax and magnitudes.

    Hi, I am quite new to astronomy and was wondering whether I had done the following question correctly (some of my answers seem unrealistic). Plus, how many significant figures should questions like these be answered to? The star Rigel in the constellation of Orion has an apparent magnitude of...
  42. MathematicalPhysicist

    Calculating the Distance of a Parallax Triple Stellar System

    The question: A triple stellar system, showing a parallax of 0.01 arcsecond, is composed of a binary system, with a distance between the two stars of 0.2 AU, and a third star, at a distance of 90 AU from the binary. This system is observed in the optical (wavelength of 55000 Angstram) with a...
  43. M

    Finding refractive index of liquid with no parallax principle

    In physics class we did an experiment where we dropped a metal pin in a beaker of water, covered half the beaker with a mirror and clamped another metal pin above the beaker. We had to find the refractive index of the water by measuring the apparent depth of the submerged pin and dividing it...
  44. D

    Help Calculating Parallax Error - Physics Forums

    Sorry didn't see this forum. I need some help with calculating simple theoretical parallax error: https://www.physicsforums.com/showthread.php?t=257036
  45. D

    Astrophysics, simple parallax error

    Homework Statement Experiment 15.2 (note this is from a book about 5 years old, and the book has no solutions!). I am simply unsure how to calculate between parallax with 1% error and parallax with 50% error etc. Please see the scan I done of the page...
  46. D

    Calculating Parallax Angle Change | Need Help!

    I am working on trying to calculate what the parallax is with the following information. Latitude of the sun declination sin of declination angular separation, sun to star I have all of this data, I'm supposed to be able to calculate it from this info. Don't know how. Lost in the...
  47. K

    Astrophysics: Parallax method

    1) From the ground of the Earth, atmospheric turbulence limits the typical best image to a full-wide-at-half-maximum (FWHM) of ~0.5 arcsceond (this is an angle). Suppose we can determine positional accuracy of stars to ~1/10 of the FWHM, to how far in light years can we use the parallax method...
  48. P

    Parallax Questions: Confirm Answers are Right?

    Can somebody please confirm that my answers are right? 1. At what time on Earth did the light start from a galaxy 2 billion light years away? My answer was 2 X 10^9 years. This problem seemed too easy to be true. 2. How far is the nearest star in kilometers (about 1.3 par sec away)? My...
  49. W

    Understanding Kepler's Laws and Parallax in Astronomy

    I have been reading into Kepler's laws lately because I am absolutely intrigued by space and the cosmos, but the second one about equal time and equal areas doesn't quite make sense on wikipedia or the physics textbook (Giancoli 3rd Edition i believe)... and can someone help me understand...
  50. S

    Parallax error - what type of error is it?

    My friends are having an argument as to whether parallax error is systematic or random error. We have tried looking all around in books/internet etc but nothing mentions what type of error it is. Any help would be appreciated :)