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Parallax is the angle between the apparent location of an object as seen from two different places: for example, given two distant trees in view, someone standing close to you might appear near one or the other tree depending upon where you are standing. Furthermore, by knowing the distance between two such places you observed from, and by observing the angular distances between the person and each such distant tree (as observed from each of your two locations), you can work out how far this person is from you. Similarly, a nearby star may appear closer to a particular more-distant star or another such star, based upon where the Earth is located, the Earth's location continually changing as it orbits. With the ability to measure very small angular distances, such angular measurement between a nearer star and more distant stars yield a determination of the distance to the nearer star (stellar parallax).
For such measurement, the largest, most-easily measured parallax angle is that from Earth at times a half a year apart, when the positions of the Earth differ by 2 AU (this distance between measurement-locations being termed the measurement's baseline). From the star's angular distances from stars further away, the angular distance across the celestial sphere of its apparent movement is worked out. Half this angular-distance measurement is cited as the parallax angle, which is what the angle would be from two positions 1 AU apart. A parsec is the distance of a star with this (1-AU based) parallax angle of 1 arcsecond. Typical "rough" capabilities of telescopes (based upon the Rayleigh Criteria):
| Instrument | angular resolution | distance |
| Ground-based telescope without adaptive optics | 1 arcsecond | 1 pc |
| With adoptive optics | 50 milliarcseconds | 20 pc |
| HST | 50 milliarcseconds | 20 pc |
| radio interferometer | 5 milliarcseconds | 200 pc |
| very-long-baseline interferometry (8000 km baseline) | 1 milliarcsecond | 1 kpc |
(Note that astronomers have ways to do better than implied by the Rayleigh criteria, one such method being averaging many independent measurements.)
Secular parallax consists of using the Sun's motion to gain a longer baseline, but the fact that the target star also has such a motion reduces the information that can be gained.
Note that within astronomy, the word parallax is sometimes combined with a modifier (e.g., spectroscopic parallax) to indicate some method of determining stellar distance that has nothing to do with the phenomenon of parallax and angles: the word parallax is used only to indicate that it is a method of determining distance.