1. Definition
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Parallax is the apparent shift in the position of an object when viewed from different viewpoints.
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A simple everyday example: if you look at your finger first with your left eye closed, then with your right eye closed, your finger appears to move relative to distant background objects. This is parallax.
2. Parallax in Astronomy
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In astronomy, parallax is used to measure the distances to nearby stars.
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Stellar parallax is the apparent angular shift of a star against the background of more distant stars, observed from two opposite points of Earth’s orbit around the Sun (six months apart).
3. Parallax Angle
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The key measurement is the parallax angle (p).
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It is half the angular difference in the star’s apparent position and is measured in arcseconds (″), where 1 arcsecond = 1/3600 of a degree.
4. How Distance Is Calculated
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The distance dd to a star in parsecs (pc) is calculated by the formula:
d=1pd = \frac{1}{p}
where:
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dd = distance in parsecs
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pp = parallax angle in arcseconds
🧮 1 parsec ≈ 3.26 light-years ≈ 3.1 × 10^13 kilometers.
5. Example Calculation
If a star has a parallax angle of 0.1 arcseconds, then:
d=10.1=10 parsecs≈32.6 light-yearsd = \frac{1}{0.1} = 10 \text{ parsecs} \approx 32.6 \text{ light-years}
6. Types of Parallax
| Type | Description |
|---|---|
| Stellar Parallax | Apparent shift of a star due to Earth’s orbit around the Sun |
| Geometric Parallax | Shift observed when viewing an object from two different points on Earth |
| Annual Parallax | Parallax caused by Earth’s yearly revolution around the Sun |
| Differential Parallax | Difference in parallax between two stars, used to measure relative distances |
7. Interesting Facts
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The first successful measurement of stellar parallax was made by German astronomer Friedrich Bessel in 1838 for the star 61 Cygni.
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Space missions like Hipparcos and Gaia have measured parallaxes for millions of stars with great precision.
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Parallax remains the most reliable and direct method to measure distances to nearby stars.