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Subject: Physics , asked on 11/5/15

# What is PARALLACTIC SECOND?

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Difference in the direction of a celestial object as seen by observers from two widely separated points, a measurement used to find a body's distance. The two positions of the observers and that of the object form a triangle; its apex angle (at the object) is twice the parallax, which becomes smaller with increasing distance. Observations for calculating the Sun's parallax can be made simultaneously from two different places on Earth's surface; that value reaches a maximum of 8.794 seconds of arc for observers at points separated by Earth's diameter. Observing the difference in an object's position as seen from Earth at points six months apart in its orbit (stellar, or annual, parallax) allows measurements of distances (e.g., of stars) too large to be made from two places on Earth's surface. The nearest star system, Alpha Centauri, has a stellar parallax of 0.76 second of arc. Highly precise parallaxes, and thus the positions, of more than 100,000 stars in the Sun's vicinity were determined from data collected by the European Space Agency's Hipparcos satellite (launched 1989).

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what is S.I unit of form factor

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1 parallatic second is the radius of a circle at the centre of circle,1AU long,subtends on angle1''
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S.I unit of form factor

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angle made by an arc of one second at a point on earth or observation point....is called an parsec...ok...

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What do you mean by -- ' x ' seconds of arc where x refers to any value?

And I didn't asked what is parallax I asked What is PARALLACTIC SECOND?

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what is parallictic angle?