Heliocentric Parallax

Heliocentric Parallax

Definition

  • The term Heliocentric Parallax refers to the apparent shift or change in position of a nearby star as observed from different points on Earth’s orbit around the sun.
  • It is coined from the Greek term “Parallaxis” which implies alteration or shift in viewing angle.

Measurement

  • Heliocentric Parallax is measured in ′′(arcseconds). The smaller the measurement in arcseconds, the greater the star’s distance from Earth.
  • The parallax angle is one-half the total angular shift. It is equivalent to the angle subtended at the star by the Earth’s mean radius.

Calculation of Distance

  • Astronomers utilise heliocentric parallax to measure the distance between Earth and nearby stars.
  • The Parallax formula is p=1/d, where ‘p’ denotes Parallax in arcseconds and ‘d’ denotes distance in parsecs.
  • The Parsec (pc) is a unit of astronomical distance equivalent to approximately 3.26 light years.

Stellar Parallax and Celestial Sphere

  • Heliocentric parallax leads to the concept of Stellar Parallax, which is the angle between two lines of sight to the star, from opposite sides of Earth’s orbit.
  • This can also be depicted on a Celestial sphere, an imaginary sphere concentric with Earth on which stars are visually plotted for observational purposes.

Accuracy and Limitations

  • The parallax method becomes less accurate for stars located at great distances as their parallax angles become extremely small and challenging to measure.
  • The maximum distance to which this method is reliably used is about 500 parsecs.
  • For stars beyond this limit, other methods such as cepheid variables and red shift are employed.

Significance of Heliocentric Parallax

  • Heliocentric Parallax is vital for establishing a star’s distance scale within the Milky Way and in other nearby galaxies, thereby helping to map the universe.
  • From the distance of a star, various properties - such as its luminosity, physical size, and temperature - can be inferred, thereby aiding our understanding of a star’s life cycle.