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.