Parallax, a term often heard but seldom understood. A phenomenon that has shaped our understanding of the universe and continues to influence modern technology. Let's delve into its essence, exploring its origins, applications, and the science that makes it possible.
Stellar parallax
Measure a star’s apparent movement against the background of more distant stars as Earth revolves around the sun
- The gold standard for measuring stellar distances because it relies solely on geometry
- To measure the distance of a star, astronomers use a baseline of 1 astronomical unit (AU), which is the average distance between Earth and the sun, about 93 million miles (150 million kilometers).
- They also measure small angles in arcseconds, which are tiny fractions of a degree on the night sky.
- If we divide the baseline of one AU by the tangent of one arcsecond, it comes out to about 19.2 trillion miles (30.9 trillion kilometers), or about 3.26 light years
Shooting for accuracy
In 1989, the European Space Agency (ESA) launched an orbiting telescope called Hipparcos (named after Hipparchus) to measure stellar distances using parallax with an accuracy of 2-4 milliarcseconds (mas), or thousandths of an arcsecond.
- The satellite has already obtained distances of 1 billion stars, about 1% of all the stars in the Milky Way, and produced spectacular 3D maps.
- The ESA’s successor mission is Gaia, launched in 2013.
3D Imaging: reproduction and display of 3D images
The key is to capture 2D images of the subject from two slightly different angles, similar to the way human eyes do, and present them in such a way that each eye sees only one of the two images.
- Most of today’s 3D televisions use an active-shutter scheme to display images for each eye that alternate at 240 Hz.
Early measures
The first known astronomical measurement using parallax occurred in 189 BC, when Hipparchus used observations of a solar eclipse from two different locations to measure the distance to the moon.
- His mistake was in assuming that the moon was directly overhead, thus miscalculating the angle difference between Hellespont and Alexandria
- In 1672, Giovanni Cassini and a colleague made simultaneous observations of Mars, with Cassini in Paris and Richer in French Guiana, and based on this, calculated Mars’ distance from Earth, allowing for the first estimation of the dimensions of the solar system
Cosmic distance
By measuring the distances to a number of nearby stars, astronomers have been able to establish relationships between a star’s color and its intrinsic brightness, i.e., the brightness it would appear to be if viewed from a standard distance.
- These stars then become “standard candles.”
- If a star is too far away to be measured by this method, astronomers can match its color and spectrum to one of the standard candles and determine the intrinsic brightness of that star and determine its distance.
