Have you ever noticed that when you are looking out of a window of a moving car nearby objects pass by your field of vision quickly, while distant ones move slowly? I assume you have, so much so that it may just seem natural to you. This is an example of parallax in everyday life. But what exactly is this thing called parallax?
Parallax is the observed shift of an object’s position when seen from different viewpoints. It is fundamental in astronomy and even photography. Moreover, it plays an essential role in depth perception and calculating distance; the differences in the images formed in our left and right eyes help our brain perceive depth and distance. You can experiment with this notion by holding one finger in front of your face and closing one eye. Afterward, switch the eye you have opened. It will seem like your finger has moved, although it hasn’t. In the same way that the concept of parallax is of utmost significance in human vision, it can affect instruments and systems such as cameras, telescopes, and surveying instruments.
The difficulty of determining distances in the Universe stems from the vast distances between celestial objects – even the closest star is 40 trillion kilometers away. After thousands of years, astronomers finally figured out how to measure the distances of the stars from our planet and make a three-dimensional map of the stars and galaxies’ position, using a few different methods such as the parallax. The first actual stellar parallaxes were measured by Friedrich Bessel, then Thomas Henderson and Friedrich Georg Wilhelm von Struve in the late 1830s. It’s important to note that before that the Greek astronomer Hipparchus used observations of a solar eclipse from two different locations to try and determine the distance between Earth and the Moon. Furthermore, Italian astronomer Giovanni Cassini in Paris and his colleague Jean Richer in Cayenne, French Guiana, made observations of Mars; later, Cassini computed the parallax using those and determined the distance between Earth and Mars.
Notably, in 2013, ESA (European Space Agency) launched a telescope named Gaia which has helped find data about our galaxy by illustrating the positions, motions, and parallaxes of more than 1 billion stars.
The image above shows the shift in a star’s position between two observations separated by six months – in this case January and July. The measurement of the parallax is used to find the distance of the celestial body from Earth and the Sun. The orbit of the Earth around the Sun is the used baseline for the studies of bodies outside the Solar System.
It is undoubtedly the case that stars and galaxies can be so distant that even with the most modern technology, it is currently impossible to measure their parallax. However, scientists can use ideas and insights derived from parallax calculations of the closer stars to determine the distances of those that are further away.
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