Parallax is the change in the relative position of an object as seen from two different locations.
Mathematically, the relationship between any two viewing angles and a distant object can be summed up in what is called the parallax angle.
As long as certain information is known – such as the angle between the visible lines and the distance between the points of view – trigonometry can be used to determine the height of the object.
Our brain does this naturally on a human level; Each of our eyes ‘sees’ objects according to their line of sight, and the difference in the position of objects is interpreted by our brain as perpendicular distance, or depth.
This is why we see things in front of us as three dimensional.
At larger scales, parallax angles can describe large distances, including those between Earth and other celestial bodies such as the Sun or other nearby stars.
How do astronomers use parallax?
In order to measure the great distances separating objects in the interior of our Sun, such as from the Earth to the Sun, early astronomers had to increase the distance between the observatories to the scale of entire continents by jumping from one part of the world to the other. .
Centuries ago, astronomers took their telescopes across the oceans to see the inner planets as they eclipsed the Sun. Such planetary transits (called transits) are frequent events that depend on perfect alignment between the Earth, the planets being observed, and the Sun.
The predicted journey of Venus in 1769 saw people all over the world, from North America to Russia, to Europe to the south of Tahiti, collecting data on the timing and short journeys of the paths apparently taken by the miniature version of Venus. the world. from one side of the Sun’s disk to the other.
Two years later a French astronomer named Jérôme Lalande used those measurements, along with others from a previous expedition, to find the distance of 153 million kilometers (95 million miles) between the Earth and the Sun; just touching the current official figure of about 149,600,000 kilometers.
For objects beyond our Sun, the distance between the observations must be greater than the size of a single planet.
Fortunately, the Earth’s orbit provides such a difference, stretching over 300 million kilometers (186 million miles).
Spaceships can also provide us with another visual site for measuring distances. NASA’s New Horizons probe, which flew by Pluto and continues to explore the outer Solar System, has done just that.
The European Space Agency’s Gaia Observatory is now engaged in the largest-ever mission to record the locations of nearly a billion objects in space.
Orbiting the Sun at a distance of about 1.5 million kilometers from Earth, Gaia will detect small differences in the visibility of these objects from different locations during its annual journey.
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Using parallax angles for Gaia measurements produces estimates of objects as far away as 30,000 light-years – the distance between Earth and the center of the Milky Way. For nearby objects, measurements can be very accurate to within 0.001 percent of the actual distance.
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