There is a change in the relative position of an object.
The relationship between two points and a distant object can be summed up in a mathematical way.
If some of the information is known, it can be used to deduce how far away the object is.
Our brains do this on a personal level, with each of our eyes seeing objects according to their own line of sight, with different positions interpreted by our brain.
We see objects in front of us as three-dimensional.
Calculating the distance between Earth and other bodies such as the Sun or other nearby stars is possible on larger scales.
Astronomers had to jump from one side of the globe to the other in order to measure the distances between objects in the Solar System.
Astronomers used to take their telescopes across the oceans to watch the planets as they passed by the Sun. A perfect alignment between Earth, the planet being observed, and the Sun is required for a planetary crossing.
Observers from all over the world gathered data on the path taken by Venus as it transited from one side of the planet to the other.
Two years later a French astronomer named Jérme Lalande applied those measurements, along with some from an earlier transit, to come up with an estimate of the distance between Earth and the Sun.
The diameter of a single planet is not enough for objects beyond our Solar System.
There is a gap of more than 300 million kilometres.
We can use spacecraft to calculate distance. NASA's NewHorizon probe did a flyby of Pluto and is continuing to explore the outer Solar System.
The European Space Agency's Gaia space observatory is attempting to record the positions of a billion objects.
At a distance of 1.5 million kilometres from Earth, Gaia will be able to see tiny differences in the apparent positions of objects.
Rough estimates on objects as far away as 30,000 light-years away from Earth will be produced by applying a conjugate angle to the measurement. Measurement could be so precise that they'll be within a hair of the actual distance.
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