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Astrometry is the oldest branch of astronomy. It studies the geometrical relationships between objects in the sky and their apparent and true motions.
To find the distance to a star we use a concept called the parallax. If we observe a star from the Earth and record its position with respect to the background stars and then repeat this measurement 6 months later, when the Earth is on the opposite side of its orbit around the Sun, we see that the position of the star appears to shift with respect to the background. This apparent angular displacement of a star is what is called the stellar parallax. By measuring the parallax, we can deduce the distance to a nearby star using simple geometry. But stellar parallax is a difficult quantity to measure as it is extremely small for all but the nearest few hundred stars.
Astrometry also determines how celestial objects are moving in space relative to each other. For this purpose it is necessary to measure two components of motion: the radial velocity which is the velocity of the star away or toward us, and the proper motion which is the motion of an object across the sky.
Click on the thumbnail below to learn how we can use stellar parallax to measure the distance to a nearby star:
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