How can we estimate the distance of a cosmic object? The method of stellar parallax exploits the revolution of the Earth around the Sun to observe the apparent shift of a nearby object against the background of distant stars. In this demonstration, three stationary bodies are placed at fixed distances from you, the observer. As you move your viewing angle left and right, notice how the nearer objects appear to shift against the fixed background (the tick marks) much more than the further objects. Stellar parallax is a fairly accurate technique for objects closer than ~100 parsecs (326 light-years) but begins to lose accuracy beyond that.
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