Parallax is a method used to measure the distances to relatively nearby objects, primarily within our galaxy.
It leverages the changing vantage point of the Earth as it orbits the Sun.
The Earth's Orbit and Changing Vantage Points
As the Earth orbits the Sun, our perspective changes throughout the year.
An observation made at one point in Earth's orbit, when compared to an observation six months later (when the Earth is on the opposite side of the Sun), reveals a shift in the apparent position of a nearby star relative to more distant background stars.
Measuring Distance Using Parallax
The amount a nearby star appears to move against the backdrop of distant stars indicates its distance from us.
The greater the apparent movement (parallax motion), the closer the star is to Earth.
Depth Perception in Space
Human eyes lack depth perception when viewing objects in outer space because our eyes are not sufficiently separated to perceive the minute differences in angles.
Parallax Angle and Its Relation to Distance
Parallax is quantified as an angle. We measure how far the nearby star moves back and forth relative to background stars.
Knowing the distance between the Earth and the Sun (1 AU) and measuring the parallax angle allows us to calculate the distance to the star using trigonometry.
The parallax angle is inversely related to the distance of the star; the farther the object, the smaller the parallax angle.
Formula
\text{distance} = \frac{1}{\text{parallax angle}}
Units of Measurement
If a star has a parallax angle of one arcsecond, it is at a distance of one parsec.
Measurements can be made with a precision of about one milliarcsecond, allowing distance determination up to approximately 1000 parsecs (1 kiloparsec).
Limitations of Parallax
Parallax is most effective for measuring distances to relatively nearby objects within our galaxy.
Beyond certain distances, the parallax angles become too small to measure accurately from Earth.
The distance from our location to the center of the galaxy is approximately 8 kiloparsecs, illustrating that parallax is useful only for our local part of the galaxy.
Other techniques are required to estimate distances to far away objects.
Absolute Distances
Parallax allows the measurement of absolute distances to celestial objects without relying on assumptions about their properties.