The distance between planets within our solar system is primarily measured using a technique called "radar ranging" or "laser ranging." Scientists bounce radio waves or laser beams off the surfaces of planets and measure the time it takes for the signals to travel back to Earth. Since the speed of light is constant, the time it takes for the signal to return allows for an accurate calculation of the distance.
When it comes to measuring the distances to stars and objects outside our solar system, a different technique called "stellar parallax" is employed. Parallax is the apparent shift in the position of an object when viewed from different locations.
To measure stellar parallax, astronomers observe a star at two different times, approximately six months apart when the Earth is on opposite sides of its orbit around the Sun. By comparing the star's apparent position against more distant background objects, astronomers can calculate the parallax angle. The parallax angle, along with basic trigonometry, provides the distance between Earth and the star.
For closer stars, stellar parallax is an effective method. However, as the distance increases, the parallax angle becomes smaller and more challenging to measure accurately. Thus, parallax is generally used to measure distances within a few hundred light-years.
For more distant objects, other methods are employed, such as:
Standard Candles: Certain types of stars, such as Cepheid variables or Type Ia supernovae, have known intrinsic brightness. By comparing their apparent brightness to their known intrinsic brightness, scientists can estimate their distance.
Spectroscopic Parallax: This method utilizes the star's spectrum to determine its luminosity. By comparing the intrinsic luminosity with the apparent brightness, the distance can be estimated.
Redshift: The light from distant galaxies and objects can be shifted towards longer wavelengths, known as redshift, due to the expansion of the universe. By measuring this redshift, astronomers can estimate the distance based on the known relationship between redshift and cosmic expansion.
It's important to note that measuring vast cosmic distances is an ongoing area of research, and scientists continually refine and develop new techniques to improve distance measurements to objects in our universe.