What is parallax?
Q: What is parallax?
A: Parallax is the perceived change in position of an object seen from two different places. It is measured by the angle between two lines of observation and can be used to determine distances.
Q: How is parallax used in astronomy?
A: In astronomy, annual parallax is the only direct way to measure distance to stars outside the solar system. Astronomers use the principle of parallax to measure distances to celestial objects including to the Moon, the Sun, and to stars beyond the Solar System.
Q: How does one calculate distance using parallax?
A: Astronomical measurement of position are taken at different times of the year. Since the Earth's orbit is known exactly, the distance from position 1 to position 2 can be worked out. The angle from the horizon to the object can be measured precisely which gives a triangle whose baseline and angles are known accurately. From this triangle, trigonometry can be used to calculate distances expressed in parsecs.
Q: Are there any limitations with using parallax for measuring distant objects?
A: Yes, it fails with objects that are so distant that earth's orbit is too small for a large enough parallax angle for accurate measurements - about 100 light-years away or more. Other methods have been invented but none are as accurate as parallax for relatively nearby objects.
Q: What satellite was used in 1989-1993 for measuring nearby stars?
A: The Hipparcos satellite was used during this time period for taking measurements over 100,000 nearby stars.
Q: What spacecraft will take similar measurements as Hipparcos did? A: Gaia (spacecraft) will take similar measurements of about a Billion stars.
