The four stars are the same size but located at different distances from us, with position 1 being the closest and position 4 being the most remote. Light years are another measure (1 parsec = 3.26 light-years), but this unit is more commonly used by the media. One parsec is the distance from the Sun to the star under consideration when the parallax angle is equal to 1 arcsecond. We can measure the distance with this method using different units, but the most commonly used one is a parsec. This gives us enough information to calculate the distance from the Earth to the star using trigonometric equations. This half-angle is known as the parallax angle and it is marked P on the illustration. In fact, we need to know half of the angle, not the entire one. We use the known distance from the Earth to the Sun (measured as 1 astronomical unit), and measure the angle formed between the line connecting the Earth at the first point of measurement, the star under consideration, and the Earth at the second point of measurement. You can see a more detailed mathematical explanation on how the distances are calculated in the article on distance, but in general, we measure these distances at two different times in the year, when the Earth is on opposite sides of the Sun (at 6-month intervals, since the Earth makes one rotation around the Sun in one year). The distance between the Sun and the star we are measuring, line AS (orange in the illustration) is equal to one parsec when P = one arcsecond A2 and A3 are the apparent positions of this star from two different observation points, relative to the white distant star DS. A is the actual position of the star, the distance to which we are measuring. Here the two positions of the Earth are marked with light blue circles, and the position of the Sun is in orange.

This tells us that we can use this phenomenon to measure how far the object (our finger) is from us. The closer your finger is to your eyes, - the larger the parallax shift relative to the remote object when you compare the view from each eye. If you now try to do the same experiment but keep your finger closer to your eyes, you will notice that the shift of your finger relative to the distant object is different. Did you notice that your pencil or finger moved relative to the other object? The fact that it moves is the manifestation of parallax. Now close this eye and open the other one. Note how far this finger is from another object in the distant background (say, a tree, if you are outside, or a piece of furniture if you are indoors). Here is an easy way to see parallax in action: hold up one finger and close one eye. It is manifested when observing an object from different points of view against a more distant background. Parallax is a geometric phenomenon used in distance calculations. We have discussed stellar parallax in the article on length and distance but let us briefly look at it here as well, because it is fundamental in measuring distances in space. In other words, a parsec is a distance, from which a disk with a diameter of one astronomical unit will have an angular size of one arcsecond. A parsec is defined as the distance at which an object has a 1-arcsecond stellar parallax. Where D is the actual distance measured in parsecs and p is the observed parallax angle measured in arcseconds.

The parallax of a celestial body can be used to find an approximate distance using the formula These two separated points are situated on the Earth’s orbit and created by two different orbital positions of Earth as described below.

Stellar parallax is the difference in direction of a star as seen from two widely separated points. It is measured by the angle or semi-angle between the two lines of sight from an observer to the object. The parallax is the apparent change in the position of an object resulting from a change in the position of the observer. Definition of Parallax and the Formula for Distance Calculation

Parsec to km