Terri Beck, age 13, of Ponca City, Oklahoma, for her question:
What is a parallax unit?
You can measure an inch worm with a ruler and the trail of a snail with a tape measure.
Road distance can be measured in terms of speed as in miles per hour. But the scope of the sky is limitless. We cannot use ordinary earthly units to cope with its cosmic dimensions.
The parallax unit is a celestial angle used to measure the distance of a star. Angles and triangles have been used to compute distances since geometry was invented. Surveyors use them to measure the curved surface of the earth. For example, suppose you know the . length of the base line of a triangle and the size of the angle at each end. The straight sides must slant upward at these angles, taper together and meet at a certain point. Froth your known base line and its two angles you can complete your triangle and give an accurate report of the length of the two sides, plus the apex angle at the top.
The parallax is the apex angle at the top of an immense celestial triangle. Like . all angles, it is measured in the degrees of a circle. Ages ago, it w4s decided to divide= the circumference of a circle into 360 equal unit degrees. When two straight lines, one from each end of a unit, are drawn to the exact center of the circle, they form an angle of one degree. And 90 degrees for a square cornered right angle, whether the circle is an inch or a billion miles wide.
Each degree is subdivided into 60 minute angles and each minute unit is subdivided into 60 second angles. When the apex angle and the length of the base line are known, the distance to the apex can be figured from the center of the base line. The larger the angle, the shorter the distance. But stars are very remote and not one is close enough to the Solar System to have a parallax angle of a whole degree. The nearest has a parallax angle of 0.760 of one second. From this angle, astronomers can compute the distance of the triple star Alpha Centaurs to be 4.29 light years, or roughly 27 million million earth miles.
The base line of the celestial triangle is our known distance from the sun, almost 93 million miles. The orbiting earth provides the basic angles needed to complete the triangle. The star to be measured is photographed against a background of more distant stars. It is photographed again six months later, when the earth has moved halfway around its orbit. From this new viewpoint, the star has shifted its position in relation to its parallax unit
Astronomers compute the base line angles from the two viewpoints of the star against its background. Now they have the base line and the two angles needed to compute the tell tale parallax angle of the star.
On a very small scale, you can test this star measuring trick. Stick a stamp on a wall and another on the tip of your pointing finger. The one on your finger is the star to be measured; the one on the wall is one of its more distant background stars. Step back, close one eye and stretch out your pointing arm bo that your stamp is lined up with the one on the wall. Now close your eye and sight with the other one. If your arm stayed stiff, the star on the wall will now be out of line, a little to one side. The distance between your sighting eyes is a base line. The two views provide two straight lines that meet and cross at your fingertip, creating a parallax angle at the apex of a triangle.