The pen is the star being studied and the poster is the trillions of stars beyond. Imagine that your left eye is the Earth on one side of the Sun and your right eye is the Earth at the opposite point in its orbit, on the other side of the Sun. That apparent movement is called parallax. Does the pen appear to move relative to the poster behind it? It should. Next, close your right eye and keep your left eye open. Now close your left eye, keeping your right eye open. Note what point on the poster is being covered by the pencil. Hold a pencil out in front of you at arms length and at eye level. To understand how parallax works, stand at one end of the room, facing a poster on an opposite wall. 9 in this case, the astronomer notes the star's location again.īecause the Earth has moved, the position of the far away star relative to the blanket of stars behind it also appears to move.
Half a year later, when the Earth is at the opposite point in its orbit (on the other side of the Sun), Aug. To measure the distance, astronomers note the location of a star on one date - say Feb. One AU is one trip from the center of the Earth to the center of the Sun.Īstronomers took (and still take) advantage of this distance to help them measure how far away stars are from the Earth. That distance - the length from the Earth's center to the Sun's center - is known as an "astronomical unit" or AU. The Earth averages a distance (radius) of 92,955,807.27 miles or 149,597,870.7 km from the Sun. What do you do when you want to measure the distance of a nearby star and there are no computers, no spacecraft and no power plants to generate electricity? Well, what Friedrich Wilhelm Bessel did in 1838 was use trigonometry and the parallax effect to calculate the distance of a star based on the distance from the Earth to the Sun.
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