Marine Glossary

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Parallax

The difference in the apparent position of an object viewed from different locations. Consider two navigators with sextants, one standing 50 feet, the other 60 feet, from a street light 30 feet high. From the navigator's perspective, the street light happens to be just below the moon, which both navigators agree from their sextant measurements is 35° 00' high. They now measure the height of the street light and compare its position in the sky to that of the moon. The farther sextant would read the height of the light to be 26° 34' high, the closer one would read 30° 59' high. One finds that the light is 8° 26' below the moon, the other 4° 01' below the moon. The difference of 4° 25' is called the parallax error.

Now the two navigators, without moving, turn their sextants to a spotlight on a mountain peak 12,000 feet high, located 5 nautical miles beyond the street light. The closer one would read a sextant height of 21° 31.3' for this light and the farther one would read 21° 30.9' for the same measurement. Now the parallax error is only 0.4', showing that the farther an object is, the smaller the parallax error.

Small parallax means the lights rays from the object to the two observers are nearly parallel. In celestial navigation, we must consider the parallax of light rays from celestial bodies since one light ray locates the object at its geographical position (GP) while the other ray locates the observer relative to the GP. If these rays are not strictly parallel, we must correct for this to get a proper position. Only the moon, Mars, and Venus, however, are close enough to the earth to require this special correction, which is part of the additional altitude correction done during a sight reduction. See Horizontal Parallax and Additional Altitude Correction.