Definition
The apparent shift in the position or reading of an object when it is viewed from different angles. In aviation instruments, parallax is the reading error that occurs when a pilot looks at a gauge or indicator from the side rather than straight on, causing the pointer to appear aligned with a different value than it actually is.
Plain English
The way a needle or marker on a dial seems to point to a different number depending on the angle you look at it from. To get the correct reading, you have to look at the instrument straight on, not from the side.
Context Anchor
Pilots may encounter parallax when reading round-dial instruments, fuel gauges, sight gauges, or any cockpit mark that must be viewed straight on.
Derivation
From the Greek 'parallaxis,' meaning 'alteration' or 'change.' The word captures the idea that the apparent position of something changes when your viewing angle changes, even though the object itself has not moved.
Why Pilots Care
Misreading an instrument due to parallax can produce an incorrect altitude, airspeed, or heading indication, leading to poor flight decisions.
Analogy
Hold a finger up in front of a clock and look at it with one eye, then the other. Your finger appears to jump against the clock face even though it has not moved. That apparent shift is parallax.
Intuition Check
Parallax is not the object actually moving. It is the viewing angle making the object appear to shift.
Example Sentence 1
The pilot leaned to center their view directly in front of the altimeter to avoid a parallax error before noting the cruise altitude.
Example Sentence 2
Viewing the airspeed indicator from the right seat introduced parallax that made the needle appear one knot higher than the actual speed.