Definition
The magnetic variation value used on aeronautical charts and in navigation databases that is fixed to a specific reference year (the epoch year) rather than updated continuously. Because Earth's magnetic field shifts over time, the variation printed for a given location reflects the magnetic field as it existed in that reference year, and it remains in use until the chart or database is republished against a new epoch year.
Plain English
The angle between true north and magnetic north is always slowly changing. Charts and navigation systems pick a specific year as their reference and use the variation values from that year. That snapshot is the epoch year variation.
Context Anchor
Seen in instrument procedure and navigation data discussions where magnetic variation changes over time must be tracked against a known reference year.
Derivation
Epoch comes from the Greek epoche, meaning a fixed point in time or a stopping point. In navigation it means the specific reference moment that data is locked to, even though the real world keeps changing after that moment.
Why Pilots Care
Using the correct epoch year value prevents heading errors that grow as the Earth's magnetic field slowly shifts.
Analogy
Think of it like a timestamp on a measurement. The number is only fully meaningful when you know the year it was measured, because the real-world value can drift over time.
Intuition Check
Do not read “epoch” as a long historical era here; it means one reference year. Do not read “variation” as just any difference; here it means the angle between true north and magnetic north.
Example Sentence 1
Because the chart was published using a 2020 epoch year variation, the magnetic course shown differs slightly from what the aircraft's modern compass reading suggests.
Example Sentence 2
Before loading the updated database, confirm the epoch year variation matches the aircraft's operating area.