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One degree of longitude corresponds to four minutes of mean solar time.
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WORKED EXAMPLE
Longitude：120 · Target longitude：121
120° → 121° · Longitude-only mean solar time difference.
Subtracts the origin longitude from the target and multiplies by four minutes. East is positive; a positive result means the target’s mean solar clock is ahead.
This isolates longitude only, excluding civil zones, DST and the equation of time. Across ±180° it keeps the signed coordinate difference, not the shortest route.
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