A bare 20″ is not “twenty seconds” of time. The double prime ″ marks an arcsecond in angular notation—or inches in another context—while a second of time uses the lowercase unit symbol s.
That makes the character itself an important clue in astronomy, navigation, geodesy, and optics, where tiny angles and time intervals can occur together. A clear unit label often resolves what the glyph alone cannot.
The short version: ″ is angle, s is time
| Reading | Correct mark | What it measures | Typical form |
|---|---|---|---|
| Plane angle | ″ | 1/60 of an arcminute; 1/3600 of a degree | 20″ |
| Time | s | SI base unit of time | 20 s |
The double prime in angular notation
A degree contains 60 arcminutes, and each arcminute contains 60 arcseconds, so one arcsecond is 1/3600 of a degree. In 30° 10′ 20″, the double-prime component is the arcsecond part of the angle, not a time value.
The character ″ is U+2033 DOUBLE PRIME. Unicode explicitly lists seconds and inches among its uses. For plane-angle notation, the BIPM writes the degree, arcminute, and arcsecond symbols directly after their numbers, so 20″ has no space before the double prime.
The related Arcminute ′ vs Minute min guide explains the same distinction one level higher in the sexagesimal angle system.
Time seconds use s
Time intervals use the SI second, written with lowercase s. In ordinary measurement notation, a duration is written 20 s, with the familiar gap between value and unit.
That spacing difference is not merely cosmetic. 20″ and 20 s are different quantity expressions: the first is an angle, the second a time interval. A system that strips or substitutes unit symbols without understanding the quantity can silently change the meaning.
Why quotation marks cause trouble
The keyboard double quote “ is U+0022 QUOTATION MARK, while the typographic right double quotation mark ” is U+201D. Neither is U+2033 DOUBLE PRIME.
Because “ is easy to type on a standard keyboard, it is often used as a plain-text approximation for arcseconds or inches. That may be practical in an ASCII-only system, but the underlying character remains a quotation mark. Smart-quote software can complicate matters further by changing U+0022 into a curly quote that still is not the double prime.
When exact Unicode text is available, use ″ for the double-prime mark rather than relying on quote substitution.
The inch problem
The double prime changes meaning with its surroundings. In 6′ 2″ it conventionally marks inches; after degrees and arcminutes in an angular coordinate, the same U+2033 character marks arcseconds.
A bare value such as 0.5″ becomes ambiguous the moment it is detached from its heading. In a telescope specification it may describe angular resolution; in a mechanical drawing it may be a half-inch dimension. The double prime alone cannot settle that question.
Nearby notation often resolves the ambiguity quickly. A preceding degree or prime mark strongly favors an angular reading, while a dimension written with feet, inches, or a drawing scale points toward length. If none of those clues is present, the column heading or unit metadata becomes essential.
Converting arcseconds to decimal degrees
One arcsecond is 1/3600 of a degree, or approximately 0.0002777778°. Converting from degrees-minutes-seconds notation to decimal degrees therefore requires arithmetic: divide arcminutes by 60 and arcseconds by 3600, then combine the values.
Changing ″ to s, a quotation mark, or a decimal point does not perform that conversion. Character cleanup and unit conversion should be treated as separate operations in data pipelines.
Label the quantity, not just the value
Keep quantity labels with the values whenever possible. In a CSV or API response, a column name such as resolution_arcsec or an explicit unit field carries information that the small raised mark cannot preserve by itself.
If the issue is the single raised mark rather than the double prime, the guide to prime, apostrophe, and right single quote can help identify the exact Unicode character before you interpret its measurement role.
Why arcseconds are convenient for small angles
Arcseconds are useful because many angular differences are much smaller than one degree. A specification or observation can therefore use values such as 2″ or 0.5″ without writing a long decimal fraction of a degree.
That compact notation is useful in astronomy and imaging, but it depends heavily on context. Once a value is copied away from its heading, the same double-prime character can just as plausibly be read as inches.
Spacing is supporting evidence
Spacing offers another clue: plane-angle notation writes 20″ with no gap, whereas a time interval is written 20 s. The difference follows the BIPM’s special formatting rule for degree, arcminute, and arcsecond.
Real-world text is often badly spaced, so this should not be the only test. The strongest interpretation comes from several clues together: the code point, the spacing, the field label, and any nearby degree or prime marks.