Both reduce to a bare letter c in plain text, and both appear in scientific writing.
The degree sign is the discriminator, and it is the thing plain text loses. °C is a temperature and needs its preceding degree sign and a space before the number; italic c is a speed and stands alone in an equation. Where formatting is stripped — a filename, a search box, a plain-text note — they become indistinguishable.
Both reduce to a bare letter c and appear together in scientific writing.
Drawn from the visual attributes recorded for each symbol. Only traits the two records disagree about are listed.
| Trait | Degree Celsius symbol | Speed of light symbol |
|---|---|---|
| Built from | circles and letter like | letter like |
| Degree Celsius symbol | Speed of light symbol | |
|---|---|---|
| What it is | A unit of temperature | A fundamental constant |
| Written form | Degree sign then capital C | Italic lower-case c, alone |
| Value | A scale, not a value | Exactly 299 792 458 m/s by definition |
| Survives plain text? | Loses the degree sign | Loses the italic |
| Relationship (from Symbolwise's records) | Commonly confused with Speed of light symbol — Both reduce to a bare letter c and appear together in scientific writing. | Commonly confused with Degree Celsius symbol — Both reduce to a bare letter c and appear in the same scientific writing. |
Wikipedia · retrieved 2026-07-31
Tertiary reference. Used for origin narrative and for units the SI does not define, never for an official-definition claim.
Wikipedia · retrieved 2026-08-01
Tertiary reference. Used for what a symbol denotes in ordinary scientific practice and for which fields reuse it, never for a claim that any body assigns the letter its meaning.
CODATA Internationally Recommended Values of the Fundamental Physical Constants
NIST / CODATA Task Group on Fundamental Constants · retrieved 2026-08-01
Authoritative for the recommended value and uncertainty of a physical constant, and for whether that value is exact by definition or measured. It fixes values; it does not decide which letter denotes which quantity, and no record cites it for a symbol's meaning.
Check the letter after the degree sign. C is Celsius, on which water freezes at 0 and boils at 100. F is Fahrenheit, on which the same points are 32 and 212. The scales coincide at −40 and nowhere else, so a number without its letter is genuinely ambiguous — and at everyday temperatures the same figure means very different weather.
A change of one kelvin and a change of one degree Celsius are exactly the same size, so a temperature *difference* may be stated in either. The scales start in different places: 0 K is absolute zero and 0 °C is the freezing point of water, 273.15 K. The written forms differ too — the kelvin takes no degree sign, so 300 K is right and 300 °K is not.
Alpha is a Greek letter and stands for a quantity — an angle, a coefficient, a significance level. The proportionality sign is an operator and stands between two quantities to say they keep a constant ratio. The drawings differ in that the proportionality sign's right-hand stroke runs out in an open curve rather than closing into the letter's tail.
The becquerel counts nuclear decays — one per second — and says nothing about harm. The sievert weights absorbed energy by the kind of radiation delivering it, and so estimates biological effect. A becquerel of one isotope and a becquerel of another can carry very different risk, which is exactly why exposure limits and medical doses are stated in sieverts and never in becquerels.