Definition
A natural day is the period of time between sunrise and sunset — the hours of daylight. It is distinguished from the civil or artificial day (midnight to midnight) and from the calendar day (the full 24-hour period conventionally assigned to a single date). The natural day measures only the daylit portion of a 24-hour cycle and varies in duration across seasons and latitudes.
Where a legal instrument, statute, or rule specifies a period of days without qualification, courts have generally treated the operative unit as the civil day (midnight to midnight) rather than the natural day. The natural day becomes legally significant when a document expressly uses the term, or when courts must interpret archaic instruments drafted when natural-day reckoning was more prevalent.
Common Language
Modern common usage (Wiktionary): "Natural day" in ordinary English typically means a full 24-hour day as experienced in ordinary life, or loosely synonymous with "calendar day."
Historical common usage (Webster's 1913): Webster's 1913 defines the natural day as the period of daylight, from sunrise to sunset, contrasted with the astronomical or civil day of 24 hours.
The gap matters for researchers: common modern usage has drifted toward treating "natural day" as roughly equivalent to "calendar day," while the legal and historical technical meaning is narrower and specifically excludes nighttime hours. An instrument using this term in a historical context almost certainly intends the sunrise-to-sunset meaning.
Common Confusion
Natural day is frequently conflated with two related but distinct concepts:
Civil day (artificial day): The full 24-hour period running midnight to midnight. This is the default unit in modern statutory and contractual computation of time.
Calendar day: A synonym for the civil day in most modern usage, encompassing both daylight and nighttime hours.
Burrill's Law Dictionary flags the confusion directly, noting that natural day "seems to be sometimes confounded with an artificial day." The older term "solar day" appears as a synonym in Burrill, reinforcing that the intended meaning is strictly the daylit period. Researchers encountering these terms in historical instruments should not assume interchangeability.
Why It Matters in Research
The practical stakes of natural day versus civil day are highest in historical documents — colonial-era instruments, early American deeds and contracts, and pre-modern statutes — where time computation conventions differed from those now codified in rules of civil procedure and statutory interpretation.
Researchers should watch for two traps. First, historical sources may use "day" alone when they mean natural day; the explicit phrase "natural day" is a signal that the drafter was conscious of the distinction and intended to exclude nighttime. Second, modern rules of time computation (such as Fed. R. Civ. P. 6 or analogous state rules) do not recognize the natural day as a unit — they operate on civil days. Importing natural-day logic into a modern procedural calculation would be error.
Corpus connections: Terms-of-days provisions in contracts, statutes of limitation, notice requirements, and performance deadlines are all areas where the natural/civil/calendar day distinction can affect outcome. Researchers working with real property instruments, early American commercial contracts, or pre-codification statutes should flag any day-computation language for close reading.
Jurisdictional variation is unlikely to be significant in modern practice, but historical variation is real: common law courts in England and early America did not uniformly apply the same conventions, and local custom sometimes governed.
Historical Dictionary Support
The four source dictionaries agree on the core definition — sunrise to sunset — and all cross-reference their entries on "Day" for the fuller treatment of day-computation law. This cross-referencing pattern is itself informative: it signals that historical lexicographers treated "natural day" as a sub-entry within the larger taxonomy of day-types rather than as a freestanding concept with independent doctrinal weight.
Burrill adds the most analytical value among the four, identifying the synonym "solar day" and flagging the confusion with artificial day, citing Dyer 131a (a sixteenth-century English law report). This suggests the definitional tension between natural and artificial day was already a recognized problem in early English common law, not a modern ambiguity.
Black's, Rapalje, and Bouvier offer identical or near-identical definitions and simply redirect to "Day." Researchers needing the fuller common-law framework for time computation should follow that cross-reference in each source, as the substantive doctrine on how courts compute periods of days appears there rather than here.
What the historical dictionaries collectively miss: none addresses how the rise of statutory time-computation rules progressively displaced natural-day reckoning in American law, nor do they signal that the term is now largely archaic in live legal practice.
Encyclopedia Cross-Reference
No Law Mind Encyclopedia entry directly addresses natural day or legal time computation. The matching entries suggested (IRS Notice of Deficiency, Naturalization Proceedings, Contracts — Definition) are not sufficiently related to warrant cross-reference here.