Definition
A solar month is a calendar month — that is, one of the twelve named months of the Gregorian calendar as they appear on the civil calendar, running from the first day of the month to the last. The term is used in legal instruments, statutes, and pleadings to distinguish this familiar calendar-based unit from a lunar month (approximately 29.5 days) or an arbitrary 30-day period. In practice, solar months vary in length: 28, 29, 30, or 31 days depending on the month and year.
The term carries no independent legal definition beyond its equivalence to "calendar month." When a legal document specifies a solar month, courts treat it identically to a calendar month.
Common Language
Modern common usage (Wiktionary): In astronomy and general usage, a solar month refers to the time the sun takes to pass through one sign of the zodiac, approximately 30.44 days — a technical astronomical measure.
Historical common usage (Webster's 1913): "The period of the sun's passing through a sign of the zodiac; the twelfth part of a solar year; a calendar month."
The gap matters: In ordinary and scientific English, "solar month" carries an astronomical flavor, suggesting a precise astronomical interval tied to the sun's apparent motion. In law, the term sheds all astronomical content and means simply a calendar month as commonly understood — nothing more precise, nothing more technical. A researcher encountering "solar month" in a legal instrument should not import astronomical precision; the operative meaning is purely civil and administrative.
Common Confusion
Solar month, calendar month, and lunar month are the three competing units that historical legal sources work to distinguish. A lunar month is approximately 29 days, 12 hours — the period between new moons. A solar or calendar month is one of the twelve named divisions of the civil year. In early common law and ecclesiastical contexts, "month" alone often meant lunar month by default, which is why courts and draftsmen began specifying "calendar month" or "solar month" explicitly to avoid ambiguity. The two terms — solar month and calendar month — are legally synonymous; the confusion arises only when a document uses "month" without qualification, which may require historical and jurisdictional interpretation to resolve.
Why It Matters in Research
The primary research trap is the unqualified word "month" in older instruments and statutes. Before the term "calendar month" became standard in drafting, "month" in common law contexts defaulted to lunar month in many English authorities. When researching pre-nineteenth-century documents — deeds, wills, statutes, pleadings — finding "solar month" is actually a clarifying signal: the drafter was deliberately invoking the calendar month and rejecting lunar reckoning. Its presence resolves ambiguity rather than creating it.
For corpus researchers, "solar month" appears most frequently as a cross-reference anchor. Every historical dictionary entry points immediately to CALENDAR MONTH as the operative definition. This means the substantive law — how months are counted, how partial months are treated, how month-based deadlines are computed — lives under CALENDAR MONTH, not under this entry. Do not expect independent doctrine at SOLAR MONTH; follow the chain to CALENDAR MONTH and, where deadline computation is at issue, to related entries on time.
Jurisdictional variation in month computation (whether a month runs from a date to the same date the following month, or by counting days) is not resolved by the label "solar month" — that question belongs to the law of time computation in the relevant jurisdiction.
Historical Dictionary Support
All five source dictionaries are in complete agreement: solar month equals calendar month, full stop. Black's (both editions), Rapalje & Lawrence, Bouvier, and Burrill each define the term in a single sentence and cross-reference CALENDAR MONTH or MONTH without elaboration. This unanimity itself is informative — the term was never contested or ambiguous in legal usage; it existed solely as an alternative label for calendar month, likely borrowed from common speech to add precision to instruments where "month" alone might be read as lunar.
Bouvier provides the most supporting authority, citing Coke on Littleton (Co. Litt. 135b), a King's Bench reporter (1 W. Bla. 450), Maule & Selwyn (1 Maule & S. 111), and Bingham (1 Bingh. 307) — all English authorities confirming that solar and calendar months are interchangeable in legal reckoning. Burrill cites the same Coke passage. No American authority diverges from this position. The historical sources collectively reflect a legal vocabulary that had stabilized by the early nineteenth century, treating "solar month" as a slightly formal or archaic synonym for the ordinary calendar month.
Jurisdictional Note
No meaningful jurisdictional variation affects the term itself. The equivalence of solar month and calendar month is consistent across American and English legal authority. Variation in how calendar months are counted for deadline or limitations purposes is a separate question governed by each jurisdiction's rules of time computation.