In the legal sense, a penumbra is a logical extension of a rule, law, or legal statement that provides people with rights not explicitly delineated in the law. This concept dates to 19th century legal precedents in the United States. Justice Oliver Wendell Holmes contributed significantly to the body of legal discussion on this concept and referred to it in several court cases. One of the most famous invocations of the legal penumbra occurred in the 1965 Griswold v Connecticut case.
Under the logic of this legal theory, a law can imply rights without stating them outright. As long as a reasonable interpretation of a law could provide for a given right, a judge could argue a legal matter falls within the penumbra of the law. While the reasoning may be somewhat shaky and the legal basis can be difficult to prove, if attorneys and judges can argue the matter persuasively, people may accept it.
The right to privacy is an excellent example of a penumbra. Many people believe this right is enshrined in the Constitution of the United States. It is actually not. Instead, judges and legal scholars argue that clauses like the First Amendment include a right to privacy in their penumbra and numerous legal cases have established a body of case law to support this belief, making it difficult to challenge. In Griswold v Connecticut, a challenge to a ban on selling contraceptives, the argument was that this law violated marital privacy and, by extension, the first amendment.
This term is borrowed from astronomy, where the penumbra is the shaded area surrounding a total eclipse. Rather than being definitively stated in a law, the rights are implied in the penumbra, making it a bit of a legal gray area. It is possible to challenge the logic an attorney or scholar uses when laying out the evidence for attaching a given right to a particular rule of law, using supporting documentation like other laws, records from people who participated in the drafting of the law, and so forth.
Legal scholars, attorneys, and judges rely on theories like this to interpret the law, adding meaning and depth to it over time. If people have to read the law literally, they may find loopholes making it difficult to judge certain kinds of cases fairly. The law often has trouble keeping pace with society, and being able to extend logical rights to people on the basis of precedent and implications in existing law is an important legal tool.