On screen, a pathologist glances at a body and announces the time of death to the minute. In reality there is no such clock. What forensic scientists produce is an estimate — a window — built by combining several changes that unfold at roughly predictable rates, each with its own uncertainty.
The early changes (first hours to ~2 days)
Three classic post-mortem changes are most useful early on:
- Body cooling (algor mortis). After death the body loses heat toward its surroundings. Formal methods (such as the Henssge nomogram) use body temperature, ambient temperature and body size to estimate the interval — but wind, clothing and water dramatically change the rate.
- Rigor mortis. Muscles stiffen over several hours, reach full rigidity, then relax again over a day or two — a rough sequence rather than a precise clock.
- Livor mortis (lividity). Blood settles under gravity, colouring the lowest parts of the body. Its onset, and whether it still shifts when the body is moved, helps bracket the interval and can reveal if a body was repositioned.
The later changes (days and beyond)
As time passes, the estimate leans on:
- decomposition, whose stages progress with temperature and environment, and
- forensic entomology — the species and developmental stage of insects colonising the body, which can indicate elapsed time over days to weeks.
Context and its limits
Scene evidence matters too: stomach contents and digestion, last-known-alive information, mail, lights, and phone activity all help. But every biological method is sensitive to temperature, environment and the individual, so a careful practitioner gives a range, not a single moment — and treats a confident-sounding exact time with suspicion.