Why isn’t Easter the same date each year? Unlike Christmas, it relies on different religious calenders and astronomy tricks, in particular the Spring Equinox and the full moon, meaning it is complex to co-ordinate. The most we achieve are close matches: a lunar month is just over four weeks of earth days; a solar year is near to 12 lunar months. But if you make that rule absolute, things gradually get out of sync.
Our own bodies share similar issues when it comes to circadian rhythms. Although individual cells isolated in a dish display a roughly 24-hour cycle, they need to be synchronised for a whole organism to work effectively. In studies where respondents ‘free-run’, ie where they are shielded from time cues – light, sound or action – the internal body clock shifts to a cycle of a little more than 24 hours, gradually losing sync with the day.
There are pathways to transmit light from the retina to the suprachiasmatic nucleus, a tiny bit of brain above the crossing of the optic nerves: the central timekeeper. Complex feedback mechanisms keep the whole thing running. For religious dates and biology, controlling cycles is a tricky business.
Dr Daniel Glaser is director of Science Gallery at King’s College London
How the brain keeps track of time | Daniel Glaser
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