The Palolo worm of Samoa and Fiji is really extraordinary in its time-keeping. Having spent most of the year lurking dully in its shoreline rock crevice, it spawns: and each year this occurs at exactly the same moment, as Anthony Smith relates in his book The Seasons (1973) 'The precise moment of spawning is at dawn, both on the day before and on the very day when the moon enters its last quarter... this Pacific worm then subsides until the right time of the right phase of the right moon on the following year.' Human awareness of the passage of time may not be quite as acute as this, though circadian and circannian rhythms are well established phenomena, for us as for other creatures. Recently we have learnt that to be sad may be the result of Seasonal Affective Disorder - and our awareness of seasonal change and variation has been heightened by what appear to be gross and alarming irregularities.

The cycle of the seasons, which in temperate climates provides an essentially reassuring framework for local anomalies, has long been a part of our consciousness, reflected as much in literature as in the way in which we take it for granted in life. From Chaucer to Keats, Shakespeare to Wordsworth, Hardy to Hughes, the seasons have been a primary component of experience, and their effects on the landscape an essential part of expression. They are almost ubiquitous, from George Herbert's 'sweet spring, full of sweet days ...