Surprisingly, the number of hours per day and days per year are changing. We're not sure what they were when life first appeared on earth about 3,800 million years ago when organisms lacked skeletal hard parts, but we do know what they have been since corals and other marine shell-bearing animals first evolved some 600 million years ago. The daily and yearly incrementally-formed growth lines on fossil invertebrates show that there were about 425 days per year and 21 hours per day 600 eons (million years) ago. (A day is based on earth's rotation period; a year is based on our orbital period).

Since then (600 million years ago), the fossil growth lines indicate that days per year have decreased and hours per day have increased; to roughly 405 days per year and 22 hours per day at about 400 million years ago to approximately 385 days per year and 23 hours per day at roughly 200 eons ago, and to 365 1/4 days per year and 24 hours per day at present. These changes are happening naturally to conventional rotational time that is based on earth's spin rate, which is slowing down, gradually due to oceanic tidal friction, and sporadically because of irregular sloshing of molten material in earth's deep outer core.

There is another, more accurate type of time that we use - atomic time. This never changes and is based on one second of time always being equal to 9,192,631,770 vibrations of the cesium atom.

There are valid reasons for keeping track of two kinds of time. Rotational time, although it is as inconstant as Juliet's moon, has been used for centuries, and is essential for astronomers in order to locate stars and other deep-space objects from a planet that is rotating eastward at 15 degrees per hour. It is also preferred by historians and geologists to record events of the past using the same time scale as their predecessors did: one second is 1/60th of a minute, 1/3,600th of an hour, 1/86,400th of a day and 1/31,557,600th of a year.

Advocates of atomic time, who deal with modern telecommunications, want consistently reliable unchanging time units such as the atomic second which always equals the same number of cesium atomic vibrations.

Since each time system has a different base, their units must be adjusted occasionally to keep the two systems synchronous with each other. This is done by adding an extra (leap) second to rotational time before it deviates more than one second from atomic time. It was last done at the last minute of the 2005 year end and resulted in a 61-second minute having occurred just before the 2006 year began.

Life having been present on earth throughout the entire 600 eons that shelled organisms bearing growth lines have existed, indicates that temperatures essential for survival also had to be present. This shows that earth has always orbited at approximately the same distance (93 million miles) from the sun, otherwise temperatures would have been too hot or too cold for life to survive. This in turn shows that the year length has never changed, because the year, earth's orbital period, would have been shorter and hotter if we were closer to the sun, and longer and colder if we were farther out.

Slowing of rotational time will continue until we spin at the same rate that the moon orbits the earth: once per month (which time unit by then will also have changed).

This ends this column - which is about time.