"You see, we reckon our day by two counts: one of them begins at noon and goes on—one, two, three, four o'clock, etc.—up to midnight; the other begins at midnight and ends at noon."
"That's simple enough. I get that all right."
"Now people didn't always do that. There were other countries that planned their day differently. The ancient Babylonians, for instance, began their day at sunrise; the Athenians and Jews at sunset; and the Egyptians and Romans at midnight."
"How funny! I thought that of course it had always been done as we do it," confessed Christopher, with frank astonishment.
"Not at all. Our present system of time-keeping has been evolved out of the past and, like many other such heirlooms, is the result of a vast amount of study. Centuries ago nobody knew how to reckon time or what to reckon it by. Some computed it by the sun and had what is known as the solar day—a span of twenty-four hours; others figured it by the moon and got a lunar day of twenty-four hours and fifty minutes; while still others resorted to the stars or constellations and reached a result known as sidereal time, a day of twenty-three hours, fifty-six minutes. Now you see there is quite a bit of difference in these various reckonings. The difference might not matter so much on land, but when one is at sea and has to compute latitude and longitude, it matters a vast deal."
"Oh!" A light of understanding was slowly dawning on the boy.
"Now," went on McPhearson, "apparent solar time is dependent on the motion of the sun and is shown by the sundial; mean solar time, on the other hand, is shown by a correct clock; and the difference between the two—or the difference between apparent time and mean time is technically known as the equation of time, and is set forth in a nautical almanac published by the government."
McPhearson waited a moment.
"And that's what mariners use?"
"Yes."