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As the position of the Sun in the sky changes, the shadows it casts change position too. A sundial can be used to tell the time by the position of the shadow cast by its gnomon (the upright part) at different times of the day.

as well. A sundial has an upright part called a gnomon that casts a shadow across a surface that is marked into 12 equal sections. Each section represents an hour, matching the 12 hours of the

night. Gnomon means “one who knows”—it can tell you what the time is by the movement of the shadow on the dial. THE MOON AND THE MONTH

As well as being able to measure a day and a year it is useful to have something in between. The Moon goes through regular phases, from full moon to new moon (when the Moon is not visible in the sky) and back to full again. This happens as the moon travels round the Earth and the angle of the Sun’s rays reflecting from it changes. The time between one full moon and the next is called a month. A month, therefore, is the time the Moon takes to go round the Earth once. THE CALENDAR Problems arose when people tried to put these different ways of measuring time together to keep a record of the changing year called a calendar. For instance, a lunar month is actually just over 29½ days, and there aren’t an exact number of lunar months in a solar year. In fact, there are 12 lunar months and 11 days in a solar year. (If you do the sum you’ll see that makes 365 days.) To complicate things still further, there aren’t an exact number of days in a year, because the Earth spins around 365¼ times as it goes around the Sun once. All this means that making calendars that work is an awkward business. The Babylonians, who lived in what is now Iraq, used the lunar month as the basis of their calendar. They added an extra month every now and again to make up a year as reckoned by the Sun. The Greeks and Romans used a similar system. By the time of Julius Caesar, the calendar was in such a mess that the months that used to fall in the winter were now happening in the autumn. 12.