4: Measuring Time in the Sky

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Time is one of the slipperiest concepts in all of science. Everyone feels that they know what time is, but when we try to measure it, we quickly run into difficulties. For early scientists and astronomers, the sky itself served as the first clock and calendar.

The sky above us is constantly changing and full of wonderful objects that never stop moving! As scientists and astronomers, one of our first tasks is to be able to say when and where something interesting happened. The ability to locate things in time and space, both in an absolute sense, and in relation to one another, is a fundamental skill. In this unit, we will explore measuring the Earth-Moon system with time, and then move on to show how science can accommodate different ideas and explanations for the same observations! Only experiments can tell us which model is correct!

4.1: The Earth Clock
The concept of time is intimately connected with astronomy, and more particularly with the spinning Earth. We divide the Earth into 24 time zones, it takes the Sun one hour to move across each one of these zones. The motion of the sundial’s shadow around the gnomon gives is the ‘clockwise’ direction (turning to the right). This motion is also intimately related to the Earth’s spinning motion on its axis.
4.2: Moonrise and Moonset
Everyone is aware that the Sun rises early each morning, the time changes a bit from season to season, but sunrise is remarkably consistent. Moonrise is no such thing! Many people know that the Moon is sometimes visible in the early morning sky, but few people take note that the Moon rises about an hour later each day. If the time of sunrise is so consistent, why is the time of moonrise so variable?