2.1 Our changing view of the sky

The apparent motion of objects in the night sky is caused by the rotation of the Earth, which turns on its axis once every 24 hours. As a result of this rotation, the Moon and other objects as seen from the northern hemisphere will rise on the eastern side of the sky, reach their highest point in the south, and set towards the west.

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As the sun sets behind Mount Teide, we're reminded that the apparent motion of the sun across the sky is actually caused by the rotation of the Earth. As the planet turns on its axis, the sun first rises in the east, comes to a highest point in the south, and then sets here in the west.

Now, after dark, of course, the Earth keeps turning. And so the moon and all of our celestial objects will follow a similar path across the sky, from the east, to a highest point in the south, and setting in the west. Looking to the north, that's the north celestial pole. And that's the one fixed point in the sky. And the entire celestial sphere appears to rotate about this point.

This is all crucial to planning your observations with Coast, because it's important to know when a particular object is going to be visible, where to find it in the night sky, and what's the best time to observe it. So for instance, if the object you're interested in is in the western part of the sky as the sun's setting, that means the object's going to be setting as well soon after dark. So you should plan to observe that one early on in the evening. Whereas if an object's in the eastern part of the sky, that means it's rising. So you can plan to observe that one later in your observing session when it's nice and high.

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If you are lucky enough to have a clear view of the night sky from home or nearby, you may like to observe this motion by looking out at hourly intervals during a clear evening and noting the positions of a bright object such as the Moon or a bright star over a period of a few hours and follow its motion across the sky.

The Earth also moves in its orbit around the Sun, causing a slower change in our view of the night sky with the seasons. Each month, the side of the Earth facing away from the Sun faces out in a different direction into space. Over the course of a year objects will rise slightly earlier each evening, with the whole pattern of constellations taking twelve months to go through one complete cycle.

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Figure _unit2.3.1 Figure 5 The Earth in orbit around the Sun. Different constellations will be visible at different times of year as the night side of the Earth looks out in different directions into space. Orion, for instance, will be visible in the winter months, but not in the summer.

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The Earth in orbit around the Sun. Different constellations will be visible at different times of year as the night side of the Earth looks out in different directions into space. Orion, for instance, will be visible in the winter months, but not in the summer.

Figure 5 The Earth in orbit around the Sun. Different constellations will be visible at different times of year as the night side of the ...

Of course not everyone will have access to a good viewing site, and the weather can also make observing difficult. Fortunately, much of the observing that you will be doing on this course will be done from the COAST observatory, which is high up and has a much higher likelihood of clear skies. It’s time for our first visit to Tenerife!