Why might you need a planisphere to help determine which constellations are visible in the sky at a certain time of year?

For example, a planisphere can help you to learn the constellations or even just identify a bright star you can see at a particular time. It can also be a useful aide-mémoire when planning an observing session. Although the two discs are pinned to each other, they can still be rotated independently of each other.

Explanation:

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