Question

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

Explanation:

Earth's orbital motion around the Sun changes the portion of the celestial sphere visible from Earth throughout the year. A planisphere maps the night sky, accounting for the date and time, to quickly show which constellations are above the horizon at a given moment, as tracking this shift manually is complex.

Answer:

Because Earth's yearly orbit around the Sun causes the visible portion of the night sky (and its constellations) to shift, and a planisphere simplifies identifying which constellations are above the horizon by accounting for the specific date and time.