Final answer:
Constellations cannot be seen all year long because of seasonal changes in the night sky caused by Earth's revolution around the Sun. At the North Pole, one can see half the stars over a year, but astronomical observations are limited in far northern latitudes during summer due to the midnight Sun.
Step-by-step explanation:
You can't see all constellations all year long due to seasonal changes in the night sky. This happens because as the Earth revolves around the Sun, we observe different segments of the night sky at different times of the year. The part of the sky we are facing at night changes as the Earth progresses along its orbital path.
As described in Figure 2.6 from your textbook, the ecliptic is the apparent path of the Sun against the background stars, which means that constellations lying along the ecliptic will be obscured by the Sun's glare during the day and are not visible.
From the North Pole, you could theoretically see half of the total stars over the course of a year as Earth rotates, since the entire celestial hemisphere is visible. During the summer months in far northern latitudes, the Sun doesn't set for an extended period, which leads to the phenomenon known as the midnight Sun, making it difficult to observe stars due to the prolonged daylight and sky brightness.