Observers in the Northern Hemisphere see the zodiacal constellation Gemini in the winter. Why do they not see it in the summer?

Earth orbits the Sun once every year. This movement defines a year and causes the changes in the night sky. Earth also rotates on its own axis once every 24 hours, which gives us day and night. The path that Earth follows around the Sun is nearly circular, but not quite—it’s actually an ellipse. This elliptical orbit means Earth’s distance from the Sun changes slightly over the year, but this variation is minor. The important aspect is that Earth’s position relative to the Sun changes continually.

Imagine being on Earth, looking up at the night sky at different times of the year. Depending on where Earth is in its orbit, different regions of space become visible.

Zodiacal constellations are a special group of constellations that lie along the ecliptic. The ecliptic is the apparent path that the Sun takes through the sky over the course of a year. These constellations are divided into twelve segments, each corresponding to a zodiac sign.

When people talk about their 'zodiac sign,' they are referring to the constellation in which the Sun was located at the time of their birth. Because the Sun moves through these constellations over the year, we see different zodiacal constellations at different times.

For instance, if we observe Gemini in the winter, it means the Sun is in the opposite part of the sky.

The night sky changes with the seasons due to Earth's orbit. As Earth travels around the Sun, the night side of our planet faces different parts of space. This means constellations you can see in winter are not the same as those you see in summer.

In winter, the night side of Earth faces the direction of the Gemini constellation, making it visible. But in summer, Earth has moved in its orbit so the Sun is in the Gemini constellation's direction. This makes Gemini invisible at night, as it is above the horizon during the day.

This seasonal cycle repeats every year, with constellations like Leo becoming visible in spring and Scorpius in the summer.

The ecliptic is the imaginary line that defines the Sun's pathway as seen from Earth. It’s based on Earth’s orbit around the Sun and marks the plane of this orbit. The zodiacal constellations are all located along the ecliptic.

The relevance of the ecliptic is in understanding why we see certain constellations at specific times. Because the ecliptic represents the Sun's path, the constellations along this line will be visible at different times throughout the year.

For example, since Gemini lies along the ecliptic, it becomes visible when Earth’s night side faces away from the Sun and towards Gemini, primarily in winter. Conversely, when the Sun is located near Gemini during the daytime in summer, this constellation disappears from the night sky.