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Earth's orbit, computer illustration. This diagram shows the Earth in winter and summer. It also shows the seasonal constellations Orion and Scorpius. In winter (in the northern hemisphere) Orion is visible because the night side of the Earth faces it. In summer it is above the horizon only during the day, so it cannot be seen. Similarly, Scorpius is only visible in summer. (KEYSTONE/SCIENCE PHOTO LIBRARY/MARK GARLICK)