When you look at the sky you're looking at the part of the sky not washed out by the sun. As the Earth goes around the sun, the night side of the Earth faces different parts of the sky. You can actually track this a bit if you go out and look at the stars each day; the movement of the Earth means that a given star will rise four minutes earlier than the day before. This means that if. for example, Sirius, the brightest star in the sky, rises at around the same time as the sun, the next day it will come up four minutes earlier, and the next day it will be four minutes earlier than that. (The first day you can see a star come up in the
pre-dawn sky is called the Heliacal rising).
You'll note that as this continues through the year, eventually the star-- Sirius in this case -- will rise when the sun sets. (Or, set around sunrise). In the northern hemisphere that means wintertime, for the most part-- Sirius is visible just about all night in December and January. Knowing what stars appear at what season was very, very important for the ancients, who used them to time plantings and harvest. Sirius is famous for this as the Egyptians used it to time the Nile floods. In Europe planting was around the Vernal Equinox, when Leo and Virgo are high in the sky at night.
The same constellations are not visible at every location on Earth, and many constellations are only visible during certain seasons. ... Because Earth is simultaneously revolving around the sun as it rotates on its axis, constellations in different parts of the sky are only visible during certain seasons.
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