The sky is not the same all year
If you step outside on a clear evening in January in the Northern Hemisphere, you are likely to see Orion, the hunter, standing high in the south. Go out at the same hour in July, and Orion is nowhere to be found. Instead, you may see Scorpius or the bright stars of the Summer Triangle. The stars themselves have not gone anywhere. What changed is which part of the sky faces away from the Sun after dark.
The main reason: Earth orbits the Sun
The Earth travels once around the Sun every year. At night, you are on the side of the planet facing away from the Sun, and you look out into space in the opposite direction from it. As Earth moves along its orbit, that opposite direction slowly turns around.
Here is a way to picture it. Stand in the middle of a room holding a lamp, with pictures hung on all the walls. If you walk in a circle around the lamp while always looking away from it, over time you would face each wall in turn. The pictures on the walls are the constellations, and the lamp is the Sun.
The sky overhead at midnight in winter is therefore the part of the sky that the Sun lies in front of in summer. In summer, those stars are in the daytime sky, drowned out by sunlight, even though they are still there above us.
A shift of about one degree per day
Earth completes its orbit in roughly 365 days, and a full circle is 360 degrees. So the direction of the Sun against the background stars shifts by close to one degree each day, which is about twice the width of the full Moon. Over a month the shift is about 30 degrees, and a year brings it all the way around.
The path the Sun appears to trace across the background stars over a year is called the ecliptic. The constellations along it make up the zodiac. The Sun is “in” a different one of them at different times of year, and those constellations are lost in the glare for about a month at a time.
Stars rise earlier each night
A related fact explains the gradual change you can notice from week to week. Earth turns once on its axis in about 23 hours and 56 minutes relative to the stars, but the Sun returns to the same spot in the sky only every 24 hours, because Earth has also moved a little along its orbit. The difference is about four minutes each day.
So any particular star rises about four minutes earlier each night. Four minutes a day adds up to about two hours per month (30 x 4 = 120 minutes), and to a full 24 hours over the year (365 x 4 minutes is about 1,460 minutes, which is about 24 hours). That is why a constellation that rises at midnight in one month rises around 10 pm a month later, and why it eventually moves into the evening sky for a season before dropping out of view.
Which constellations stay all year?
Not every constellation comes and goes. Where you live determines how much of the sky you can see.
- Circumpolar constellations. Near the celestial poles, stars circle around without ever setting. At mid-northern latitudes, Ursa Major (with the Big Dipper), Ursa Minor and Cassiopeia stay above the horizon all night every night, although their position changes through the year.
- Seasonal constellations. Those farther from the pole rise and set, and so are visible only during part of the year after dark.
- Hidden constellations. From the Northern Hemisphere, you can never see the Southern Cross. From far southern places, you cannot see the Big Dipper.
At the equator, you can eventually see the whole sky over the course of a year. At the poles, you see only one half.
Hemisphere differences
The seasons of the year are reversed between the hemispheres, but the stars do not simply swap. Someone in Australia sees Orion in the evening during their summer, around January, because the constellation is on the same side of the sky as it is for northerners at that time. It appears upside down compared with the view from the north. The southern sky also contains constellations and features, such as the Southern Cross and the Magellanic Clouds, that people at northern latitudes never see.
Not caused by the tilt
Many people link the changing sky to Earth’s tilted axis, which does cause the seasons. The tilt affects how high the Sun climbs and how long the days last, which does influence when it gets dark. But the shift in constellations is mostly a consequence of Earth’s orbital motion. Without the orbit, the same stars would be overhead at the same time every night. It is a useful distinction for anyone using a star chart.
Slow changes over thousands of years
Earth’s axis also wobbles like a spinning top, completing one cycle in about 26,000 years. This effect, called precession, slowly shifts which stars lie near the celestial pole and moves the seasonal timing of constellations. In addition, the stars themselves drift, so shapes change over tens of thousands of years. Over a human lifetime, the changes are far too small to notice.
How people have used the pattern
Seasonal star patterns have been used for thousands of years as a calendar, for example to tell farmers when to plant. Many of the constellation traditions in use today trace back in part to ancient Mesopotamia, where people tracked the sky carefully. Sailors in the Age of Exploration depended on the stars to find their way, and some animal migration is thought to involve stellar cues as well, with certain night-migrating birds, for instance, thought to orient using star patterns. Today satellites handle most navigation, so GPS navigation has largely replaced the stars for travelers.
Planets and the Moon
One more point to avoid confusion: planets and the Moon do not follow the seasonal pattern of the constellations. They move against the stellar background on their own schedules, which is why a bright planet can appear in different constellations in different years. The Moon takes about a month to pass through the zodiac, and the Moon phases follow a separate cycle. In the same way, solar eclipses happen when the Moon lines up with the Sun, whatever the constellations are doing.