•A couple of weeks have passed. a star that was just over a tree in the east when the Sunset. If you happen to be out at the same time, you’d expect that star to be in the same place. But it’s not. It’s a bit higher above the tree. And if you look west, stars that were well off the horizon just after sunset last week are now lower. If you wait a month, this effect is even more pronounced; all new constellations will be visible in the sky after sunset. This is because the Earth is going around the Sun, changing our viewpoint on the sky.

•The Earth takes a year to orbit the Sun once. Every day, it moves a little bit along with its orbit. And as it does, from Earth’s perspective, distant stars appear to move their positions relative to the sun. one day we might see a star very near the Sun, but the next day the angle is a bit bigger. about six months after we first saw it, the star is directly opposite the Sun in the sky. Then the angle starts to shrink again as the star approaches the Sun from the opposite side, until, after a full year, the cycle repeats. What this means to you is that the stars appear to rise and set at different times over the year. Stars in the east rise about four minutes earlier every night and stars in the west set four minutes earlier. A constellation that was entirely below the eastern horizon at sunset one month might be completely visible after sunset the next month.

•Another way to think about it is that the stars appear to be fixed, and as the Earth circles the Sun, the Sun moves through the stars over the year, making a complete circle around the sky once per year. The path it takes is a reflection of the Earth’s path around the Sun, a line in the sky. We call that line the ecliptic. That means the Sun passes through the same constellations in the sky every year. We give those constellations a special name: the zodiac.

•Every year, during a given month, the Sun will appear to be in a certain zodiacal constellation, from Sagittarius e.g. over a year the sun returns to Sagittarius, even though we talk about this process in terms of the sun’s movement, it’s the path traveled by the Earth that creates this effect, as our perspective moves with it. And of course, the planets move in the sky as well. they orbit the Sun, too, and they do so in approximately the same plane the Earth does.

•the solar system from the side, would look flat! So to us, on Earth, the planets go around leisure the sky for over a year, and they also appear to change their positions relative to the Sun and the stars. The inner planets, Mercury and Venus, move so rapidly you can see their motion after a single night. The outer planets are more leisurely, but wait long enough and they too will be seen to move, sliding through the constellations.

•globes are modeling the Earth are tilted because the Earth is tilted. The Earth spins on its axis once per day and orbits the Sun once per year. But the Earth’s axis is tilted concerning its orbital plane by 23.5 degrees. And this has a profound effect on our planet.

•In June and July, the Earth’s north pole is tipped toward the Sun. Six months later it’s pointed away. This affects the path the Sun takes across our sky. Instead of it taking the same path every day, in the northern summer, when we’re tipped toward the Sun, the Sun takes a higher path in the sky. Because that path is longer, the days are longer, too. Six months later, in December and January, the Earth’s pole is tipped away. The Sun takes a lower path in the sky, and because the path is shorter days are shorter too. That’s why we have seasons!

•When the Sun is up high in the sky it shines straight down on the ground, heating it better, and days are longer so it has more time to heat us. It gets hot. In the winter, it’s the reverse: The Sun is lower so it can’t warm us up as efficiently, and it has less time to do so. It gets cold. There you go seasons.

•The Earth’s axis is tipped. If it weren’t, the seasons wouldn’t change month to month. It’s the angle of the sun’s rays that make winter cold and summer hot, not our distance from the sun. when it’s summer in the northern hemisphere, it’s winter in the southern. When the north pole is tipped toward the Sun, the south pole is tipped away, so northern and southern hemisphere seasons are opposite each other.

•The north pole’s not always going to point toward the sun in June, and Polaris is not always going to be the North Star. That’s because our planet’s axis is moving. Have you ever seen a spinning top start to wobble, its axis moving in a slow circle even as the top, itself spins? This is called precession, and the Earth does it too! Our planet spins on its axis once per day, but the axis wobbles, making a very slow circle that takes 26,000 years to complete. This affects a lot of what we see in the sky. For example, Polaris won’t always be the pole star! Every year, the pole points a little farther from that star, making a big circle 47 degrees across.

•For ancient Egyptians, the star Thuban was the pole star, and in about 11,000 years that position will be held by the bright star Vega. Also, the date the Sun is in a particular zodiac constellation changes slowly due to precession as well. When the ancients first thought up this idea, the Sun was in Aries on March 22, the vernal equinox (what some people call the first day of spring). But due to precession, it’s now in Pisces! That’s why your astrological sign doesn’t match where the Sun is in the sky.