In this lesson, students model the apparent movement of stars across the night sky to describe why stars are visible at night, but not during the daytime. They focus on the crosscutting concept of Patterns when they describe how the movement of stars across the sky repeats night after night. They also engage with the crosscutting concept of Cause and Effect when they explain the reason for this apparent movement, and why stars are not visible by day.
Whole Class Activity:
This short video shows star trails being made. It is a permanently mounted video camera that is constantly taking pictures in order to create "trails" as the stars appear to move across the sky.
This short video will show you that the earth is what moves (spins), not the stars in the sky. Each video camera has a "clock drive mount" on it that moves at the exact same rate of speed as the earth turns!
Why do stars and constellations rise and set each night?
The North Star, Polaris, lies very near the rotation axis of the celestial sphere, right above the Earth’s north pole. Since it’s almost right on the north celestial pole, Polaris appears to stay nearly fixed in the sky all night and all year, just as the Earth’s north pole stays fixed as the rest of the Earth’s surface moves around it. Any other star on the celestial sphere south of Polaris rotates in circles of increasing diameter about the rotation axis. It’s the same with the south celestial pole. Stars above the Earth’s equator trace out the largest circles around the sky during their daily motion across the celestial sphere as the Earth turns. And south of the equator, stars trace out circles with smaller apparent diameters as they lie closer to the south celestial pole. The images to the right give you a better idea of how the stars appear to rotate during the day.
Star trails caused by the apparent rotation of the celestial sphere around a celestial pole ...
Like the stars and planets, the Sun also appears to move on the celestial sphere. If you measure the time when the sun is highest in the sky, you will find it takes exactly 24 hours for the Sun to move all the way around the celestial sphere and return to its highest point. In fact, that’s how we define a “day”, or what astronomers formally call a solar day.
How about the stars? If you go out at night and select a star to observe, and measure its position on the celestial sphere, you will find it takes 24 hours to move all the way around the sky and get back to the same spot.
Well, almost 24 hours.
If you measure accurately, you’ll find it takes only 23 hours and 56 minutes for a star to get back to the same position in the sky as it was the night before. That’s because, during the day, the Earth revolved around the Sun by 1/365 of its orbit. So each day, you look in a slightly different direction in space, and this causes every star to appear to rise 4 minutes earlier each night. In two weeks, the star rises about an hour earlier; in one month the star rises 2 hours earlier, and in 12 months, it appears to move all the way around the sky back to the position at which you first saw it the previous year.
This apparent motion where the stars rise a little earlier each night, which is caused by the Earth’s revolution around the Sun, explains why the stars you see in the night sky in each season are different than the stars you saw during the last season.
Why can't you see stars during the day?
Why do stars glow at night and not during the day?"
Stars do glow during the day, but we can't see them because of the glare of sunlight. When the sun is up, the blue color in sunlight gets scattered all over the atmosphere, turning the sky the familiar bright blue color. This blue light is much brighter than the faint light coming from the stars, so it prevents us from seeing them.
If you were standing on the Moon, for instance, where there is no atmosphere, you would see the stars both day and night.
Video Evidence of Learning:
- Create a video that demonstrates the following concepts from today's lesson. Use any models, props, videos, or photos that you might need:
- Demonstrate and explain how stars appear to travel through the night-time sky in a predictable daily pattern. What does this have to do with the rotation of the earth?
- Explain why stars are visible at night, but not during the daytime.
- Explain why the sun, moon, and many night-time stars appear to rise and set, while some night-time stars appear to travel in circles.
- Describe and explain any other concepts that you learned during today's lesson.
- No reflection questions today (since the video synthesizes your knowledge well).