Q&A: Where the Sun Goes at Night

Question: My grandson just started an astronomy unit at school, and his teacher tried to explain where the Sun goes at night, but he didn't get what she said about the Earth spinning. So he asked me, and I told him it's really the Sun that's moving. I've actually seen it move when I watch sunsets. Can you confirm that? — WB, Crawford, TX

Answer: It sure looks like the Sun is moving. And it's hard to believe the Earth is moving, since we can't feel its motion, but that's exactly what's happening. The animation above shows an observer standing in southern Asia. The spinning Earth carries that observer around in one rotation every day (24 hours). The view is looking down on the North Pole. From this perspective, Earth rotates counter-clockwise.

During that time the observer experiences day and night, depending on which side of the Earth he's on. The side that faces the Sun is day, the side that faces away from the Sun is night. Sunset happens at the top (in the animation), and sunrise at the bottom. Note: The Sun is shown much closer to Earth (and much smaller) than it really is.

Here's a great exercise for your brain that improves your ability to visualize. While watching the animation, imagine you are that person riding around on the spinning Earth. To your eyes, what would the Sun look like it's doing? When would the Sun be directly overhead? When would you lose sight of it? When would it reappear? This visualization process is called changing your frame of reference. You'll learn more about that if and when you take a course in physics.

So Earth does spin. Why you don't feel this spinning, as you do on a merry-go-round, is harder to explain. What it comes down to is how fast you're changing direction. The technical term for this is centripetal acceleration. The math might look complicated, but all you need to know is: centripetal acceleration measures how fast you are changing direction.

Riding on a fast spinning playground style merry-go-round, you change direction by 360° in about 2-3 seconds. That's fast enough for you to feel a force, so you have to hang on to the bars or you'll fall off. And that's why you don't find this type of ride at many playgrounds these days — too many people have been injured on them.

On the spinning Earth it takes 24 hours for the same 360° change, and that's just too gradual for you to feel. The amount of force felt by a 50 kg (110 pound) person riding along on Earth's equator is only 1.7 newtons (6 ounces). For comparison, on some amusement park rides that move you in a circle really fast, the force can be hundreds of times greater. You might be familiar with the infamous Gravitron™.

Finally, to directly answer your question about "where the Sun goes at night," click on the thumbnail below. It's a screen capture from my tablet computer. I have an astronomy app that shows what's up in the sky wherever I point the device. Similar (free) apps are available for all tablets and smart phones. Instead of pointing my tablet up at the sky, I pointed it downward (below the horizon) shortly after sunset. It clearly shows the Sun on the "other side" of Earth.

So next time you watch a beautiful sunset, think about the fact that the Sun isn't really moving. You are being carried to the east by the rotating Earth, and it just looks like the Sun is moving.

Next Week in Sky Lights ⇒ The International Dateline Explained

Quintuple Conjunction in the West
The International Dateline Explained

Comments are closed.