Beyond Earth · Lesson 3 of 7
The Sun and Our Solar System
“Use the nearest star to understand light, distance, and the two motions of Earth.”
• Describe the Sun as a star and explain why it appears larger and brighter than other stars. • Explain the Sun’s role as the chief source of light and heat for Earth. • Describe the Solar System and distinguish revolution from rotation. • Interpret the Earth–Sun distance and the astronomical unit as a scale comparison.
Our nearest star
The Sun is a very hot ball of gas that produces its own light and heat. It is the closest star to Earth, so it looks much larger and brighter than other stars. Those distant stars have not vanished during the day: the sunlit sky is too bright for us to see their faint points of light. Looking smaller does not mean a distant star is necessarily physically smaller; some are bigger than the Sun.
The star at the centre of our Solar System and the main source of Earth’s light and heat.
Sunlight supports plants, which provide food and oxygen for many living things. The Sun’s heating also influences weather, winds, seasons, and the water cycle. Earth’s suitable conditions depend on a combination of factors, so do not reduce every process to sunlight alone. Cultures have long honoured the Sun; in India it is known as Sūrya.
A large star at a great distance
The Sun is roughly 100 times Earth’s diameter, yet from here it appears as a disk in the sky because it is about 150 million kilometres away. Distance is easier to compare when we use a convenient unit. One astronomical unit, written au, is approximately the Earth–Sun distance. The nearest star beyond the Sun, Proxima Centauri, is about 269,000 au away in the chapter’s comparison. That helps explain why stars still look like points.
A unit of distance approximately equal to the average distance from Earth to the Sun, about 150 million kilometres.
Problem
Earth is about 1 au from the Sun, while Proxima Centauri is about 269,000 au away. What does that say about how the two stars look?
- 1.An au is one Earth–Sun distance, not one kilometre.
- 2.Proxima Centauri is hundreds of thousands of Earth–Sun distances away.
- 3.Its great distance is one reason it appears as a point while the nearby Sun appears as a disk. The comparison does not measure their actual diameters.
What belongs to the Solar System?
Earth is one of the objects associated with the Sun. The Sun, the eight planets, their moons, and numerous smaller bodies such as asteroids and comets make up the Solar System. Many of these bodies travel around the Sun. The Sun is its largest and heaviest object and produces nearly all its energy. Planets and moons are visible mainly because they reflect sunlight; they do not shine in the same way that a star does.
The Sun and the planets, moons, and smaller bodies associated with it.
Two motions of Earth
Earth goes around the Sun in nearly one year. This motion is called revolution. At the same time, Earth turns about its own axis; one turn takes approximately 24 hours, a day. These are simultaneous motions of the same planet, not two names for one motion. Other planets also rotate about their axes while revolving around the Sun.
The movement of an object around another object; Earth revolves around the Sun.
The turning of an object about its own axis; Earth completes approximately one rotation in 24 hours.
Revolution is a journey around the Sun; rotation is a turn about an axis. A diagram on a flat page is only a simplified view of a three-dimensional system.
Quiz
Why does the Sun appear larger than other stars?
Why are other stars generally not visible during the day?
About how far is the Sun from Earth?
Which motion takes Earth nearly one year?
Which statement about planets and stars is sound?
Practice Problems
- Explain why the Sun is a star even though it looks different from the night-sky stars.
- Describe two ways that energy from the Sun matters to life or processes on Earth.
- Explain what 1 au means and compare it qualitatively with 269,000 au.
- Draw Earth turning about its axis while travelling around the Sun. Label rotation and revolution and their approximate times.
- List the major kinds of Solar System members, indicating which produces its own light and which mostly reflect sunlight.
- A student claims that stars cease to exist each morning. Use the brightness of the daytime sky to respond.
Key Takeaways
• The Sun is our nearest star and the main source of Earth’s light and heat. • One au is about the Earth–Sun distance; other stars are vastly farther away. • The Solar System includes the Sun, planets, moons, and smaller bodies. • Earth revolves around the Sun in nearly one year and rotates in about 24 hours.