## Distances And Size

The average distance between Earth and the Sun, called the astronomical unit (AU), is about 150 million km (93 million miles).

Astronomers calculate this distance from planetary data obtained by radar ranging. They use the astronomical unit as a measure of distance in the solar system (Table 8.1).

The Sun is a huge gaseous sphere. We see the apparent surface layer in the sky. Its radius (R0) is about 696,000 km (432,000 miles). From Earth, the Sun's angular diameter = 32' (about V2°) looks deceptively equal to the full Moon's. This illusion occurs because the Sun is 400 times farther away.

WARNING: You could permanently blind your eyes if you observe the Sun without first taking proper precautions! You should never look at the Sun directly and never look at the Sun through an optical instrument unless special solar filters properly cover the full aperture.

A way to observe the Sun is to project its image onto a screen and look only at the Sun's image on the screen (Figures 4.2 and 4.3).

About how many minutes does it take for sunlight to travel 1 AU? Tip: distance = speed x time. You can rewrite this: time = distance/speed._

Answer: About 8.3 minutes. (That means that if the Sun stopped shining, you would not know about it until 8.3 minutes later.)

Solution: The speed of light is ~ 300,000 km (186,000 miles) per second and

1 AU = 150,000,000 km 150,000,000 km 93,000,000 miles

300,000 km/second 186,000 miles/second

= 500 seconds or 8.3 minutes

Sun image

### Reflecting I telescope

Figure 4.2. Projected image of the Sun focused on a screen behind the eyepiece of a small reflecting telescope. A way to avoid looking at the Sun while pointing a telescope is to use the shadow of the telescope cast on the screen as a guide.

Figure 4.3. Projected image of the Sun focused on a screen behind the eyepiece of a small refracting telescope. A way to avoid looking at the Sun while pointing a telescope is to use the shadow of the telescope cast on the screen as a guide.