Our place in space

Look up on a clear night and every star you can see with your own eyes belongs to one galaxy, the Milky Way. You are standing on a spinning ball of rock, circling one ordinary star among billions. This lesson finds out exactly where.

Every star you can see with your own eyes belongs to the Milky Way.

Look

Stand outside on a clear night. The ground under your feet is part of a rocky ball 12,742 km wide. That ball, Earth, is spinning once a day and racing around a star, the Sun, once a year.

The Sun is so big that about 1.3 million Earths would fit inside it. It is so heavy that it holds almost all the mass of everything that orbits it: planets, moons, dwarf planets, asteroids and comets. Together, the Sun and everything held by its gravity is the solar system.

In the orrery above, the yellow dot in the middle is the Sun. Press Play and watch the planets move. The closer a planet is to the Sun, the faster it goes around:

• Mercury takes 88 days.
• Earth takes one year.
• Neptune, the farthest planet, takes almost 165 years.

Our solar system is one of billions of star systems in a huge spiral of stars called the Milky Way. On a very dark night you can see it as a faint band of light across the sky. Every star you can see with your own eyes belongs to it.

Understand

Distances in space are so large that kilometres become awkward, so astronomers use bigger rulers.

The astronomical unit (au) is roughly the average distance between Earth and the Sun. Since 2012 it has an exact value: 149,597,870,700 metres, about 149.6 million km. With it, the solar system becomes easy to picture:

World	Average distance from the Sun
Mercury	0.39 au
Earth	1.00 au
Jupiter	5.2 au
Neptune	30.1 au

Light-time is the time light needs to cover a distance. Light moves at 299,792 km per second, so:

time = distance ÷ speed of light

For 1 au: 149,597,871 km ÷ 299,792 km/s ≈ 499 s, or 8 minutes 19 seconds. When you look at the Sun (never directly, it damages your eyes), you see light that left it about eight minutes ago. Sunlight takes over four hours to reach Neptune.

The light-year is the distance light travels in one year: about 9.46 trillion km, or 63,241 au. The nearest star after the Sun, Proxima Centauri, is about 4.2 light-years away. The Milky Way is roughly 100,000 light-years across.

Master

The au used to be measured: it was defined through the Gaussian gravitational constant, so its length in metres was a quantity astronomers had to determine. Radar ranging of Venus and spacecraft tracking in the late twentieth century pinned it down to a few metres. In 2012 the International Astronomical Union made it a fixed number (Resolution B2), so the au is now simply a unit, like the metre.

What we actually measure very precisely today is the Sun’s gravitational parameter, GM☉ = 1.32712440041 × 10²⁰ m³ s⁻², from the motion of planets and spacecraft. The product G × M is known far better than either G or M alone, because G is one of the least precisely known constants in physics. That is why orbital mechanics works with μ = GM throughout, as you will see in the Navigator path.

Two practical consequences:

1. Earth’s distance is not constant. Earth’s orbit has an eccentricity of about 0.0167, so its distance from the Sun ranges from about 147.1 million km (early January, perihelion) to 152.1 million km (early July, aphelion). Seasons are not caused by this: January is winter in the northern hemisphere. The real cause is covered in the Seasons lesson.
2. Light-time is a real engineering limit. A radio command to a rover on Mars takes between about 3 and 22 minutes one way, depending on where both planets are in their orbits. Spacecraft far away must be able to act on their own.