The Universe and the Solar System
Senior 5 · Geography
The Universe and the Solar System
Lesson 1 of 3
25 min
On a clear night away from town lights, the sky over Rwanda shows a pale band of light stretching from one horizon to the other. Look at it through binoculars and it breaks up into countless faint stars. That band is the Milky Way: our own galaxy, seen from inside. Every star you can see with the naked eye belongs to it, and the Sun is one of them. Geography begins its study of the Earth here, because the Earth's place among these bodies controls its day and night, its seasons and its climate.
Definition
Universe: all of space and everything in it: matter, energy, stars, planets and galaxies. Galaxy: a huge system of stars, gas and dust held together by gravity. Example: the Milky Way, which contains roughly 100 to 400 billion stars.
Definition
Star: a ball of hot gas that produces light and heat by nuclear fusion in its core. Example: the Sun. Constellation: a group of stars that form a pattern in the sky as seen from the Earth, used to name regions of the sky. The stars in a constellation are usually at very different distances from us. Example: Orion, which is overhead from Rwanda on evenings in January.
Definition
Light-year (ly): the distance light travels in one year, used for distances between stars. Light travels about 300,000 km every second, so
Astronomical unit (AU): the mean distance from the Earth to the Sun, about 149.6 million km, used inside the solar system.
Astronomers group galaxies by shape. Spiral galaxies have a bright central bulge and arms that wind out from it; the Milky Way is a spiral with a bar of stars across its centre. Elliptical galaxies are smooth oval swarms of old stars with little gas. Irregular galaxies have no definite shape. The Milky Way is about 100,000 light-years across, and the Sun lies in one of its arms, about 26,000 light-years from the centre (NASA). Our nearest large neighbour, the Andromeda Galaxy, is about 2.5 million light-years away, so the light reaching us tonight left it about 2.5 million years ago.
Rwanda's position, about 2° south of the equator, gives its people an unusual view: during a year they can see almost the whole sky. The Southern Cross (Crux), used for finding south, rises above the southern horizon, while the Plough, part of the northern constellation Ursa Major, appears low in the north. Someone in Europe never sees the Southern Cross, and someone in Cape Town never sees all of the Plough.
Definition
Solar system: the Sun and every body held in orbit around it by its gravity: eight planets and their moons, dwarf planets, asteroids, comets and meteoroids. Planet: a body that orbits the Sun, is massive enough to have pulled itself into a round shape, and has cleared its orbit of other bodies. Pluto does not meet the last condition and has been called a dwarf planet since 2006.
The Sun holds 99.8% of the solar system's mass. It is a medium-sized star about 1.39 million km across, 109 times the Earth's diameter, with a surface temperature of about 5,500 °C. It is the source of almost all the energy that drives the Earth's winds, ocean currents, water cycle and plant growth.
| Planet | Mean distance from Sun (million km) | Distance (AU) | Diameter (km) | Time for one orbit | Time for one rotation |
|---|---|---|---|---|---|
| Mercury | 57.9 | 0.39 | 4,879 | 88 days | 58.6 days |
| Venus | 108.2 | 0.72 | 12,104 | 225 days | 243 days (retrograde) |
| Earth | 149.6 | 1 | 12,756 | 365.25 days | 23.9 hours |
| Mars | 228.0 | 1.52 | 6,792 | 687 days | 24.6 hours |
| Jupiter | 778.5 | 5.2 | 142,984 | 11.9 years | 9.9 hours |
| Saturn | 1,432.0 | 9.58 | 120,536 | 29.4 years | 10.7 hours |
| Uranus | 2,867.0 | 19.2 | 51,118 | 84 years | 17.2 hours (retrograde) |
| Neptune | 4,515.0 | 30.1 | 49,528 | 165 years | 16.1 hours |
Data: NASA Planetary Fact Sheet. Retrograde means the planet spins in the opposite direction to most others.
The four inner (terrestrial) planets, Mercury, Venus, Earth and Mars, are small, rocky and dense, with few or no moons. The four outer (giant) planets are far larger: Jupiter and Saturn are gas giants made mostly of hydrogen and helium, and Uranus and Neptune are ice giants. All four have ring systems and many moons. Between Mars and Jupiter lies the asteroid belt, and beyond Neptune the Kuiper belt of icy bodies, including Pluto.
Worked example 1: light time from the Sun
Problem. How long does sunlight take to reach the Earth?
Solution.
So we see the Sun as it was about 8 minutes ago. Light from Neptune's distance (30.1 AU) would take minutes, more than 4 hours.
Worked example 2: a scale model of the solar system
Problem. In a model at a scale of 1 to 1,000,000,000, the Sun is a ball 1.39 m across. How big is the Earth and how far away is it? How far away is Neptune?
Solution. Divide every real length by . Earth: , a marble. Its distance: . Neptune: away. The solar system is almost entirely empty space.
Warning
Common mistakes
Tip
Memory hook. The order of the planets: "My Very Educated Mother Just Served Us Noodles": Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
CrashCourse: "Introduction to the Solar System: Crash Course Astronomy #9". Watch for how the solar system formed from a spinning disc, which explains why the inner planets are rocky and the outer ones are giants.
Practice
Summary
Sources and credits
Written by Ganzaa. Planetary data: NASA Planetary Fact Sheet, NASA Goddard Space Flight Center; Milky Way figures from NASA Science. Figure 1 was drawn by Ganzaa from those data.
Photo: "Andromeda Galaxy (with h-alpha)", Adam Evans, Wikimedia Commons, CC BY 2.0, unaltered, caption by Ganzaa.
Video: "Introduction to the Solar System: Crash Course Astronomy #9", CrashCourse, embedded from YouTube.
