Chapter 15. Earth in the Universe

Earth, Moon and Sun. Earth rotates once about every 24 hours and orbits the Sun once about every 365.25 days. Its axis is tilted about 23.4°, which causes the seasons. The Moon orbits Earth about every 27.3 days, and its phases arise from our changing view of its sunlit half [17].

Eclipses. A solar eclipse occurs when the Moon passes between the Sun and Earth; a lunar eclipse occurs when Earth passes between the Sun and Moon.

The Solar System. The Sun, eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune), their moons, dwarf planets, asteroids and comets. The average Earth–Sun distance, one astronomical unit (AU), is about 149.6 million km, and sunlight takes about 8 minutes 20 seconds to reach Earth [17].

Stars. Stars shine by nuclear fusion in their cores. The Sun is a medium-sized star. Massive stars may end as supernovae, leaving neutron stars or black holes.

Galaxies and the universe. Our galaxy, the Milky Way, is a barred spiral containing hundreds of billions of stars. The universe is about 13.8 billion years old and has been expanding since the Big Bang [17][18].

Distances. A light-year is the distance light travels in one year, about 9.46 trillion km.

Review: Why do we have seasons? (Answer: because Earth’s axis is tilted relative to its orbit, not because its distance from the Sun changes much.)