The Earth's atmosphere is a thin layer of ___________________________ that surrounds the Earth. It composed of 78% ___________________________, 21% oxygen, 0.9% argon, 0.03% carbon dioxide, and trace amounts of other gases.

The Earth has one ___________________________. The moon is about one quarter of the diameter of the Earth. The moon may have once been a part of the Earth; it may have been broken off the ___________________________ during a catastrophic collision of a huge body with the Earth billions of years ago.

On average, the Earth orbits at ___________________________ miles (149,600,000 km) from the Sun. The Earth is closest to the Sun (this is called perihelion) around January 2 each year (when we are 91.4 million miles = 147.1 million km from the Sun); it is farthest away from the Sun (this is called ___________________________) around ___________________________ 2 each year (when we are 94.8 million miles = 152.6 million km from the Sun).

Each day on Earth takes 23.93 ___________________________. Each year on Earth takes 365.26 Earth days (that is, it takes the Earth 365.26 days to orbit the Sun once). The Earth's rotation is slowing down very slightly over time, about one second every 10 years.

The Earth's axis is tilted from perpendicular to the plane of the ecliptic by 23.45°. This tilting is what gives us the four ___________________________ of the year: Summer, Spring, Winter and ___________________________. Since the axis is tilted, different parts of the globe are oriented towards the Sun at different times of the year; this affects the amount of ___________________________ each receives.