1.1a Earths Sun is an average-sized star. The Sun is more than a million times greater
in volume than Earth.
1.1b Other stars are like the Sun but are so far away that they look like points of light.
Distances between stars are vast compared to distances within our solar system.
1.1c The Sun and the planets that revolve around it are the major bodies in the solar sys-
tem. Other members include comets, moons, and asteroids. Earths orbit is nearly circular.
1.1d Gravity is the force that keeps planets in orbit around the Sun and the Moon in
orbit around the Earth.
1.1e Most objects in the solar system have a regular and predictable motion. These
motions explain such phenomena as a day, a year, phases of the Moon, eclipses, tides,
meteor showers, and comets.
1.1f The latitude/longitude coordinate system and our system of time are based on
1.1g Moons are seen by reflected light. Our Moon orbits Earth, while Earth orbits the
Sun. The Moons phases as observed from Earth are the result of seeing different por-
tions of the lighted area of the Moons surface. The phases repeat in a cyclic pattern in
about one month.
1.1h The apparent motions of the Sun, Moon, planets, and stars across the sky can be
explained by Earths rotation and revolution. Earths rotation causes the length of one
day to be approximately 24 hours. This rotation also causes the Sun and Moon to appear
to rise along the eastern horizon and to set along the western horizon. Earths revolution
around the Sun defines the length of the year as 365 1/4 days.
1.1i The tilt of Earths axis of rotation and the revolution of Earth around the Sun cause
seasons on Earth. The length of daylight varies depending on latitude and season.
1.1j The shape of Earth, the other planets, and stars is nearly spherical.