1.1a Most objects in the solar system are in regular and predictable motion.

¥ These motions explain such phenomena as the day, the year, seasons, phases of the

moon, eclipses, and tides.

¥ Gravity influences the motions of celestial objects. The force of gravity between two

objects in the universe depends on their masses and the distance between them.

1.1b Nine planets move around the Sun in nearly circular orbits.

¥ The orbit of each planet is an ellipse with the Sun located at one of the foci.

¥ Earth is orbited by one moon and many artificial satellites.

1.1c Earth’s coordinate system of latitude and longitude, with the equator and prime

meridian as reference lines, is based upon Earth’s rotation and our observation of the

Sun and stars.

1.1d Earth rotates on an imaginary axis at a rate of 15 degrees per hour. To people on

Earth, this turning of the planet makes it seem as though the Sun, the moon, and the

stars are moving around Earth once a day. Rotation provides a basis for our system of

local time; meridians of longitude are the basis for time zones.

1.1e The Foucault pendulum and the Coriolis effect provide evidence of Earth’s


1.1f Earth’s changing position with regard to the Sun and the moon has noticeable


¥ Earth revolves around the Sun with its rotational axis tilted at 23.5 degrees to a line

perpendicular to the plane of its orbit, with the North Pole aligned with Polaris.

¥ During Earth’s one-year period of revolution, the tilt of its axis results in changes in

the angle of incidence of the Sun’s rays at a given latitude; these changes cause variation in the heating of the surface. This produces seasonal variation in weather.

1.1g Seasonal changes in the apparent positions of constellations provide evidence of

Earth’s revolution.

1.1h The Sun’s apparent path through the sky varies with latitude and season.

1.1i Approximately 70 percent of Earth’s surface is covered by a relatively thin layer of

water, which responds to the gravitational attraction of the moon and the Sun with a

daily cycle of high and low tides.